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Copper vapor laser application for surface monitoring of divertor and first wall in ITER
Fusion Engineering and Design 60 (2002) 141– 155 www.elsevier.com/locate/fusengdes
Copper vapor laser application for surface monitoring of divertor and first wall in ITER O.I. Buzhinskij a,*, N.N. Vasiliev b, A.I. Moshkunov c, I.A. Slivitskaya c, A.A. Slivitsky c a b
Troitsk Institute for Inno6ation and Fusion Research, 142190 Troitsk, Moscow, Russia Nuclear Fusion Institute, Russian Research Center ‘Kurchato6 Institute’, Moscow, Russia c Joint-Stock Company, Altech, Russia Received 5 October 2001; accepted 26 November 2001
Abstract The availability of copper vapor laser system for surfaces surveying of International Thermonuclear Experimental Reactor (ITER) divertor and first wall during plasma discharge are justified. The construction concept of laser projector with image intensity amplification of the objects removed at a distance up to 20 m under intensive plasma background conditions is developed. The preliminary optical scheme for divertor and first wall surface monitoring from upper port is developed. Diffraction angle resolution at the object space is estimated (equal 1.18 × 10 − 5 rad).The preliminary estimation of laser viewing system power parameters is carried out. Laser system operation under intensive plasma background conditions (Tth/n = 104 eV — typical thermonuclear temperature). Power relation signal/noise, calculated at the image intensity amplifier input is equal S/N =104, i.e. plasma background radiation can be neglected in the considered scheme. Hence, the proposed technique application for monitoring of ITER discharge chamber internal surfaces is possible during running cycles, and in pauses especially. Experimental tests of laser system without scanning mirror and plasma noise background are carried out. Objects were located at a distance about 20 m from laser and amplifier. Probing laser had the next operating parameters: average radiation power, 10 W; pulse repetition rate, 10 kHz; output beam diameter, 14 mm. Linear resolution at object’s plane about 1 mm was obtained. Degradation of resolution in comparison with estimated value— 0.24 mm is related with insufficient degree of the image contrast, as the laser amplifier tube had unnormalized noise level. Method determined rate of the protecting cover erosion of surveyed surfaces is proposed. Under given measurement method application the laser viewing system can record decreasing of protection cover depth about 0.5 mm, and available value of protection cover erosion rate is equal 0.5 mm/ms. Object’s surface observe methods are considered. Laser beam noninterlaced raster scanning on surveyed surface is proposed. The laser beam moving velocity is determined by required accuracy of the image reproduction. Pointing direction definition method based on checkpoint grids is offered. Quadratic pyramidal deepening center may be a checkpoint grid node of pointing direction system. Accuracy of pointing direction positioning at the object equals 2 mm provided an even decrease of protection cover thickness near the given deepening. © 2002 Elsevier Science B.V. All rights reserved.
* Corresponding author. Tel.: + 7-95-334-0538; fax: + 7-95-334-5510. E-mail address: [email protected] (O.I. Buzhinskij). 0920-3796/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0920-3796(01)00610-X
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Keywords: Cu-laser; Divertor tile; First wall; Tokamak
1. Introduction Surveying of surfaces of International Thermonuclear Experimental Reactor (ITER) divertor and first wall during plasma discharge would be very interesting and useful. It is a difficult problem because of plasma screening effects of the surveying surfaces. Traditional optical methods, as a rule, do not allow observing any object through brightly radiating plasma layer, because background light radiation intensity usually is much more than light intensity, scattered by the observed object surface. Light amplification phenomenon employed for active optical medium allows other decisions of any diagnostic problems, in particular, under intensive plasma background radiation conditions. Since the mid 1970s the new metal vapor laser class, possessing very high light signal gain occurred [1], chances of new optical systems construction appeared due to the use of object’s image light intensity amplification [2]. Using modern laser engineering, it is possible to survey surfaces of ITER divertor and first wall and to study time evolution of the most interesting regions of the discharge chamber during all running cycles. The idea is as follows. The surveyed object is illuminated by laser radiation pulse which is not absorbed by plasma and which peak intensity has a much higher value than background intensity within a given solid angle. During illumination pulse width it is possible to view an object of interest for us. To obtain the object’s image it is necessary to collect laser radiation scattered on the object’s surface, to amplify it selectively, and to focus it at the recording device surface. It can be carried out if the laser amplifier medium signal gain would be sufficiently high. The laser mode operation can be as pulse-repetition mode to give the possibility to register the surveying object changes in real time. The pulse copper vapor laser represents a sufficiently proved device that is produced in small
series, as in Russia, and abroad. These lasers various modifications are used for different applications [3]. For examples, medicine, different technological applications, laser isotope isolation methods [4], atmospheric laser probing, plasma diagnostic, advertising and show business and so on. Copper vapor laser is most preferable to use in proposed diagnostics due to a lot of obvious advantages: Visible radiation at green (510.6 nm) and yellow (578.2 nm) wavelengths; High average generation power (10–600) W [4]; Pulsed operation mode with typical pulse duration (10–20) ns; High pulse repetition rate from 10 up to 50 kHz; Possibility of obtaining of the radiation divergence close to diffractional limit; Extremely high small signal gain in laser medium— more than 100 dB/m; Excellent optical homogeneity of laser medium. Two last features of copper vapor laser— high gain value and optical homogeneity of laser medium— are especially important for chosen application. Using similar laser medium for analysis of scattered radiation, it is possible to receive intensity amplification effect of surveyed surface image with a simultaneous spectral and time filtration of noise. It is obtained due to application of identical laser mediums, both in a source of probing radiation, and in analyzing amplifying optical system. Wave front, passing through the laser amplifier, will keep the information about surveyed surface image completely. Amplifier output radiation intensity is more than sufficient to use standard TV CCD chambers as analyzing devices. In the given report a feasibility to apply the proposed technique for ITER plasma facing surfaces surveying is discussed both during operation cycles, and in pauses between them. For this goal the arrangement of the zones representing the
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greatest interest for surveying, and ports, suitable for installation of send– receive optics is considered. The discharge cavity typical sizes significant to a given method are determined. The laser system block-scheme description is given. Estimations of the spatial resolution for one variant of the optical scheme as well as validation of plasma optical properties and the system energetic parameters are carried out. Feasibility of in situ wall protection erosion rate measurement (including disruption conditions) is discussed. The optical scheme of laser viewing system placed at the upper diagnostic port is given. The optimal arrangement variant of laser system in upper and equatorial ITER ports is considered. Observation time of the divertor and more parts of the chamber’s internal surfaces are estimated. The preliminary analysis of operation speciality of the given laser system in varying magnetic field conditions is performed. The design basic directions for real system construction of the divertor and first wall ITER monitoring are determined.
2. Design features of ITER discharge chamber
2.1. ITER plasma facing components surfaces of greatest interest for sur6eying It is known, that ITER divertor plates and adjoining to them areas of the first wall will operate at the most intense conditions. Thus a survey of them may turn out the most appreciable. Analyzing arrangements of plasma faced components inside of the chamber one can remarks that part of the divertor plasma facing areas are seen best of all from the upper diagnostic ports. The invisible (from the upper ports) part of them could be observed through the divertor ports. The first wall surface areas of torus are well visible from equatorial ports at probing beam incident angles band (0 –45) degrees.
2.2. Internal ca6ity specific sizes of the discharge chamber Let’s define specific optical path lengths for the laser beams outgoing from various ports. Under
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divertor viewing from the upper port the vertical beam passes through plasma volume one direction distance about 8– 9 m. Under inner covering viewing from equatorial ports in the planes close to horizontal, the maximal one pass length through plasma volume will be about 15 m. On the basis of these data in the subsequent calculations we shall accept one pass length through plasma volume equals the maximum of those listed above, i.e. 15 m. Detailed description of laser viewing systems arrangement is given in Section 4.
3. The description of laser monitoring system for viewing of ITER discharge chamber internal surface
3.1. Block-scheme of laser 6iewing system Let’s consider main design principles of the viewing system based on copper vapor laser. As is described earlier, the object’s information may be received as a result of scattered laser radiation collecting with the subsequent power amplification and the image projection at CCD matrix connected to the computer. Copper vapor laser operates in a pulse mode; lifetime of inverse population density of the upper laser level is equal to 10–12 ns. Therefore effective amplification of the object’s scattered light is possible only if this light will return into the laser medium during 10 ns. It means, that the distance between the laser and object should be less than 1.5 m. To survey more removed objects it is necessary to use a system containing two identical laser media, one of which serves for illumination, and another for amplification of scattered radiation. The time delay between pumping pulses of these lasers is determined by optical path lengths from probing laser up to observed object and back up to the amplifier. Advantage of such system is the ability of different distances tuning up to object at constant amplification efficiency. Besides, it is possible to optimize operating conditions for each laser separately. The block scheme of such a system is represented in Fig. 1. Copper vapor laser beam (1) goes through the optical path (9) to the researched object (5), situated in plasma volume (6). The laser radiation
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scattered by the object, passed through plasma volume and wall aperture (7), collects by receiving objective (8). Then the scattered light passes through the optical path (9) and after matching optics (10) enters the copper vapor laser amplifier (2). The objective (11) projects surveyed object’s image on CCD matrix receiving plate (12); CCD signal is sent to the computer (13). The computer synchronizes lasers, controls optics, registers and processes images. Probing and object’s scattered laser beams pass through the plasma volume. This radiation is scattered by plasma components, absorbed and refracted; besides polarization of light is changed. Some questions of laser beams propagation are considered in Section 3.2. Intensive plasma radiation is noise component at amplifier input. Ratio signal/noise definition at laser amplifier input is considered in Section 3.4.
where Ne is electron density (Ne = 5×1014 cm − 3), e is electron charge (e =1.60× 10 − 19 coul), me is electron mass (me = 9.11× 10 − 31 kg), m0 is electric constant (m0 = 8.85× 10 − 12 farad m − 1). For comparison cyclic frequency for green wavelength of copper vapor laser is equal g = 3.70× 1015 Hz, and for yellow wavelength— y = 3.26× 1015 Hz. Since the given laser frequencies don’t match with plasma absorption spectrum, ( g \ pe and y \ pe), the laser radiation absorption by plasma can be neglected. The refraction index of plasma for laser wavelengths is equal
3.2. Optical properties of plasma
Thus, for considered conditions plasma can be supposedly transparent at copper vapor laser wavelengths with refraction index n 1. The operation of the given laser system is practically independent on light polarization transform under the pass through plasma, since radiation of this laser possess natural polarization. Besides, the optical scheme of the laser system is polarization-independent.
Let’s consider the plasma transparency inside ITER discharge chamber for a visible range of wavelengths. The plasma cutting frequency (Langmuir oscillations frequency) is: pe =
'
Ne e 2 =1.26×1012 Hz, m0me
ng = ny =
' '
1−
2pe = 0.99999994, 2g
1−
2pe = 0.99999993. 2y
Fig. 1. Copper vapor laser viewing system. Block scheme. 1, Copper vapor laser generator; 2, copper vapor laser amplifier; 3, pumping pulse source; 4, pumping pulse source; 5, under investigation object; 6, plasma volume; 7, wall aperture; 8, receiving objective; 9, optical path; 10, matching optics; 11, lens; 12, CCD matrix; 13, computer; 14, synchronizer; 15, tunable time delay.
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1.22uy = 1.18× 10 − 5 rad, DR
3.3. The description of the optical scheme 6ariant
0 =
As an example we shall consider one of possible variants of the given scheme (see Fig. 1). Laser generator output beam (1) has a diameter d0 =14 mm and angular divergence q= 1.44 ×10 − 3 rad. Pulse energy of laser is equal to 1 mJ. After passing distance L= 25 m (including 15 m through plasma as it was marked in Section 2.2), laser beam illuminates surveyed object. The laser spot diameter at the object’s surface can be obtained as:
where uy = 578.2 nm is wavelength of laser radiation, DR = 60 mm is receiving objective diameter. We shall assume, that plasma is optically homogeneous medium with refraction index n= 1. In this case extreme linear resolution at the object located at a distance from receiving objective R0 = (5–20) m is equal Dx0 = (0.06–0.24) mm. Diaphragm diameter at plasma volume boundary as was specified earlier, is equal Dw = 58 mm. If optical path protection against neutrons flux will require to reduce diameter of this diaphragm, for example, to value Dw = 10 mm, extreme angular resolution at space of object is equal C0 = 7.05× 10 − 5 rad. Accordingly, extreme linear resolution at object, will be equal Dx0 = (0.35–1.41) mm.
Di = d0 + qL
Di = 50 mm
Let’s assume, that laser light is scattered by surveyed surface within solid angle 2y. As already mentioned it was marked above, plasma can be supposedly transparent for green and yellow wavelengths of laser radiation. Diaphragm is placed on the boundary of plasma volume in flash with the port-plug first wall. The receiving objective collects scattered radiation passed through the diaphragm. The diaphragm diameter is equal Dw =58 mm, receiving objective is placed outside of plasma volume at distance R0 =20 m from observed object and at distance Rpl =5 m from the diaphragm plane. We shall accept receiving objective aperture equal DR = 60 mm. It is necessary to note, that features of the optical scheme angular selection determine sizes of diameters Dw and DR at a preset value of diameter Di, and one of system spatial filter elements is the laser amplifier. After passing through receiving objective scattered radiation is propagated along optical path channels which design is defined by radiating safety requirements of external premises. The optical path includes some mirrors and vacuum windows. We shall assume, that this path transmittance is equal }opt =0.1. Then radiation enters the laser amplifier through matching optics. Light beam, past through the amplifier, transfers the information about the object. The object’s image is projected on CCD matrix receiving plane by objective. Such a manner of optical system performance (7, 8, 10, 2, 11 see Fig. 1) is chosen that image aberration can be neglected. Then angular resolution in space of object corresponds to Rayleigh diffraction for yellow laser wavelength
3.4. Laser system power parameters estimation Normal operation of the given system is possible only provided that laser radiation is not absorbed by plasma, and that plasma radiation energy at laser wavelengths will have a lower level of object’s scattered laser radiation energy at amplifier input. For ratio signal/noise estimation of the describing system (see Fig. 1), at first we shall determine the useful signal level at the laser amplifier input. Let’s assume that object’s scattered radiation intensity inside solid angle of 2y is constant and does not depend on scattering angle. We shall accept integral scattering factor equal = 0.1. Then, laser radiation energy, scattered by object’s surface elementary unit (for example, area of 1 mm2), is determined at amplifier input as ER = Et}opt
S pSR }opt Si2yR 20
ER = 5.73× 10 − 16 J
where Et = 1 mJ — probing laser pulse energy (see Section 3.3), Sp = 1 mm2 — object’s surface elementary unit area, SR =
yD 2R =28.27 cm2 − recieving objective area, 4
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yD 2i = 19.64 cm2 4 laser radiation spot area at the object surface. Si =
The ratio Si/Sp is equal to number of object’s surface elementary units within laser spot. As plasma radiation for given viewing system is noise, it is necessary to determine, what part of this radiation energy is emitted at laser wavelengths (ug = 510.6 nm, uy =578.2 nm) during generation pulse inside receiving objective effective solid angle. We use an absolutely black body model for plasma radiation parameters calculation. Spectral distribution function of radiation energy flux is defined by Plank radiation law (the radiation which are emitted from surface unit during time unit in wavelengths interval unit in a solid angle of 2y) and has the following form: F(u, Ts)=
2yhc 2u − 5 ; hc exp −1 kuTs
where h is Plank constant (h = 6.626 ×10 − 34 J s); c is light velocity in vacuum (c =2.998 ×108 m s − 1); k is Boltzmann constant (k= 1.381 × 10 − 23 J K − 1); T is temperature (T =3 ×107 K). For tokamak thermonuclear plasma radiation intensity is 1/w times less than black body radiation intensity. w:
2×10 − 11(NiNe)2/7 , Tth/n
w =5 ×10 − 7,
where Ni =Ne =5× 1014 cm − 3 is ions and electrons densities, Tth/n =104 eV is typical thermonuclear temperature [5]. Therefore plasma radiation energy which are emitted by surface unit 1 cm2 (placed at diaphragm plane Dw) during laser generation pulse within laser line Doppler width for green and yellow wavelengths, respectively, is equal zg = wF(ug, Ts)Du~ zg = 1.75×10 − 11 J · cm − 2 zy = wF(uy, Ts)Du~ zy = 1.0610 − 11 J cm − 2
where Du is copper vapor laser generation line width (Du = 2.025× 10 − 10 cm), ~ is laser generation pulse duration (~= 15 ns). Accordingly, plasma radiation energy, summarized for two wavelengths, at laser amplifier input is equal EB = (zg + zy)Sw
Sp Ssel }opt Si 2yR 2pl
EB = 5.70× 10 − 20 J where Sw is diaphragm area at plasma boundary, Si/Sp is the number of object’s surface elementary units within laser spot, Rpl is distance from a diaphragm up to receiving objective (Rpl = 5 m), }opt is optical path transmittance (}opt = 0.1), Ssel is equivalent receiving objective area, defined as Ssel =
y(qselRpl)2 , 4
where qsel is spatial selection angle of the optical scheme, counted for receiving objective plane (qsel = 3.5× 10 − 3 rad). Optical path contains a vacuum window made of fused silica, assume that of KU-1 or KS-4V, which after an irradiation becomes a source of photoluminescence radiation in the wide spectrum range. This radiation at laser amplifier input is the additive to plasma noise. However, preliminary estimations based on report data [6] have shown, that the phenomenon of window photoluminescence for the given optical scheme can be neglected. Thus, the power relation signal/noise, probable at the image intensity amplifier input is equal S/N=
ER = 104, EB
i.e. plasma background radiation can be neglected in the considered scheme. Hence, the proposed technique application for monitoring of ITER discharge chamber internal surfaces is possible during running cycles, and in pauses especially.
3.5. Selection of object’s surface sur6eying method and pointing direction definition technique Early described optical system included two opposing laser beams propagating inside suffi-
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Fig. 2. Copper vapor laser viewing system, placed at upper port. 1, Laser; 2, vacuum window; 3, upper diagnostic port; 4, first mirror; 5, aperture; 6, divertor.
ciently small solid angles. This way allows high quality receiving of object’s image. Optical axes of probing and scattered beams are crossed in the center of surveying surface place of diameter 50 mm. However the given place is only a small part of all surfaces, which is necessary for monitoring. There are two ways of observing area increasing. We shall consider each of them. The first way consists of enlarging illuminated surface area N times. Then, under keeping of a signal/noise ratio, it is necessary to raise probing laser radiation power N times, too. In Section 3.4 calculation for the laser average power of radiation 10 W is given at pulse repetition rate 10 kHz. The magnification of illuminated spot diameter, for example in 40 times, i.e. up to 2 m that corresponds to ITER divertor ring width, will demand raising of probing laser radiation power up to 16000 W. Achievement of such level of average power for copper vapor laser is a problem. Except for it, under change to wideangle optics the optical scheme aberrations will worsen. Thus, this way appears technically difficult, expensive and does not solve a problem of the large areas analysis as a whole. The second way is based on use of laser beam noninterlaced raster scanning on surveyed surface. In this case the laser beam moves in space within pyramid volume. Pyramid top lays on a
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surface of the scanner mirror, and the top plane angle corresponds to oscillation amplitude of the scanner mirror (the top angle is equal 2h, see Figs. 2 and 13). The laser beam moving velocity is determined by the given degree of spots overlapping from pulse to pulse. Spots overlapping degree is defined by required accuracy of the image reproduction. The probing laser beam and scattered radiation beam are propagated through the same optical path. So, this way has a number of advantages, though it is not easy, too. The optics of scanning system should not worsen the quality of the object’s image. The scanner should operate at a temperature about 150 °C and in magnetic fields (B=0.1 T; dB/dt =0.1 T s − 1). (The estimation of copper vapor laser stable running in magnetic fields is given in Section 4.5.) For the given observing method the appropriate program of registration, processing and image identification should be developed. Computer control of the scanner will allow establishing pointing direction at the object with the accuracy defined scanner backlash. We admit distance from the scanner up to an object of 20 m, then at scanner angular backlash of 10 s linear positioning accuracy at object’s plane will equal 2 mm. If the system includes a few consistently located scanners the mistake is accordingly increased. On the other hand, to define the pointing direction it is possible to use any marks that may be concrete design units features and specially made checkpoint grids. Checkpoint grid manufacture on observed surfaces would help to solve other important problem on inspection of wall protection erosion rate. This technique will be useful under keeping of measurements described in Section 3.6.
3.6. To the problem of di6ertor plates protection co6er erosion measurement at use of 6iewing system with the image intensity laser amplifier It is known, that one of the most important problems at experiments performance with ITER is quantitative principles definition of plasma facing components protection erosion process,
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first of all divertor, as in normal operation modes, and —especially—in failure modes. Application of image intensity laser amplifier operating in pulse-frequency mode enables process viewing of erosion on any allocated place in-situ even during failure. Essentially it may look as follows. Beforehand, any certain relief, for example, pyramidal deepening, is made at the protection cover zone, allocated for monitoring. In process of protection cover ablation the relief configuration will change. This change in value will depend on erosion depth. Availability of the timebase will allow determination of process kinetic parameters. Viewing method resolution will specify concrete values of technique sensitivity. The spatial resolution of the optical scheme without scanning is considered in Section 3.3. Diffraction limited linear resolution at object located at distance from receiving objective 20 m is equal 0.24 mm. Under scanning scheme addition spatial resolution at object’s plane may worsen up to 1 mm. We shall consider the reference point option proposed above. Let us have quadratic pyramidal deepening in protection cover. Pyramid height is about 5 mm and pyramid vertex angle equals 120°, then square side value equals 17 mm. This value decreases due to ablation. Viewing system will record this decreasing plane projection that is perpendicular to pointing direction. In other words, recording value will depend on the incident angle of probing laser beam at researched surface. Let ablation depth be about 1 mm. If laser beam incident angle equals 30°, then the decrease of surface square side is about 3 mm. If laser beam incident angle equals 45°, then the decrease of the surface square side is about 2.5 mm. Thus, under given measurement method application the laser viewing system can record the decrease of protection cover depth is about 0.5 mm. Time resolution of the method will be defined by laser pulse repetition rate and predetermined pulses number within time averaging interval of recorded information. Let under positioning of viewing system on the same reference point, time averaging object’s image is registered during ten consecutive laser pulses. Then, under laser pulse repetition rate
equals 10 kHz the computer will reproduce staging variation pattern of object’s surface image with step 0.001 s. Therefore, available value of protection cover erosion rate is equal 0.5 mm ms − 1. Except for that, quadratic pyramidal deepening center may be a checkpoint grid node of the pointing system. It was mentioned in Section 3.5. Accuracy of pointing direction positioning at the object is equal 2 mm provided even decreasing of protection cover thickness near the given deepening.
4. Laser system complex for surface monitoring of all divertor and first wall in ITER
4.1. Laser 6iewing system module, placed at upper diagnostic port As it was specified in Section 2.1, divertor and adjoining areas of first wall shielding will operate in the most intense conditions. Though divertor plates have the complex form, the most part of its surfaces (however not all) it is possible to see from above. Scheme of laser system arrangement at the upper diagnostic port is given in Fig. 2. The module containing laser generator, amplifier and basic optical scheme (1) is located behind the first vacuum boundary. Laser radiation passes through vacuum window (2), is reflected from flat mirror (4), and then passes through diaphragm (5) located at protective panel of upper diagnostic port. If mirror (4) turns around of two mutually perpendicular axes at angle 9h/2, (h8°), laser beam will scan the square of size 2.6 m× 2.6 m at divertor surface. In more detail the optical scheme with scanning mirror is given in Fig. 3. The given scheme specifies a relative positioning of optical elements. Functionally it is described in Section 3.1. For confirmation of the spatial resolution estimation of optical scheme (see Fig. 3) model tests of laser system without scanning mirror and plasma noise background are carried out. Objects were located at distance about 20 m from laser and amplifier. Probing laser had the next operating parameters: average radiation power, 10 W;
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Fig. 3. Copper vapor laser viewing system. Variant of optical scheme. 1, Laser generator tube; 2, laser amplifier tube; 3, resonator mirror; 4, attenuator; 5, concave mirror; 6, CCD matrix; 7, concave mirror; 8 – 10, plane mirrors; 11, vacuum window; 12, plane mirror; 13, aperture; 14, divertor.
pulse repetition rate, 10 kHz; output beam diameter, 14 mm. Under investigation object was illuminated by laser beam spot of diameter 50 mm. Receiving objective (clear aperture, 60 mm) was located at the distance of 20 m from the object’s plane. Amplified object’s image was projected at the screen. Linear resolution at object’s plane about 1 mm was obtained. Degradation of resolution in comparison with estimated value— 0.24 mm (see Section 3.3) is related with insufficient degree of the image contrast, as the laser amplifier tube had unnormalized noise level. Note, that scanning mirror (12) (see Fig. 3) is placed close to plasma volume. If under these conditions the mirror surface will deteriorate, it is necessary to locate this mirror at a more protected zone. One of the protection methods of the first mirror is moving it from plasma boundary at a distance of about 1 m inside the upper port box. The scanning mirror (1) unit located at a distance of 300 mm from input diaphragm of diameter 150 mm is represented in Fig. 4. The diaphragm size is increased in comparison with a computed value of 58 mm in view of mirror scanning at angle 9h/2 (h 8°). For protection of mirror’s reflecting surface the conic channel—blind by length on axis 1000 mm is arranged. It is supposed that dispersion of particles flows and radiations on rough
conic surface will not allow its collimation into diaphragm aperture. However, results of work [7] and personal discussion with the author guided the conclusion about insufficient protection of reflecting surface of scanning mirror (1) in design shown in Fig. 4. It proves to be true that the tests that have been carried out in similar dimensional conditions by Dr V. Voitseny. The samples of mirrors made of the SS316 substrate sputtered by Rh film of various thickness were tested. The most proof samples had reflectance of 75% for green area of spectrum. Other decision of the first mirror protection is proposed in design represented in Fig. 5(a). It is obvious, that the scanning block design should provide reliable protection of all elements. Its durability should correspond to operational
Fig. 4. Upper diagnostic port aperture. 1, Scanning mirror.
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Fig. 5. (a,b) Copper vapor laser viewing system with scanning module, placed at upper port. 1, Laser; 2, vacuum window; 3, upper diagnostic port; 4, scanning block; 5, first mirror; 6, aperture; 7, scanner; 8, divertor.
Fig. 6. Scanning module, placed at upper port. 1, Laser; 2, vacuum window; 3, upper diagnostic port; 4, scanning module; 5, first mirror; 6, aperture; 7, scanner.
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Fig. 7. General ITER torus view. 1, Laser module; 2, combined scanning module; 3, upper port; 4, equatorial port; 5, divertor port; 6, blanket module.
characteristics of the rest laser system components. Diaphragm of diameter 60 mm (6) is placed at the distance 1000 mm from the edge of plasma, the first mirror (5) of size (750× 750) mm is located at the distance 2500 mm from diaphragm plane. The first mirror builds the diaphragm image at the plane of two-coordinate scanner mirror (7). Laser beam passes through scanner, matching optics and enters into the optical system of laser module. The probing beam passes the same optical way from laser to divertor. Partitioning of probing and scattered light beams in the laser module is the same, as well as in scheme Fig. 3. This variant is lacking, because the part of scanning block is out of inclined upper port channel. Design updating is allowable for decreasing gabarit dimensions. Namely, the diaphragm size and the distance separating it from plasma remain constant, and
Fig. 8. Laser viewing system working from upper port. 1, Laser module; 2, combined scanning module; 3, upper port; 4, laser lighting of the divertor plates.
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Fig. 9. Optical scheme of scanning module. 1, First mirror; 2, scanner; 3, common optical path; 4, vacuum window; 5, laser module.
first mirror focal length is decreasing. This variant represents Fig. 5(b). Further design development of scanning module will be based on this modification. In more detail scanning module with laser are shown in Fig. 6. As is mentioned above, the scanning area from one rectangular mirror covers divertor strip width and looks like a square in size 2.6 m× 2.6 m. The circumference of the divertor ring on average line is approximately 30 m. And in project ITER it is supposedly five upper diagnostic viewing ports in regular intervals located on a circle. Hence, from one port it should be visible not less 6 m of divertor ring arc. It is possible, if combined scanning module design is executed as the united body including three identical scanning modules (Fig. 6). The combined scanning module is located at the plasma end of the upper port. General ITER torus view, the arrangement of the combined scanning module and the laser module are shown in Fig. 7. Laser system located in the upper port surveys about 7 m arc length on an average line of divertor ring; that is shown in Fig. 8. The optical scheme of the combined scanning module is shown in Fig. 9. The laser beam passes common path of matching optics, then it passes in sequence each rectangular mirror scanning scheme path to the divertor. Two laser systems placed in the corresponding upper ports, survey about 13 m divertor ring arc length that is shown in Fig. 10.
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4.2. Laser 6iewing system module, placed in equatorial port The laser system described in Section 4.1 may be used to observe the first wall protection surface. The scanning module consists of the four first mirrors and is placed in equatorial port. And the laser module is at the end of an equatorial tube similar to accommodation in Section 4.1. Zones of the surveying of the equatorial scanning module at the equatorial plane are shown in Fig. 11. The length of each viewing arc is equal 4.4 m. On a vertical the field of the observe covers all of the first wall from above up to the divertor upper part. Circumference of torus equatorial section is equal to 52.5 m. From one equatorial port the laser system sees totally 8.8 m arc length. Hence, from six equatorial ports it is possible to see all torus external walls. The installation general view is submitted in Fig. 12. Combined scanning module places in equatorial port. Solid fields of scanning by a laser beam from four scanning submodules are schematically represented in Fig. 12. Symmetric solid fields of scanning by the laser beam emergent from four scanning modules are shown in Fig. 13. In more detail the combined scanning module design consisted of four rectangular mirrors is shown in Fig. 14.
The envelope of the torus surface zone illuminated by laser light is shown in Fig. 15. Crosshatched blanket modules are illuminated by the upper and lower scanning modules.
4.3. Work description of laser system complex for surface monitoring Laser systems for surfaces conditions monitoring of divertor and part of first wall of ITER are considered in Sections 4.1 and 4.2. Divertor is surveyed from five upper inclined ports. All external parts of the chamber’s first wall surface are visible from six equatorial ports located regularly on a circle of equatorial torus section. All modules of laser systems operate independently from each other and may have the common control. Let’s estimate common surfaces observe time of the divertor and external part of the first wall surface. From one upper port the area equal to the sum of the areas of three squares with the side 2.6 m is surveyed. We remind, that observe way is based on use of laser beam noninterlaced raster scanning on researched surface with moving velocity appropriate to the given degree of spots overlapping from pulse to pulse. Spots overlapping degree is defined by required accuracy of the image reproduction. Let the laser beam spot move along the square side with spots overlapping a degree
Fig. 10. Two laser viewing systems placed in upper ports.
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Fig. 11. Torus equatorial section plane. 1, Lighting field; 2, equatorial port; 3, first mirror.
equal 0.5 from spot diameter. Spot diameter is equal 50 mm. Then it will be placed 105 spots on one square side. Spots rows have the same overlapping against each other, equal 0.5 from spot diameter. On all areas of a square it will be placed 11025 spots. It is sent ten probing laser pulses in one viewing point. Laser pulses repetition rate is equal 10 kHz. Hence, under the above listed conditions look-up time of one square 2.6 m×2.6 m is equal 11 s. Accordingly, consecutive look-up time of three squares from one upper port is equal 33 s. If all five upper laser ports operate simultaneously all the visible divertor surfaces are surveyed for 33 s, too. Now, let’s estimate operating time of laser viewing system from equatorial port. The combined scanning module contains four mirrors consistently running with one laser module. Each mirror illuminates a square with side 3.7 m, placed at a distance equal to 15 m from the equatorial port. Look-up time of such a square is equal 22 s. Then, consecutive look-up time of four squares from one equatorial port is equal 88 s. If all six laser equatorial ports operate simultaneously all external parts of the chamber’s first wall surface are surveyed also for 88 s. Let’s assume, that all laser systems operate simultaneously. Hence, common observed time of divertor surface and an external part of chamber’s first wall surface is equal 88 s. In this case the
complex structure includes 11 two-tube laser modules. Five of them are located in the upper ports and six in equatorial ports.
4.4. Running features of laser 6iewing system in magnetic, temperature and 6ibrating fields of ITER One of the essential factors that may influence operation stability of laser viewing system is the presence of strong non-stationary magnetic fields. A problem of magnetic field effect on the copper vapor laser activity can be arrived, when the laser is used as a brightness amplifier for the
Fig. 12. Laser system, placed in equatorial port. 1, Laser module; 2, combined scanning module; placed in equatorial port; 3, upper port; 4, divertor port; 5, blanket module; 6, solid laser lighting scanning fields.
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Fig. 13. Solid laser lighting scanning fields. 1, Combined scanning module, placed in equatorial port; 2, blanket module; 3, solid laser lighting scanning fields.
ITER viewing system. The laser has to be installed in one of the ports, where magnetic field is about 0.1 T that can be able to spoil discharge parameters. Magnetic field effect can become apparent as following: 1. Decreasing of discharge plasma electrical conductivity in direction that is transversal to magnetic field in (1+i 2) factor, where i = · ~ is Hall number. For copper vapor laser discharge conditions (neon as gas medium, p =40 kPa), magnetic field of 0.1 T and electron temperature of 1 eV the Hall number would be about 0.25. So this effect can be ignored as first estimation, if the laser is located far enough from poloidal field coils.
Fig. 15. Surveying zones of chamber surface for equatorial port viewing system. 1, Divertor; 2, blanket module; 3, laser lighting of blanket module (crosshatched plates); 4, solid laser lighting scanning fields.
2. Discharge plasma convection stimulated by pressure gradient 9p = J× B. For the case when the electrical field is directed along the discharge tube and magnetic field is directed across to axis of the tube, the discharge column will became bend and a part of the discharge plasma column located out of the tube axis will be rotated around the tube axis. So the discharge plasma could contact the tube wall and discharge will be abrupt. But possibly this convection has no time to be developed because of the discharge is very short ( 50 ns). There is no possibility to carry out simple quantitative estimations of last effects on this study. More comprehensive analysis will be required in future, including (maybe) direct experiment. Under design development of laser viewing system for maintenance of required accuracy and stability of its operation as a whole it is necessary to take into account influence of thermal and vibrating fields and its changing at different stages of running cycle ITER. In particular, updating of laser tubes cooling system for working at vacuum conditions is required.
5. Structure of monitoring laser system for ITER Fig. 14. Combined scanning module of equatorial port. 1, Mirrors of combined scanning module, placed in equatorial port; 2, blanket module; 3, laser lighting zones of blanket modules.
The real laser viewing system includes: Copper vapor laser for illumination, developed for operation at ITER conditions.
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Optical path of illumination and scattered radiation formation, including scanning module, system of mirrors, a vacuum window, objectives with the variable focal length, developed for operation at ITER conditions. Copper vapor laser amplifier, developed for operation at ITER conditions. Image registration system with CCD-camera. Computer for control of lasers, scanners, optics, registration and processing of the image. The program of control of lasers, scanners, optics. The program of registration, processing and image identification.
6. The conclusion
1. The optical scheme variant of combining scanning module for panoramic image generation of surveyed surface is developed. Optical scheme is shown in Fig. 9. 2. The laser systems arrangements in the upper and equatorial ports for all divertor visible surfaces and external part of first wall surface monitoring are offered. For this purpose in part it is required to occupy five upper and six equatorial ports (see Figs. 10 and 12). 3. The full survey time estimation of divertor and external part of the first wall surface is carried out. Observe time of divertor is equal to 33 s. Observe time of first wall external part is equal to 88 s. Thus, results of executed works confirm that the given method may be used for monitoring of divertor and first wall surfaces during working cycle ITER. For real system development the following steps of work performance is necessary: The laser system prototyping for method approbation and laboratory testing at one of operating tokamaks.
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Development and tests of definition methods of sheeting erosion rate. Optical properties studying of researched surfaces. Development, manufacturing and tests of scanning module, capable to work with the required accuracy at real conditions of ITER. Amendment of photoluminescence energy value of the irradiated vacuum windows. Amendment of transparency value of the irradiated vacuum windows. Optimization of laser amplifier operation modes to achieve required sensitivity and the interface with a CCD-matrix. Measurements of plasma radiation intensity level at probing laser wavelengths. Study of vibrations, temperature and magnetic fields influence on operating stability of laser viewing system for ITER. Development of the automated control system of lasers, scanners, optics, registration and processing of the information and image identification.
References [1] G.G. Petrash, Pulsed gas-discharge lasers, Sov. Phys.-Usp. 14 (1972) 747 – 765. [2] K.I. Zemskov, A.A. Isaev, M.A. Kazaryan, G.G Petrash, Laser projection microscope, Sov. J. Quant. Elect. 1 (1974) 14. [3] O.I. Buzhinskij, Evolution of examinations of the Cu-laser and possibility of its practical application, Rev. of Kurchatov Institute Of Atomic Energy (now RRC ‘Kurchatov Institute’), 1983 (in Russian). [4] O.P. Maximov, V.A. Mishin, A.I. Moshkunov, I.A. Slivitskaya, A.A. Slivitsky, V.A. Shutov, 600W CVL System, Tech. Digest of International Conference Lasers’97, December 1997, New Orleans. [5] A.M. Prokhorov (Ed.), Physical Encyclopedia, vol. 2, 1990, pp. 109. [6] I.V. Moskalenko, N.A. Molodtsov, N.Yu. Vukolov, Results of Study Photoluminescence Induced in Pre-Irradiated Optical Materials under UV Radiation, Moscow, 2001. [7] V.S. Voitseny, Latest data from simulation experiments in Kharkow, 2001.
Copper vapor laser application for surface monitoring of divertor and first wall in ITER O.I. Buzhinskij a,*, N.N. Vasiliev b, A.I. Moshkunov c, I.A. Slivitskaya c, A.A. Slivitsky c a b
Troitsk Institute for Inno6ation and Fusion Research, 142190 Troitsk, Moscow, Russia Nuclear Fusion Institute, Russian Research Center ‘Kurchato6 Institute’, Moscow, Russia c Joint-Stock Company, Altech, Russia Received 5 October 2001; accepted 26 November 2001
Abstract The availability of copper vapor laser system for surfaces surveying of International Thermonuclear Experimental Reactor (ITER) divertor and first wall during plasma discharge are justified. The construction concept of laser projector with image intensity amplification of the objects removed at a distance up to 20 m under intensive plasma background conditions is developed. The preliminary optical scheme for divertor and first wall surface monitoring from upper port is developed. Diffraction angle resolution at the object space is estimated (equal 1.18 × 10 − 5 rad).The preliminary estimation of laser viewing system power parameters is carried out. Laser system operation under intensive plasma background conditions (Tth/n = 104 eV — typical thermonuclear temperature). Power relation signal/noise, calculated at the image intensity amplifier input is equal S/N =104, i.e. plasma background radiation can be neglected in the considered scheme. Hence, the proposed technique application for monitoring of ITER discharge chamber internal surfaces is possible during running cycles, and in pauses especially. Experimental tests of laser system without scanning mirror and plasma noise background are carried out. Objects were located at a distance about 20 m from laser and amplifier. Probing laser had the next operating parameters: average radiation power, 10 W; pulse repetition rate, 10 kHz; output beam diameter, 14 mm. Linear resolution at object’s plane about 1 mm was obtained. Degradation of resolution in comparison with estimated value— 0.24 mm is related with insufficient degree of the image contrast, as the laser amplifier tube had unnormalized noise level. Method determined rate of the protecting cover erosion of surveyed surfaces is proposed. Under given measurement method application the laser viewing system can record decreasing of protection cover depth about 0.5 mm, and available value of protection cover erosion rate is equal 0.5 mm/ms. Object’s surface observe methods are considered. Laser beam noninterlaced raster scanning on surveyed surface is proposed. The laser beam moving velocity is determined by required accuracy of the image reproduction. Pointing direction definition method based on checkpoint grids is offered. Quadratic pyramidal deepening center may be a checkpoint grid node of pointing direction system. Accuracy of pointing direction positioning at the object equals 2 mm provided an even decrease of protection cover thickness near the given deepening. © 2002 Elsevier Science B.V. All rights reserved.
* Corresponding author. Tel.: + 7-95-334-0538; fax: + 7-95-334-5510. E-mail address: [email protected] (O.I. Buzhinskij). 0920-3796/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0920-3796(01)00610-X
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Keywords: Cu-laser; Divertor tile; First wall; Tokamak
1. Introduction Surveying of surfaces of International Thermonuclear Experimental Reactor (ITER) divertor and first wall during plasma discharge would be very interesting and useful. It is a difficult problem because of plasma screening effects of the surveying surfaces. Traditional optical methods, as a rule, do not allow observing any object through brightly radiating plasma layer, because background light radiation intensity usually is much more than light intensity, scattered by the observed object surface. Light amplification phenomenon employed for active optical medium allows other decisions of any diagnostic problems, in particular, under intensive plasma background radiation conditions. Since the mid 1970s the new metal vapor laser class, possessing very high light signal gain occurred [1], chances of new optical systems construction appeared due to the use of object’s image light intensity amplification [2]. Using modern laser engineering, it is possible to survey surfaces of ITER divertor and first wall and to study time evolution of the most interesting regions of the discharge chamber during all running cycles. The idea is as follows. The surveyed object is illuminated by laser radiation pulse which is not absorbed by plasma and which peak intensity has a much higher value than background intensity within a given solid angle. During illumination pulse width it is possible to view an object of interest for us. To obtain the object’s image it is necessary to collect laser radiation scattered on the object’s surface, to amplify it selectively, and to focus it at the recording device surface. It can be carried out if the laser amplifier medium signal gain would be sufficiently high. The laser mode operation can be as pulse-repetition mode to give the possibility to register the surveying object changes in real time. The pulse copper vapor laser represents a sufficiently proved device that is produced in small
series, as in Russia, and abroad. These lasers various modifications are used for different applications [3]. For examples, medicine, different technological applications, laser isotope isolation methods [4], atmospheric laser probing, plasma diagnostic, advertising and show business and so on. Copper vapor laser is most preferable to use in proposed diagnostics due to a lot of obvious advantages: Visible radiation at green (510.6 nm) and yellow (578.2 nm) wavelengths; High average generation power (10–600) W [4]; Pulsed operation mode with typical pulse duration (10–20) ns; High pulse repetition rate from 10 up to 50 kHz; Possibility of obtaining of the radiation divergence close to diffractional limit; Extremely high small signal gain in laser medium— more than 100 dB/m; Excellent optical homogeneity of laser medium. Two last features of copper vapor laser— high gain value and optical homogeneity of laser medium— are especially important for chosen application. Using similar laser medium for analysis of scattered radiation, it is possible to receive intensity amplification effect of surveyed surface image with a simultaneous spectral and time filtration of noise. It is obtained due to application of identical laser mediums, both in a source of probing radiation, and in analyzing amplifying optical system. Wave front, passing through the laser amplifier, will keep the information about surveyed surface image completely. Amplifier output radiation intensity is more than sufficient to use standard TV CCD chambers as analyzing devices. In the given report a feasibility to apply the proposed technique for ITER plasma facing surfaces surveying is discussed both during operation cycles, and in pauses between them. For this goal the arrangement of the zones representing the
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greatest interest for surveying, and ports, suitable for installation of send– receive optics is considered. The discharge cavity typical sizes significant to a given method are determined. The laser system block-scheme description is given. Estimations of the spatial resolution for one variant of the optical scheme as well as validation of plasma optical properties and the system energetic parameters are carried out. Feasibility of in situ wall protection erosion rate measurement (including disruption conditions) is discussed. The optical scheme of laser viewing system placed at the upper diagnostic port is given. The optimal arrangement variant of laser system in upper and equatorial ITER ports is considered. Observation time of the divertor and more parts of the chamber’s internal surfaces are estimated. The preliminary analysis of operation speciality of the given laser system in varying magnetic field conditions is performed. The design basic directions for real system construction of the divertor and first wall ITER monitoring are determined.
2. Design features of ITER discharge chamber
2.1. ITER plasma facing components surfaces of greatest interest for sur6eying It is known, that ITER divertor plates and adjoining to them areas of the first wall will operate at the most intense conditions. Thus a survey of them may turn out the most appreciable. Analyzing arrangements of plasma faced components inside of the chamber one can remarks that part of the divertor plasma facing areas are seen best of all from the upper diagnostic ports. The invisible (from the upper ports) part of them could be observed through the divertor ports. The first wall surface areas of torus are well visible from equatorial ports at probing beam incident angles band (0 –45) degrees.
2.2. Internal ca6ity specific sizes of the discharge chamber Let’s define specific optical path lengths for the laser beams outgoing from various ports. Under
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divertor viewing from the upper port the vertical beam passes through plasma volume one direction distance about 8– 9 m. Under inner covering viewing from equatorial ports in the planes close to horizontal, the maximal one pass length through plasma volume will be about 15 m. On the basis of these data in the subsequent calculations we shall accept one pass length through plasma volume equals the maximum of those listed above, i.e. 15 m. Detailed description of laser viewing systems arrangement is given in Section 4.
3. The description of laser monitoring system for viewing of ITER discharge chamber internal surface
3.1. Block-scheme of laser 6iewing system Let’s consider main design principles of the viewing system based on copper vapor laser. As is described earlier, the object’s information may be received as a result of scattered laser radiation collecting with the subsequent power amplification and the image projection at CCD matrix connected to the computer. Copper vapor laser operates in a pulse mode; lifetime of inverse population density of the upper laser level is equal to 10–12 ns. Therefore effective amplification of the object’s scattered light is possible only if this light will return into the laser medium during 10 ns. It means, that the distance between the laser and object should be less than 1.5 m. To survey more removed objects it is necessary to use a system containing two identical laser media, one of which serves for illumination, and another for amplification of scattered radiation. The time delay between pumping pulses of these lasers is determined by optical path lengths from probing laser up to observed object and back up to the amplifier. Advantage of such system is the ability of different distances tuning up to object at constant amplification efficiency. Besides, it is possible to optimize operating conditions for each laser separately. The block scheme of such a system is represented in Fig. 1. Copper vapor laser beam (1) goes through the optical path (9) to the researched object (5), situated in plasma volume (6). The laser radiation
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scattered by the object, passed through plasma volume and wall aperture (7), collects by receiving objective (8). Then the scattered light passes through the optical path (9) and after matching optics (10) enters the copper vapor laser amplifier (2). The objective (11) projects surveyed object’s image on CCD matrix receiving plate (12); CCD signal is sent to the computer (13). The computer synchronizes lasers, controls optics, registers and processes images. Probing and object’s scattered laser beams pass through the plasma volume. This radiation is scattered by plasma components, absorbed and refracted; besides polarization of light is changed. Some questions of laser beams propagation are considered in Section 3.2. Intensive plasma radiation is noise component at amplifier input. Ratio signal/noise definition at laser amplifier input is considered in Section 3.4.
where Ne is electron density (Ne = 5×1014 cm − 3), e is electron charge (e =1.60× 10 − 19 coul), me is electron mass (me = 9.11× 10 − 31 kg), m0 is electric constant (m0 = 8.85× 10 − 12 farad m − 1). For comparison cyclic frequency for green wavelength of copper vapor laser is equal g = 3.70× 1015 Hz, and for yellow wavelength— y = 3.26× 1015 Hz. Since the given laser frequencies don’t match with plasma absorption spectrum, ( g \ pe and y \ pe), the laser radiation absorption by plasma can be neglected. The refraction index of plasma for laser wavelengths is equal
3.2. Optical properties of plasma
Thus, for considered conditions plasma can be supposedly transparent at copper vapor laser wavelengths with refraction index n 1. The operation of the given laser system is practically independent on light polarization transform under the pass through plasma, since radiation of this laser possess natural polarization. Besides, the optical scheme of the laser system is polarization-independent.
Let’s consider the plasma transparency inside ITER discharge chamber for a visible range of wavelengths. The plasma cutting frequency (Langmuir oscillations frequency) is: pe =
'
Ne e 2 =1.26×1012 Hz, m0me
ng = ny =
' '
1−
2pe = 0.99999994, 2g
1−
2pe = 0.99999993. 2y
Fig. 1. Copper vapor laser viewing system. Block scheme. 1, Copper vapor laser generator; 2, copper vapor laser amplifier; 3, pumping pulse source; 4, pumping pulse source; 5, under investigation object; 6, plasma volume; 7, wall aperture; 8, receiving objective; 9, optical path; 10, matching optics; 11, lens; 12, CCD matrix; 13, computer; 14, synchronizer; 15, tunable time delay.
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1.22uy = 1.18× 10 − 5 rad, DR
3.3. The description of the optical scheme 6ariant
0 =
As an example we shall consider one of possible variants of the given scheme (see Fig. 1). Laser generator output beam (1) has a diameter d0 =14 mm and angular divergence q= 1.44 ×10 − 3 rad. Pulse energy of laser is equal to 1 mJ. After passing distance L= 25 m (including 15 m through plasma as it was marked in Section 2.2), laser beam illuminates surveyed object. The laser spot diameter at the object’s surface can be obtained as:
where uy = 578.2 nm is wavelength of laser radiation, DR = 60 mm is receiving objective diameter. We shall assume, that plasma is optically homogeneous medium with refraction index n= 1. In this case extreme linear resolution at the object located at a distance from receiving objective R0 = (5–20) m is equal Dx0 = (0.06–0.24) mm. Diaphragm diameter at plasma volume boundary as was specified earlier, is equal Dw = 58 mm. If optical path protection against neutrons flux will require to reduce diameter of this diaphragm, for example, to value Dw = 10 mm, extreme angular resolution at space of object is equal C0 = 7.05× 10 − 5 rad. Accordingly, extreme linear resolution at object, will be equal Dx0 = (0.35–1.41) mm.
Di = d0 + qL
Di = 50 mm
Let’s assume, that laser light is scattered by surveyed surface within solid angle 2y. As already mentioned it was marked above, plasma can be supposedly transparent for green and yellow wavelengths of laser radiation. Diaphragm is placed on the boundary of plasma volume in flash with the port-plug first wall. The receiving objective collects scattered radiation passed through the diaphragm. The diaphragm diameter is equal Dw =58 mm, receiving objective is placed outside of plasma volume at distance R0 =20 m from observed object and at distance Rpl =5 m from the diaphragm plane. We shall accept receiving objective aperture equal DR = 60 mm. It is necessary to note, that features of the optical scheme angular selection determine sizes of diameters Dw and DR at a preset value of diameter Di, and one of system spatial filter elements is the laser amplifier. After passing through receiving objective scattered radiation is propagated along optical path channels which design is defined by radiating safety requirements of external premises. The optical path includes some mirrors and vacuum windows. We shall assume, that this path transmittance is equal }opt =0.1. Then radiation enters the laser amplifier through matching optics. Light beam, past through the amplifier, transfers the information about the object. The object’s image is projected on CCD matrix receiving plane by objective. Such a manner of optical system performance (7, 8, 10, 2, 11 see Fig. 1) is chosen that image aberration can be neglected. Then angular resolution in space of object corresponds to Rayleigh diffraction for yellow laser wavelength
3.4. Laser system power parameters estimation Normal operation of the given system is possible only provided that laser radiation is not absorbed by plasma, and that plasma radiation energy at laser wavelengths will have a lower level of object’s scattered laser radiation energy at amplifier input. For ratio signal/noise estimation of the describing system (see Fig. 1), at first we shall determine the useful signal level at the laser amplifier input. Let’s assume that object’s scattered radiation intensity inside solid angle of 2y is constant and does not depend on scattering angle. We shall accept integral scattering factor equal = 0.1. Then, laser radiation energy, scattered by object’s surface elementary unit (for example, area of 1 mm2), is determined at amplifier input as ER = Et}opt
S pSR }opt Si2yR 20
ER = 5.73× 10 − 16 J
where Et = 1 mJ — probing laser pulse energy (see Section 3.3), Sp = 1 mm2 — object’s surface elementary unit area, SR =
yD 2R =28.27 cm2 − recieving objective area, 4
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yD 2i = 19.64 cm2 4 laser radiation spot area at the object surface. Si =
The ratio Si/Sp is equal to number of object’s surface elementary units within laser spot. As plasma radiation for given viewing system is noise, it is necessary to determine, what part of this radiation energy is emitted at laser wavelengths (ug = 510.6 nm, uy =578.2 nm) during generation pulse inside receiving objective effective solid angle. We use an absolutely black body model for plasma radiation parameters calculation. Spectral distribution function of radiation energy flux is defined by Plank radiation law (the radiation which are emitted from surface unit during time unit in wavelengths interval unit in a solid angle of 2y) and has the following form: F(u, Ts)=
2yhc 2u − 5 ; hc exp −1 kuTs
where h is Plank constant (h = 6.626 ×10 − 34 J s); c is light velocity in vacuum (c =2.998 ×108 m s − 1); k is Boltzmann constant (k= 1.381 × 10 − 23 J K − 1); T is temperature (T =3 ×107 K). For tokamak thermonuclear plasma radiation intensity is 1/w times less than black body radiation intensity. w:
2×10 − 11(NiNe)2/7 , Tth/n
w =5 ×10 − 7,
where Ni =Ne =5× 1014 cm − 3 is ions and electrons densities, Tth/n =104 eV is typical thermonuclear temperature [5]. Therefore plasma radiation energy which are emitted by surface unit 1 cm2 (placed at diaphragm plane Dw) during laser generation pulse within laser line Doppler width for green and yellow wavelengths, respectively, is equal zg = wF(ug, Ts)Du~ zg = 1.75×10 − 11 J · cm − 2 zy = wF(uy, Ts)Du~ zy = 1.0610 − 11 J cm − 2
where Du is copper vapor laser generation line width (Du = 2.025× 10 − 10 cm), ~ is laser generation pulse duration (~= 15 ns). Accordingly, plasma radiation energy, summarized for two wavelengths, at laser amplifier input is equal EB = (zg + zy)Sw
Sp Ssel }opt Si 2yR 2pl
EB = 5.70× 10 − 20 J where Sw is diaphragm area at plasma boundary, Si/Sp is the number of object’s surface elementary units within laser spot, Rpl is distance from a diaphragm up to receiving objective (Rpl = 5 m), }opt is optical path transmittance (}opt = 0.1), Ssel is equivalent receiving objective area, defined as Ssel =
y(qselRpl)2 , 4
where qsel is spatial selection angle of the optical scheme, counted for receiving objective plane (qsel = 3.5× 10 − 3 rad). Optical path contains a vacuum window made of fused silica, assume that of KU-1 or KS-4V, which after an irradiation becomes a source of photoluminescence radiation in the wide spectrum range. This radiation at laser amplifier input is the additive to plasma noise. However, preliminary estimations based on report data [6] have shown, that the phenomenon of window photoluminescence for the given optical scheme can be neglected. Thus, the power relation signal/noise, probable at the image intensity amplifier input is equal S/N=
ER = 104, EB
i.e. plasma background radiation can be neglected in the considered scheme. Hence, the proposed technique application for monitoring of ITER discharge chamber internal surfaces is possible during running cycles, and in pauses especially.
3.5. Selection of object’s surface sur6eying method and pointing direction definition technique Early described optical system included two opposing laser beams propagating inside suffi-
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Fig. 2. Copper vapor laser viewing system, placed at upper port. 1, Laser; 2, vacuum window; 3, upper diagnostic port; 4, first mirror; 5, aperture; 6, divertor.
ciently small solid angles. This way allows high quality receiving of object’s image. Optical axes of probing and scattered beams are crossed in the center of surveying surface place of diameter 50 mm. However the given place is only a small part of all surfaces, which is necessary for monitoring. There are two ways of observing area increasing. We shall consider each of them. The first way consists of enlarging illuminated surface area N times. Then, under keeping of a signal/noise ratio, it is necessary to raise probing laser radiation power N times, too. In Section 3.4 calculation for the laser average power of radiation 10 W is given at pulse repetition rate 10 kHz. The magnification of illuminated spot diameter, for example in 40 times, i.e. up to 2 m that corresponds to ITER divertor ring width, will demand raising of probing laser radiation power up to 16000 W. Achievement of such level of average power for copper vapor laser is a problem. Except for it, under change to wideangle optics the optical scheme aberrations will worsen. Thus, this way appears technically difficult, expensive and does not solve a problem of the large areas analysis as a whole. The second way is based on use of laser beam noninterlaced raster scanning on surveyed surface. In this case the laser beam moves in space within pyramid volume. Pyramid top lays on a
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surface of the scanner mirror, and the top plane angle corresponds to oscillation amplitude of the scanner mirror (the top angle is equal 2h, see Figs. 2 and 13). The laser beam moving velocity is determined by the given degree of spots overlapping from pulse to pulse. Spots overlapping degree is defined by required accuracy of the image reproduction. The probing laser beam and scattered radiation beam are propagated through the same optical path. So, this way has a number of advantages, though it is not easy, too. The optics of scanning system should not worsen the quality of the object’s image. The scanner should operate at a temperature about 150 °C and in magnetic fields (B=0.1 T; dB/dt =0.1 T s − 1). (The estimation of copper vapor laser stable running in magnetic fields is given in Section 4.5.) For the given observing method the appropriate program of registration, processing and image identification should be developed. Computer control of the scanner will allow establishing pointing direction at the object with the accuracy defined scanner backlash. We admit distance from the scanner up to an object of 20 m, then at scanner angular backlash of 10 s linear positioning accuracy at object’s plane will equal 2 mm. If the system includes a few consistently located scanners the mistake is accordingly increased. On the other hand, to define the pointing direction it is possible to use any marks that may be concrete design units features and specially made checkpoint grids. Checkpoint grid manufacture on observed surfaces would help to solve other important problem on inspection of wall protection erosion rate. This technique will be useful under keeping of measurements described in Section 3.6.
3.6. To the problem of di6ertor plates protection co6er erosion measurement at use of 6iewing system with the image intensity laser amplifier It is known, that one of the most important problems at experiments performance with ITER is quantitative principles definition of plasma facing components protection erosion process,
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first of all divertor, as in normal operation modes, and —especially—in failure modes. Application of image intensity laser amplifier operating in pulse-frequency mode enables process viewing of erosion on any allocated place in-situ even during failure. Essentially it may look as follows. Beforehand, any certain relief, for example, pyramidal deepening, is made at the protection cover zone, allocated for monitoring. In process of protection cover ablation the relief configuration will change. This change in value will depend on erosion depth. Availability of the timebase will allow determination of process kinetic parameters. Viewing method resolution will specify concrete values of technique sensitivity. The spatial resolution of the optical scheme without scanning is considered in Section 3.3. Diffraction limited linear resolution at object located at distance from receiving objective 20 m is equal 0.24 mm. Under scanning scheme addition spatial resolution at object’s plane may worsen up to 1 mm. We shall consider the reference point option proposed above. Let us have quadratic pyramidal deepening in protection cover. Pyramid height is about 5 mm and pyramid vertex angle equals 120°, then square side value equals 17 mm. This value decreases due to ablation. Viewing system will record this decreasing plane projection that is perpendicular to pointing direction. In other words, recording value will depend on the incident angle of probing laser beam at researched surface. Let ablation depth be about 1 mm. If laser beam incident angle equals 30°, then the decrease of surface square side is about 3 mm. If laser beam incident angle equals 45°, then the decrease of the surface square side is about 2.5 mm. Thus, under given measurement method application the laser viewing system can record the decrease of protection cover depth is about 0.5 mm. Time resolution of the method will be defined by laser pulse repetition rate and predetermined pulses number within time averaging interval of recorded information. Let under positioning of viewing system on the same reference point, time averaging object’s image is registered during ten consecutive laser pulses. Then, under laser pulse repetition rate
equals 10 kHz the computer will reproduce staging variation pattern of object’s surface image with step 0.001 s. Therefore, available value of protection cover erosion rate is equal 0.5 mm ms − 1. Except for that, quadratic pyramidal deepening center may be a checkpoint grid node of the pointing system. It was mentioned in Section 3.5. Accuracy of pointing direction positioning at the object is equal 2 mm provided even decreasing of protection cover thickness near the given deepening.
4. Laser system complex for surface monitoring of all divertor and first wall in ITER
4.1. Laser 6iewing system module, placed at upper diagnostic port As it was specified in Section 2.1, divertor and adjoining areas of first wall shielding will operate in the most intense conditions. Though divertor plates have the complex form, the most part of its surfaces (however not all) it is possible to see from above. Scheme of laser system arrangement at the upper diagnostic port is given in Fig. 2. The module containing laser generator, amplifier and basic optical scheme (1) is located behind the first vacuum boundary. Laser radiation passes through vacuum window (2), is reflected from flat mirror (4), and then passes through diaphragm (5) located at protective panel of upper diagnostic port. If mirror (4) turns around of two mutually perpendicular axes at angle 9h/2, (h8°), laser beam will scan the square of size 2.6 m× 2.6 m at divertor surface. In more detail the optical scheme with scanning mirror is given in Fig. 3. The given scheme specifies a relative positioning of optical elements. Functionally it is described in Section 3.1. For confirmation of the spatial resolution estimation of optical scheme (see Fig. 3) model tests of laser system without scanning mirror and plasma noise background are carried out. Objects were located at distance about 20 m from laser and amplifier. Probing laser had the next operating parameters: average radiation power, 10 W;
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Fig. 3. Copper vapor laser viewing system. Variant of optical scheme. 1, Laser generator tube; 2, laser amplifier tube; 3, resonator mirror; 4, attenuator; 5, concave mirror; 6, CCD matrix; 7, concave mirror; 8 – 10, plane mirrors; 11, vacuum window; 12, plane mirror; 13, aperture; 14, divertor.
pulse repetition rate, 10 kHz; output beam diameter, 14 mm. Under investigation object was illuminated by laser beam spot of diameter 50 mm. Receiving objective (clear aperture, 60 mm) was located at the distance of 20 m from the object’s plane. Amplified object’s image was projected at the screen. Linear resolution at object’s plane about 1 mm was obtained. Degradation of resolution in comparison with estimated value— 0.24 mm (see Section 3.3) is related with insufficient degree of the image contrast, as the laser amplifier tube had unnormalized noise level. Note, that scanning mirror (12) (see Fig. 3) is placed close to plasma volume. If under these conditions the mirror surface will deteriorate, it is necessary to locate this mirror at a more protected zone. One of the protection methods of the first mirror is moving it from plasma boundary at a distance of about 1 m inside the upper port box. The scanning mirror (1) unit located at a distance of 300 mm from input diaphragm of diameter 150 mm is represented in Fig. 4. The diaphragm size is increased in comparison with a computed value of 58 mm in view of mirror scanning at angle 9h/2 (h 8°). For protection of mirror’s reflecting surface the conic channel—blind by length on axis 1000 mm is arranged. It is supposed that dispersion of particles flows and radiations on rough
conic surface will not allow its collimation into diaphragm aperture. However, results of work [7] and personal discussion with the author guided the conclusion about insufficient protection of reflecting surface of scanning mirror (1) in design shown in Fig. 4. It proves to be true that the tests that have been carried out in similar dimensional conditions by Dr V. Voitseny. The samples of mirrors made of the SS316 substrate sputtered by Rh film of various thickness were tested. The most proof samples had reflectance of 75% for green area of spectrum. Other decision of the first mirror protection is proposed in design represented in Fig. 5(a). It is obvious, that the scanning block design should provide reliable protection of all elements. Its durability should correspond to operational
Fig. 4. Upper diagnostic port aperture. 1, Scanning mirror.
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Fig. 5. (a,b) Copper vapor laser viewing system with scanning module, placed at upper port. 1, Laser; 2, vacuum window; 3, upper diagnostic port; 4, scanning block; 5, first mirror; 6, aperture; 7, scanner; 8, divertor.
Fig. 6. Scanning module, placed at upper port. 1, Laser; 2, vacuum window; 3, upper diagnostic port; 4, scanning module; 5, first mirror; 6, aperture; 7, scanner.
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Fig. 7. General ITER torus view. 1, Laser module; 2, combined scanning module; 3, upper port; 4, equatorial port; 5, divertor port; 6, blanket module.
characteristics of the rest laser system components. Diaphragm of diameter 60 mm (6) is placed at the distance 1000 mm from the edge of plasma, the first mirror (5) of size (750× 750) mm is located at the distance 2500 mm from diaphragm plane. The first mirror builds the diaphragm image at the plane of two-coordinate scanner mirror (7). Laser beam passes through scanner, matching optics and enters into the optical system of laser module. The probing beam passes the same optical way from laser to divertor. Partitioning of probing and scattered light beams in the laser module is the same, as well as in scheme Fig. 3. This variant is lacking, because the part of scanning block is out of inclined upper port channel. Design updating is allowable for decreasing gabarit dimensions. Namely, the diaphragm size and the distance separating it from plasma remain constant, and
Fig. 8. Laser viewing system working from upper port. 1, Laser module; 2, combined scanning module; 3, upper port; 4, laser lighting of the divertor plates.
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Fig. 9. Optical scheme of scanning module. 1, First mirror; 2, scanner; 3, common optical path; 4, vacuum window; 5, laser module.
first mirror focal length is decreasing. This variant represents Fig. 5(b). Further design development of scanning module will be based on this modification. In more detail scanning module with laser are shown in Fig. 6. As is mentioned above, the scanning area from one rectangular mirror covers divertor strip width and looks like a square in size 2.6 m× 2.6 m. The circumference of the divertor ring on average line is approximately 30 m. And in project ITER it is supposedly five upper diagnostic viewing ports in regular intervals located on a circle. Hence, from one port it should be visible not less 6 m of divertor ring arc. It is possible, if combined scanning module design is executed as the united body including three identical scanning modules (Fig. 6). The combined scanning module is located at the plasma end of the upper port. General ITER torus view, the arrangement of the combined scanning module and the laser module are shown in Fig. 7. Laser system located in the upper port surveys about 7 m arc length on an average line of divertor ring; that is shown in Fig. 8. The optical scheme of the combined scanning module is shown in Fig. 9. The laser beam passes common path of matching optics, then it passes in sequence each rectangular mirror scanning scheme path to the divertor. Two laser systems placed in the corresponding upper ports, survey about 13 m divertor ring arc length that is shown in Fig. 10.
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4.2. Laser 6iewing system module, placed in equatorial port The laser system described in Section 4.1 may be used to observe the first wall protection surface. The scanning module consists of the four first mirrors and is placed in equatorial port. And the laser module is at the end of an equatorial tube similar to accommodation in Section 4.1. Zones of the surveying of the equatorial scanning module at the equatorial plane are shown in Fig. 11. The length of each viewing arc is equal 4.4 m. On a vertical the field of the observe covers all of the first wall from above up to the divertor upper part. Circumference of torus equatorial section is equal to 52.5 m. From one equatorial port the laser system sees totally 8.8 m arc length. Hence, from six equatorial ports it is possible to see all torus external walls. The installation general view is submitted in Fig. 12. Combined scanning module places in equatorial port. Solid fields of scanning by a laser beam from four scanning submodules are schematically represented in Fig. 12. Symmetric solid fields of scanning by the laser beam emergent from four scanning modules are shown in Fig. 13. In more detail the combined scanning module design consisted of four rectangular mirrors is shown in Fig. 14.
The envelope of the torus surface zone illuminated by laser light is shown in Fig. 15. Crosshatched blanket modules are illuminated by the upper and lower scanning modules.
4.3. Work description of laser system complex for surface monitoring Laser systems for surfaces conditions monitoring of divertor and part of first wall of ITER are considered in Sections 4.1 and 4.2. Divertor is surveyed from five upper inclined ports. All external parts of the chamber’s first wall surface are visible from six equatorial ports located regularly on a circle of equatorial torus section. All modules of laser systems operate independently from each other and may have the common control. Let’s estimate common surfaces observe time of the divertor and external part of the first wall surface. From one upper port the area equal to the sum of the areas of three squares with the side 2.6 m is surveyed. We remind, that observe way is based on use of laser beam noninterlaced raster scanning on researched surface with moving velocity appropriate to the given degree of spots overlapping from pulse to pulse. Spots overlapping degree is defined by required accuracy of the image reproduction. Let the laser beam spot move along the square side with spots overlapping a degree
Fig. 10. Two laser viewing systems placed in upper ports.
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Fig. 11. Torus equatorial section plane. 1, Lighting field; 2, equatorial port; 3, first mirror.
equal 0.5 from spot diameter. Spot diameter is equal 50 mm. Then it will be placed 105 spots on one square side. Spots rows have the same overlapping against each other, equal 0.5 from spot diameter. On all areas of a square it will be placed 11025 spots. It is sent ten probing laser pulses in one viewing point. Laser pulses repetition rate is equal 10 kHz. Hence, under the above listed conditions look-up time of one square 2.6 m×2.6 m is equal 11 s. Accordingly, consecutive look-up time of three squares from one upper port is equal 33 s. If all five upper laser ports operate simultaneously all the visible divertor surfaces are surveyed for 33 s, too. Now, let’s estimate operating time of laser viewing system from equatorial port. The combined scanning module contains four mirrors consistently running with one laser module. Each mirror illuminates a square with side 3.7 m, placed at a distance equal to 15 m from the equatorial port. Look-up time of such a square is equal 22 s. Then, consecutive look-up time of four squares from one equatorial port is equal 88 s. If all six laser equatorial ports operate simultaneously all external parts of the chamber’s first wall surface are surveyed also for 88 s. Let’s assume, that all laser systems operate simultaneously. Hence, common observed time of divertor surface and an external part of chamber’s first wall surface is equal 88 s. In this case the
complex structure includes 11 two-tube laser modules. Five of them are located in the upper ports and six in equatorial ports.
4.4. Running features of laser 6iewing system in magnetic, temperature and 6ibrating fields of ITER One of the essential factors that may influence operation stability of laser viewing system is the presence of strong non-stationary magnetic fields. A problem of magnetic field effect on the copper vapor laser activity can be arrived, when the laser is used as a brightness amplifier for the
Fig. 12. Laser system, placed in equatorial port. 1, Laser module; 2, combined scanning module; placed in equatorial port; 3, upper port; 4, divertor port; 5, blanket module; 6, solid laser lighting scanning fields.
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Fig. 13. Solid laser lighting scanning fields. 1, Combined scanning module, placed in equatorial port; 2, blanket module; 3, solid laser lighting scanning fields.
ITER viewing system. The laser has to be installed in one of the ports, where magnetic field is about 0.1 T that can be able to spoil discharge parameters. Magnetic field effect can become apparent as following: 1. Decreasing of discharge plasma electrical conductivity in direction that is transversal to magnetic field in (1+i 2) factor, where i = · ~ is Hall number. For copper vapor laser discharge conditions (neon as gas medium, p =40 kPa), magnetic field of 0.1 T and electron temperature of 1 eV the Hall number would be about 0.25. So this effect can be ignored as first estimation, if the laser is located far enough from poloidal field coils.
Fig. 15. Surveying zones of chamber surface for equatorial port viewing system. 1, Divertor; 2, blanket module; 3, laser lighting of blanket module (crosshatched plates); 4, solid laser lighting scanning fields.
2. Discharge plasma convection stimulated by pressure gradient 9p = J× B. For the case when the electrical field is directed along the discharge tube and magnetic field is directed across to axis of the tube, the discharge column will became bend and a part of the discharge plasma column located out of the tube axis will be rotated around the tube axis. So the discharge plasma could contact the tube wall and discharge will be abrupt. But possibly this convection has no time to be developed because of the discharge is very short ( 50 ns). There is no possibility to carry out simple quantitative estimations of last effects on this study. More comprehensive analysis will be required in future, including (maybe) direct experiment. Under design development of laser viewing system for maintenance of required accuracy and stability of its operation as a whole it is necessary to take into account influence of thermal and vibrating fields and its changing at different stages of running cycle ITER. In particular, updating of laser tubes cooling system for working at vacuum conditions is required.
5. Structure of monitoring laser system for ITER Fig. 14. Combined scanning module of equatorial port. 1, Mirrors of combined scanning module, placed in equatorial port; 2, blanket module; 3, laser lighting zones of blanket modules.
The real laser viewing system includes: Copper vapor laser for illumination, developed for operation at ITER conditions.
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Optical path of illumination and scattered radiation formation, including scanning module, system of mirrors, a vacuum window, objectives with the variable focal length, developed for operation at ITER conditions. Copper vapor laser amplifier, developed for operation at ITER conditions. Image registration system with CCD-camera. Computer for control of lasers, scanners, optics, registration and processing of the image. The program of control of lasers, scanners, optics. The program of registration, processing and image identification.
6. The conclusion
1. The optical scheme variant of combining scanning module for panoramic image generation of surveyed surface is developed. Optical scheme is shown in Fig. 9. 2. The laser systems arrangements in the upper and equatorial ports for all divertor visible surfaces and external part of first wall surface monitoring are offered. For this purpose in part it is required to occupy five upper and six equatorial ports (see Figs. 10 and 12). 3. The full survey time estimation of divertor and external part of the first wall surface is carried out. Observe time of divertor is equal to 33 s. Observe time of first wall external part is equal to 88 s. Thus, results of executed works confirm that the given method may be used for monitoring of divertor and first wall surfaces during working cycle ITER. For real system development the following steps of work performance is necessary: The laser system prototyping for method approbation and laboratory testing at one of operating tokamaks.
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Development and tests of definition methods of sheeting erosion rate. Optical properties studying of researched surfaces. Development, manufacturing and tests of scanning module, capable to work with the required accuracy at real conditions of ITER. Amendment of photoluminescence energy value of the irradiated vacuum windows. Amendment of transparency value of the irradiated vacuum windows. Optimization of laser amplifier operation modes to achieve required sensitivity and the interface with a CCD-matrix. Measurements of plasma radiation intensity level at probing laser wavelengths. Study of vibrations, temperature and magnetic fields influence on operating stability of laser viewing system for ITER. Development of the automated control system of lasers, scanners, optics, registration and processing of the information and image identification.
References [1] G.G. Petrash, Pulsed gas-discharge lasers, Sov. Phys.-Usp. 14 (1972) 747 – 765. [2] K.I. Zemskov, A.A. Isaev, M.A. Kazaryan, G.G Petrash, Laser projection microscope, Sov. J. Quant. Elect. 1 (1974) 14. [3] O.I. Buzhinskij, Evolution of examinations of the Cu-laser and possibility of its practical application, Rev. of Kurchatov Institute Of Atomic Energy (now RRC ‘Kurchatov Institute’), 1983 (in Russian). [4] O.P. Maximov, V.A. Mishin, A.I. Moshkunov, I.A. Slivitskaya, A.A. Slivitsky, V.A. Shutov, 600W CVL System, Tech. Digest of International Conference Lasers’97, December 1997, New Orleans. [5] A.M. Prokhorov (Ed.), Physical Encyclopedia, vol. 2, 1990, pp. 109. [6] I.V. Moskalenko, N.A. Molodtsov, N.Yu. Vukolov, Results of Study Photoluminescence Induced in Pre-Irradiated Optical Materials under UV Radiation, Moscow, 2001. [7] V.S. Voitseny, Latest data from simulation experiments in Kharkow, 2001.