- Email: [email protected]
Recent developments for plasma edge diagnostics using atomic beams
574
Journal
of Nuclear
Materials
162-164
(1989) 574-581
North-Holland,
RECENT DEVELOPMENTS
FOR PLASMA
EDGE DIAGNOSTICS
USING ATOMIC
Amsterdam
BEAMS
A. POSPIESZCZYK ‘, F. AUMAYR *, H.L. BAY ‘, E. HINTZ I, P. LEISMANN 3, Y.T. LIE ‘. G.G. ROSS 4, D. RUSBOLDT ‘, R.P. SCHORN ‘, B. SCHWEER’ and H. WINTER ’ ’ Institiit ftir Plasmaphysik,
Kernforschungsanlage Jiilich GmbH, Association Euratom - KFA, D-51 7 Jiilich, P. 0. Box 1913, Fed. Rep. Germany ’ Institut ftir Allgemeine Physik, Technische Universitiit Wien, A-1040 Wien, Austria 3 Institut ftir Experimenialphysik V, Ruhr-Universittit, D-463 Bochum, Fed Rep. Germany 4 INRS-&ergie, Universiti du QuPbec, Varennes, Canada
Key words:
atomic
beams,
optical
spectroscopy,
boundary
layer, tokamak
Li-atom beams with velocities of 1.5X10’ cm/s and 1 X106 cm/s have been used to measure n, profiles in the density range 10” -10” cme3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5-100 eV (e.g. carbon and helium), also radial T, profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.
1. Introduction The determination of well resolved spatial profiles of the electron density, the electron temperature, and the concentrations of various impurity ions in the boundary layer of a plasma is of great importance for the understanding of plasma-wall interactions. Electron density and impurity ion concentration profiles are needed for the understanding of transport mechanisms in the boundary layer. The knowledge of the electron temperature distribution can for example contribute significantly to the evaluation of spectral line intensities and to the interpretation of radiation losses. Passive spectroscopy, probes or laser scattering techniques are also used for these purposes, but they show some serious restrictions concerning applicability and spatial resolution. In order to overcome these difficulties, several active diagnostic techniques which make use of injected atomic beams have been installed on TEXTOR. They are not only located at different poloidal and toroidal positions 0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
but cover also a wide energy range, which facilitates profile measurements in regimes with strongly different densities. Some of them have been already successfully employed on different plasma devices [1,2], but the simultaneous use of sources ranging from thermal energies up to keV injection energies offers unique possibilities for the investigation of plasma boundary processes on TEXTOR. In the following the diagnostic techniques will be described and some typical results obtained with TEXTOR will be presented.
2. Principles of the methods The methods are mainly based on the following principles (more detailed descriptions can be found elsewhere [3,4]). When a particle beam with a velocity u, enters a plasma with a density profile n,(r) and a temperature profile T,(r), its attenuation and the emitted photon intensity can be described as follows. B.V.
515
A. Pospieszczyk et al. / Recent developments for plasma edge diagnostics
The attenuation of an atom beam of a density nA( r) by ionizing collisions with electrons of density n,(r) is given by --c-n
1
dn,(r)
nA(r)
(we> e
dr
(1)
CA
where (arue) is the ionization function averaged over a Maxwellian distribution of electron velocities u,. The integral of eq. (1) is: nA(r)
=
A determination of the electron temperature profile can be based on the fact that for certain elements the ionization rate and - preferably - also the excitation rate for the observed spectral line depends strongly on the electron temperature. By differentiation of eq. (2) and making use of eq. (1) we obtain the following expression: T,(r)
fiA
n,(r)
-~rnA(r’)n._(r’)~ dr’. -~
For atom energies in the keV range an additional (urvP) for ionization by protons has to be taken account. The local intensity of photon emission per unit angle following electronic excitation of a level given by
term into solid is
m
1 IA(‘)
=
(2)
~ne(r)nA(r)(almve)~
where (u,,v,) is the excitation function of level m from level 1, taking into account branching ratio and averaging over a Maxwellian distribution of electron velocities v,. We assume that the population of level m is only due to electronic collisions from the ground state and depopulation only occurs by spontaneous emission. The last assumption, however, is - in the case of lithium already noticeably violated for densities above 5 x 1012 cmm3, whereas excitation by proton collisions has to be taken into account for beam velocities in the keV regime. To fulfil the different requirements use is made of specific injection techniques [5]. For the determination of the electron density profile n,(r), we eliminate nA( r) from eq. (la) and (2) and obtain: EAIA(r) n,(r)
=
(3) (%u.,/b~lA,rr)
dr’
’
For the case of Li- or Na-atom beams, this formula can be simplified to: 5AAlA(r) n,(r)
(4)
= (“lve)~rzA(r’)
dr’
’
since the ratio (ulve)/(ulmv,) is only weakly temperature dependent for T, > 10 eV [4]. With the precautions mentioned above the method for determining n,(r) by using eq. (4) can be used for all lithium injection techniques with their different beam energies. If necessary one can apply (relatively fast converging) iteration methods.
t
-~- 1
dzA,(r)
1
Z,(r)
dr
-~ n,(r)
1 I (ei,v,>
h(r)
dr
d( eimve > dr 1)
= (erv,> =f(T,).
(5)
The right term depends on temperature only and permits evaluation of the T, profile if the respective logarithmic derivatives for IA(r) and n,(r) are known. The knowledge of the term in [ ] is not necessary if ( ulmve) is only a weak function of r (i.e. T,) and it can therefore be omitted for the first step. Then only a relative measurement of the radial emission profile is needed for the determination of T,(r). The electron density profile is determined as outlined above. The procedure using eq. (5) has on the other hand only a limited accuracy in regions of the emission profile where the beam attenuation is small, i.e. in excitation dominated parts where T, and n, are low. In order to overcome this difficulty one has - in a second step - to compare a computed emission profile according to eq. (2) (including the beam attenuation expressed by eq. (1)) with the measured radial intensity profile to obtain the best fit for T, especially in the region near the liner. Another possibility for the determination of T,(r) is offered by measuring the emission profiles of spectral lines of injected atom species, the ratio of which shows a strong dependence on the electron temperature. An appropriate candidate for such a procedure in the plasma boundary is helium [6]. Injected fast Li atoms (lo-30 kev) colliding with impurity ions provide for electron capture by the latter, resulting in strong characteristic line emission from the higher excited states, which in the plasma itself are produced at a much lower rate. This permits localized impurity ion detection with excellent signal-to-noise ratio, if a sufficiently intense neutral beam is applied. Absolute emission cross sections for such capture processes have been determined for the particularly important low-Z impurity ions He2+ [7], Cq+ (4 = 3-6) and Oq+ (q = 3-7) [8]. With these data a straightforward determination of impurity ion concentration pro-
A. Pospieszczyk et al. / Recent developments for plusma edge diagnostics
576 laser
ablation n,(r),T,ir)
3. Diagnostic arrangements
ILlI, CI1
thermal
beam. LIIAI
30 keV beam IL111 C (4, ne(rl ’
thermal
beam.oven n,(r)
ALT-II toroldal
limiter
Around TEXTOR presently lithium atoms are injected at four locations. Fig. 1 shows a projection of these systems into one poloidal cross section. Although they are all located at different toroidal positions, this fact plays most probably a minor role when all blades of the belt limiter ALT-II are in the same radial position and form the main plasma limiting element. Therefore major differences should appear only in the poloidal direction. 3.1. Thermal sources (fig. 2)
(LII)
Fig. 1. Projection of all diagnostics using atom beams into one cross section showing their location in poloidal direction.
files becomes possible, if one measures the calibrated ratio of the charge exchange and the lithium emission signals as the Li flux and the observation geometry cancel out. When the local Li-beam density is measured in parallel, e.g. by use of laser induced fluorescence techniques or by means of beam attenuation modelling, also an absolute determination of an impurity ion density profile ni(r) can be made. Application of lithium beam charge exchange spectroscopy (Li-CXS) may also be useful for measuring hydrogen ion density profiles
Lithium atoms with thermal energies around 550 o C are produced at two different toroidal angles. The penetration depth of these atoms is about GA/( atu,) = n, x I = 2 x 101* cm-*. The first unit is a conventionally built oven, containing pure metallic lithium. Many results obtained with this technique have already been published (see e.g. ref. [3]). The second one is a newly developed source consisting of an Al/Li alloy which is heated up to 580 o C. It is based on the fact that the diffusion flux of Li from the bulk to the surface is higher than the evaporation flux. As shown in fig. 2a, a stainless steel housing, which is heated by a thermocoax, contains a disk of an Al/Li alloy. This material is commercially available * and has a bulk concentration of 9 at% of lithium. The diameter of the hole, through which the beam is emitted is 5 mm; the hole can be closed by a shutter. One second before * Pechiney
[91.
vacuum-vessel
Al/Li
80/90F.
terference
filter
oven 15.talnl steel)
Fig. 2. Setup of the thermal
Al/Li
and He injection
system on TEXTOR.
A. hspieszczyk
the start of the discharge this shutter is opened for a period longer than the duration of the discharge. With a the~~uple the temperature is stabilized within a range of 5” C in order to keep the flux of Li atoms constant. For a sufficiently high Li-emission signal the oven has to deliver a total flux of 2.0 X 1014/s. TO achieve this value, it has to be moved to a position close to the liner radius. The atomic beam is well collimated to a PWHM of 18 mm at a distance of 150 mm. The Al/Li oven was operated in TEXTOR for 150 h in the temperature range of 540 to 580 ’ C. The Li inventory in the Al should be sufficient to maintain the necessary Lie flux for another 300 h. The oven can easily be replaced, since it is mounted on a movable supporting system, which can be retracted behind a valve. Below the Al/Li oven there is also mounted a 4 mm inner diameter tube, which allows the injection of helium gas into the boundary layer. A He beam is generated by means of a collimator consisting of a multichannel system (+ = 50 pm, I = 6 mm). This hole structure collimates the He beam to about 20 mm fwhm at 150 mm distance. At TEXTOR a volume of 2 litre was filled with He up to a pressure of 30 mb. Through a valve which is opened simultaneously with the shutter of the Li beam the gas enters the tube. The flux density at the collimated hole structure can be controlled by a needle valve Iocated before the main valve. For the first measurements in TEXTOR a relatively high total helium atom flux of 4.0 x lO'*/s wasderived from the pressure decay in the reservoir. This high flux was needed to overcome the problems of a nonlinear baseline due to electrical pickup during the first operation period of this system. It seems possible to reduce the injected He flux by a factor of 100, when the electrical pick-up is minimized and other observation wavelengths are chosen. For the recording of the emitted spatial intensity profile I,(r) a 128 Si-diode array camera * * (f= 1.4 or 0.95 and an integration time of ~=0.5 ms per readout) is used in comb~ation with appropriate interference filters f T = 50%). In the case of injected lithium atoms we observe the resonance line (2pzPo-2s2S) at 6708 A with a transition probability (m = 1) of A,, = 3.7 X 10’s_l. The value for (ulzoe) can be taken either from the formula of van Regemorter [lo] or from an experimental dete~nation [ll], the values of which agree well for the accuracy needed. The rate coefficient (atiu,) was taken from Bell et al. [12]. The spatial resolution is 1.2 mm in radial and 12 mm in toroidal
** Reticon array camera, G128/10.
577
et al. / Recent developments for plasma edge diagnostics
OPTICAL LlGHTGUlOE
Fig. 3. Setup of the laser ablation system on TEXTOR.
direction due to the rectangular size of each Si element. The emission profiles could be recorded in a data acquisition system with a maximum repetition rate of 2 kHz. The HeI-line, which is used for the temperature determination in the case of the collimated helium gas stream is at h = 5016 A (3ptPia--2~‘~) and has a transition probability of 1.34 x 10’ s-l. The values for (ut,,,~~) and (u,Q) are taken from ref. [13] and ref. [12], respectively.
In order to increase the penetration depth of the injected particles one has to increase their velocities. In the eV range this is achieved on TEXTOR by the laser blow off technique. The focussed light of a ruby laser (1 J output energy) ablates small portions (+ = 1-4 mm) from the rear side of a glass substrate. We have used a carbon layer of 5000 A for the T, measurement and a LiF layer of 500 A thickness on top for the n, measurement. The ablated atoms travel 110 cm to the plasma boundary with a velocity of ijh = 8 x lo5cm/s, which results in a penetration depth of about 1 x 10” cm-‘. ne m~s~ements and T,me~~ern~ts are possible up to several centimeters beyond the main limiter edge. Also, a change of the atomic species injected is accomplished easily by changing the coatings of the glass substrate. One disadvantage of this technique is the short duration of the particle pulse permitting the deter~~tion of T,(r) and n,(r) only at a single time
578
A. Pospieszczyk
et al. / Recent developments for plasma edge diagnostics
Analyzing
Neutraltsatlon
Magnet
Cell
Faraday
Cup
.i
n
I~
I1T!!i i-
”/~--/
Magnet
‘,
‘,
Ion’Dump
Vessel
Fig. 4. Setup of the high energetic lithium injection system on TEXTOR. during a discharge. The observation system in the case of the ablation is in principle identical with that described in section 3.1) with the further simplification that only a recording over the length of the injection pulse is needed. Because of the larger penetration depth we had to make use of a different camera objective so that the spatial resolution is now 1.5 mm in radial and 15 mm in toroidal direction. For CI we have used a line at h = 9095 A (3p3P2-3s3P:) with a transition probability of 1.9 X 10’ by electron S -l, the upper level of which is only excited collisions. The values for (u~,,,u,) and (aIu,) were again taken from ref. [13] and ref. [12], respectively. In
--
order to determine the beam velocities of the injected lithium and carbon atoms, two photomultipliers are used with interference filters in front, which observe the same volume as the camera and measure the time of flight of the atoms between the blow off process and the beginning of light emission in the plasma boundary. More details concerning the determination of T,(r) by this technique can be found in ref. [14]. 3.3. High energetic
sources
(fig. 4)
The system for the determination centration profiles n,(r) consisted Li I light He I light
of impurity conof a high energy
int.
int.
i’ ,
/
,
i
40
52
44
line? plasma
radius
[cm1
Fig. 5. Spatial emission profiles for Li and He measured with 100 Hz repetition rate for a single scan and the derived TINand profiles. (I,, = 340 kA, central ?ie = 3.6X lOI cm-‘).
7”
519
A. Pospieszczyk et al. / Recent developments jar plasma edge diagnostics
Li-beam source, which for the experiments reported here was run with a voltage of 15 keV, an equivalent Li* current of about l-10 mA at the plasma edge and a repetition frequency of 500 Hz. The emitted radiation was detected by a large-aperture scanning system followed by interference filters and photomultipliers. By using a beam splitter radiation at two wavelengths could be observed simultaneously. The one was the Li I resonance line already mentioned, the other corresponded to the 6H-71 transition of CV at 4946 A excited in the charge exchange process: C5+ + Li* + C4+* + Li+ and used for the determination of C5+ concentration profiles. In particular, the C5+ concentration (n=s+/n,) can be determined from the ratio of the CV signals and Li I signals and the correspon~ng excitation rate coefficients without knowledge of the absolute Li* flux. More details concerning this technique can be found in ref. [15].
-<
16%
! /
\
al
80-l 4. Rem&a Some representative results obtained with the three diagnostic systems described above shall illustrate their capabilities. For the thermal sources the spatial emission profiles for Li and He measured with 100 Hz repetition rate for a single scan are shown in fig. 5. Because He has a much higher ionization energy (24.48 eV) than Li (5.37 eV) it is penetrating further into the plasma. The electron density profile derived from the lithium emission profile by using eq. (4) and some values of the electron temperature in the range of 46 to 48 cm derived from the helium emission profile by using eq. (5) are plotted in the same figure. Although the temperature values evaluated from the helium line emission seem to be very reasonable, they have still to be treated with caution, because the cross sections for the mechanisms, which populate the excited level 3p”Pf are only partly known. A full analysis in the whole range of overlap of the helium and Lithium signals (from 52 to 42 cm) can first be made, when in the future the He-emission signals will be measured with a higher precision (see section 3). For the present analysis we estimate a probable error of +2X% for the T, values. A demonstration of the accessible range and a representative example for the evaluation of an electron density and temperature profile obtained by the laser ablation technique is shown in fig. 6. An obvious feature are two clearly distinct e-folding lengths for T,, i.e. about 2 cm in the boundary and 10 cm between 47 and 49 cm in the scrape-off layer. Also included in these
0 40
44
48
52
rkml
Fig. 6. Electron density (a) and electron temperature profiles (b) measured by laser ablation and a movable Langmuir probe (probe data: boxes). (Zp = 340 kA, central ii, = 3.7 x lOI cm-3).
plots are points, which were obtained by a movable Langmuir probe in the outer scrape off layer [16]. The agreement there is good. Small deviations are due to the different poloidal locations of the two systems. There are basically two factors, which affect the accuracy of the determined density profiles, besides the possible systematic errors concerning atomic cross sections, positioning etc.: the signal to noise ratio and the background subtraction from the camera signal. We found that major errors of about 20% can only be expected for densities above 8 x 1012 cmm3. For densities lower than 6 X 10” cmp3, the errors concerning faulty background subtraction and noise on the signal can practically be neglected Which also accounts for larger errors at higher densities is the fact that the velocity dist~bution of the injected atom beam is
580
lG5
Time [ set] Fig. 7. The CSt density-
re lated photomultiplier output
106
1.07
0
signal at 4946 A as a function of discharge time together with the Lie-beam at a radial position of 40 cm. (I, = 350 kA, central fie = 1.21 x 1013 cm- ‘).
changed during its attenuation. Therefore all thermal and suprathermal beams penetrate somewhat further into the plasma than one would predict from their original velocity distributions. This effect, which is analytically treated in ref. [17], will be included in the evaluation process in the future. We have also tried to estimate the accuracy of our T, determination. An unavoidable error is given by the inaccuracy of the ionization and excitation functions and possibly by density dependent effects. This could lead to a systematic deviation (possibly about 20%), but the general behavior would remain unchanged. The evaluation procedure itself is mainly influenced by the choice of the background subtraction, both for the lithium and the carbon intensity profile. Therefore the errors are larger at both ends of the temperature profile. First measurements involving the high-energetic Li beam have been performed with an injection energy of 15 keV and an equivalent Lie current at the plasma edge of about 1 mA. In fig. 7 the C5+ density-related photomultiplier signal at 4946 ,& is plotted versus discharge time together with the Lie-beam monitor output for a radial position of 40 cm. The correlation between monitor and both multiplier signals is clearly visible. Although the charge exchange signal amounts to only 5% of the background radiation level, it can be evaluated making use of the Lie-beam modulation. It turned out that this value corresponded to a C5+ concentration of about 5% determined as described in section 3. For this discharge the enhancement in the soft X-ray continuum yields a central carbon concentration of l-2%, but it may very well be higher at the plasma edge. With an injected Lie-beam current of more than 10 mA at 30
monitor
keV the signal to noise ratio will improve significantly. also due to the increase in the charge exchange emission cross sections with beam energy [8]. Hence, it is expected that radial concentration profiles of several ionic states of the light impurities carbon, oxygen, and helium can be measured with Li-activated charge exchange spectroscopy.
5. Conclusions We have shown that TEXTOR is equipped with boundary layer diagnostics which can widely fulfill the needs for data important for the investigation of hydrogen and impurity transport and release mechanisms. Especially in the case of a toroidal limiter configuration (ALT-II), density and temperature profiles can be obtained also at different pdoidal locations. As all diagnostic systems are based on visible spectroscopic means, their interference with the plasma itself can be kept to a minimum. They even allow investigations some centimeters beyond the limiter radius in the main plasma. However, the accuracy of the method is always strongly dependent on the accuracy of atomic data (excitation and ionization rates) used. Therefore better model calculations are desirable taking especially density dependent effects into account.
References [I] K. Kadota, Tsuchida L260.
K. Matsunaga, H. Igushi, M. Fujiwara, K. and J. Fujita, Jpn. J. Appl. Phys. 21 (1982)
A. Pospieszczyk et al. / Recent developments for plasma edge diagnostics [2] K. McCormick and the ASDEX Team, Rev. Sci. Instr. 56 (1985) 1063. [3] P. Bogen, H. Hartwig, E. Hintz, K. Hothker, Y.T. Lie, A. Pospieszczyk, U. Samm and W. Bieger, J. Nucl. Mater. 128 & 129 (1984) 157. [4] P. Bogen and E. Hintz, in: Physics of Plasma-Wall Interactions in Controlled Fusion, Eds. D.E. Post and R. Behrisch (Plenum Press, New York, 1986) p. 211. [5] A. Pospieszczyk and G.G. Ross, Rev. Sci. Instr. 59 (1988) 605. [6] N. Brenning, J. Quant. Spect. Radiat. Tranf. 24 (1980) 298. [7] F. Aumayr et al, in preparation. [8] A. Brazuk et al., Phys. Lett. 1OlA (1984) 139. [9] F. Aumayr and H. Winter, Ann. Phys. 42 (1985) 228. lo] H. van Regemorter, Astrophys. J. 136 (1962) 906. ‘111 D. Leep and A. Gallagher, Phys. Rev. A10 (1974) 1082. j12] K.L. Bell, H.B. Gilbody, J.G. Hughes, A.E. Kinston and F.L. Smith, J. Phys. Chem. Ref. Data 12 (1983) 891.
581
[13] 1.1. Sobelman, L.A. Vainshtein and E.A. Yukov, in: Excitation of Atoms and Broadening of Spectral Lines, Springer Series in Chemical Physics, Vol. 7 (1981). [14] A. Pospieszczyk and G.G. Ross, in: Proc. 14th Europ. Conf. on Controlled Fusion Plasma Physics, Madrid, Spain, June 1987, p. 1280. [15] H.L. Bay, E. Hintz, P. Leismann, D. Rusbtildt, F. Aumayr and H. Winter, Proc. 14th Europ. Conf. on Controlled Fusion Plasma Physics, Madrid, Spain, June 1987, p. 1276; Berichte der KemforschungsanIage Jtilich, No. Jtil-2139 (July 1987). [16] W.J. Corbett et al., in: these Proc. (PSI-8) J. Nucl. Mater. 162-164 (1989) 221. [17] K. Guenther et al., in: these Proc. (PSI-8) J. Nucl. Mater. 162-164 (1989) 562.
Journal
of Nuclear
Materials
162-164
(1989) 574-581
North-Holland,
RECENT DEVELOPMENTS
FOR PLASMA
EDGE DIAGNOSTICS
USING ATOMIC
Amsterdam
BEAMS
A. POSPIESZCZYK ‘, F. AUMAYR *, H.L. BAY ‘, E. HINTZ I, P. LEISMANN 3, Y.T. LIE ‘. G.G. ROSS 4, D. RUSBOLDT ‘, R.P. SCHORN ‘, B. SCHWEER’ and H. WINTER ’ ’ Institiit ftir Plasmaphysik,
Kernforschungsanlage Jiilich GmbH, Association Euratom - KFA, D-51 7 Jiilich, P. 0. Box 1913, Fed. Rep. Germany ’ Institut ftir Allgemeine Physik, Technische Universitiit Wien, A-1040 Wien, Austria 3 Institut ftir Experimenialphysik V, Ruhr-Universittit, D-463 Bochum, Fed Rep. Germany 4 INRS-&ergie, Universiti du QuPbec, Varennes, Canada
Key words:
atomic
beams,
optical
spectroscopy,
boundary
layer, tokamak
Li-atom beams with velocities of 1.5X10’ cm/s and 1 X106 cm/s have been used to measure n, profiles in the density range 10” -10” cme3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5-100 eV (e.g. carbon and helium), also radial T, profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.
1. Introduction The determination of well resolved spatial profiles of the electron density, the electron temperature, and the concentrations of various impurity ions in the boundary layer of a plasma is of great importance for the understanding of plasma-wall interactions. Electron density and impurity ion concentration profiles are needed for the understanding of transport mechanisms in the boundary layer. The knowledge of the electron temperature distribution can for example contribute significantly to the evaluation of spectral line intensities and to the interpretation of radiation losses. Passive spectroscopy, probes or laser scattering techniques are also used for these purposes, but they show some serious restrictions concerning applicability and spatial resolution. In order to overcome these difficulties, several active diagnostic techniques which make use of injected atomic beams have been installed on TEXTOR. They are not only located at different poloidal and toroidal positions 0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
but cover also a wide energy range, which facilitates profile measurements in regimes with strongly different densities. Some of them have been already successfully employed on different plasma devices [1,2], but the simultaneous use of sources ranging from thermal energies up to keV injection energies offers unique possibilities for the investigation of plasma boundary processes on TEXTOR. In the following the diagnostic techniques will be described and some typical results obtained with TEXTOR will be presented.
2. Principles of the methods The methods are mainly based on the following principles (more detailed descriptions can be found elsewhere [3,4]). When a particle beam with a velocity u, enters a plasma with a density profile n,(r) and a temperature profile T,(r), its attenuation and the emitted photon intensity can be described as follows. B.V.
515
A. Pospieszczyk et al. / Recent developments for plasma edge diagnostics
The attenuation of an atom beam of a density nA( r) by ionizing collisions with electrons of density n,(r) is given by --c-n
1
dn,(r)
nA(r)
(we> e
dr
(1)
CA
where (arue) is the ionization function averaged over a Maxwellian distribution of electron velocities u,. The integral of eq. (1) is: nA(r)
=
A determination of the electron temperature profile can be based on the fact that for certain elements the ionization rate and - preferably - also the excitation rate for the observed spectral line depends strongly on the electron temperature. By differentiation of eq. (2) and making use of eq. (1) we obtain the following expression: T,(r)
fiA
n,(r)
-~rnA(r’)n._(r’)~ dr’. -~
For atom energies in the keV range an additional (urvP) for ionization by protons has to be taken account. The local intensity of photon emission per unit angle following electronic excitation of a level given by
term into solid is
m
1 IA(‘)
=
(2)
~ne(r)nA(r)(almve)~
where (u,,v,) is the excitation function of level m from level 1, taking into account branching ratio and averaging over a Maxwellian distribution of electron velocities v,. We assume that the population of level m is only due to electronic collisions from the ground state and depopulation only occurs by spontaneous emission. The last assumption, however, is - in the case of lithium already noticeably violated for densities above 5 x 1012 cmm3, whereas excitation by proton collisions has to be taken into account for beam velocities in the keV regime. To fulfil the different requirements use is made of specific injection techniques [5]. For the determination of the electron density profile n,(r), we eliminate nA( r) from eq. (la) and (2) and obtain: EAIA(r) n,(r)
=
(3) (%u.,/b~lA,rr)
dr’
’
For the case of Li- or Na-atom beams, this formula can be simplified to: 5AAlA(r) n,(r)
(4)
= (“lve)~rzA(r’)
dr’
’
since the ratio (ulve)/(ulmv,) is only weakly temperature dependent for T, > 10 eV [4]. With the precautions mentioned above the method for determining n,(r) by using eq. (4) can be used for all lithium injection techniques with their different beam energies. If necessary one can apply (relatively fast converging) iteration methods.
t
-~- 1
dzA,(r)
1
Z,(r)
dr
-~ n,(r)
1 I (ei,v,>
h(r)
dr
d( eimve > dr 1)
= (erv,> =f(T,).
(5)
The right term depends on temperature only and permits evaluation of the T, profile if the respective logarithmic derivatives for IA(r) and n,(r) are known. The knowledge of the term in [ ] is not necessary if ( ulmve) is only a weak function of r (i.e. T,) and it can therefore be omitted for the first step. Then only a relative measurement of the radial emission profile is needed for the determination of T,(r). The electron density profile is determined as outlined above. The procedure using eq. (5) has on the other hand only a limited accuracy in regions of the emission profile where the beam attenuation is small, i.e. in excitation dominated parts where T, and n, are low. In order to overcome this difficulty one has - in a second step - to compare a computed emission profile according to eq. (2) (including the beam attenuation expressed by eq. (1)) with the measured radial intensity profile to obtain the best fit for T, especially in the region near the liner. Another possibility for the determination of T,(r) is offered by measuring the emission profiles of spectral lines of injected atom species, the ratio of which shows a strong dependence on the electron temperature. An appropriate candidate for such a procedure in the plasma boundary is helium [6]. Injected fast Li atoms (lo-30 kev) colliding with impurity ions provide for electron capture by the latter, resulting in strong characteristic line emission from the higher excited states, which in the plasma itself are produced at a much lower rate. This permits localized impurity ion detection with excellent signal-to-noise ratio, if a sufficiently intense neutral beam is applied. Absolute emission cross sections for such capture processes have been determined for the particularly important low-Z impurity ions He2+ [7], Cq+ (4 = 3-6) and Oq+ (q = 3-7) [8]. With these data a straightforward determination of impurity ion concentration pro-
A. Pospieszczyk et al. / Recent developments for plusma edge diagnostics
576 laser
ablation n,(r),T,ir)
3. Diagnostic arrangements
ILlI, CI1
thermal
beam. LIIAI
30 keV beam IL111 C (4, ne(rl ’
thermal
beam.oven n,(r)
ALT-II toroldal
limiter
Around TEXTOR presently lithium atoms are injected at four locations. Fig. 1 shows a projection of these systems into one poloidal cross section. Although they are all located at different toroidal positions, this fact plays most probably a minor role when all blades of the belt limiter ALT-II are in the same radial position and form the main plasma limiting element. Therefore major differences should appear only in the poloidal direction. 3.1. Thermal sources (fig. 2)
(LII)
Fig. 1. Projection of all diagnostics using atom beams into one cross section showing their location in poloidal direction.
files becomes possible, if one measures the calibrated ratio of the charge exchange and the lithium emission signals as the Li flux and the observation geometry cancel out. When the local Li-beam density is measured in parallel, e.g. by use of laser induced fluorescence techniques or by means of beam attenuation modelling, also an absolute determination of an impurity ion density profile ni(r) can be made. Application of lithium beam charge exchange spectroscopy (Li-CXS) may also be useful for measuring hydrogen ion density profiles
Lithium atoms with thermal energies around 550 o C are produced at two different toroidal angles. The penetration depth of these atoms is about GA/( atu,) = n, x I = 2 x 101* cm-*. The first unit is a conventionally built oven, containing pure metallic lithium. Many results obtained with this technique have already been published (see e.g. ref. [3]). The second one is a newly developed source consisting of an Al/Li alloy which is heated up to 580 o C. It is based on the fact that the diffusion flux of Li from the bulk to the surface is higher than the evaporation flux. As shown in fig. 2a, a stainless steel housing, which is heated by a thermocoax, contains a disk of an Al/Li alloy. This material is commercially available * and has a bulk concentration of 9 at% of lithium. The diameter of the hole, through which the beam is emitted is 5 mm; the hole can be closed by a shutter. One second before * Pechiney
[91.
vacuum-vessel
Al/Li
80/90F.
terference
filter
oven 15.talnl steel)
Fig. 2. Setup of the thermal
Al/Li
and He injection
system on TEXTOR.
A. hspieszczyk
the start of the discharge this shutter is opened for a period longer than the duration of the discharge. With a the~~uple the temperature is stabilized within a range of 5” C in order to keep the flux of Li atoms constant. For a sufficiently high Li-emission signal the oven has to deliver a total flux of 2.0 X 1014/s. TO achieve this value, it has to be moved to a position close to the liner radius. The atomic beam is well collimated to a PWHM of 18 mm at a distance of 150 mm. The Al/Li oven was operated in TEXTOR for 150 h in the temperature range of 540 to 580 ’ C. The Li inventory in the Al should be sufficient to maintain the necessary Lie flux for another 300 h. The oven can easily be replaced, since it is mounted on a movable supporting system, which can be retracted behind a valve. Below the Al/Li oven there is also mounted a 4 mm inner diameter tube, which allows the injection of helium gas into the boundary layer. A He beam is generated by means of a collimator consisting of a multichannel system (+ = 50 pm, I = 6 mm). This hole structure collimates the He beam to about 20 mm fwhm at 150 mm distance. At TEXTOR a volume of 2 litre was filled with He up to a pressure of 30 mb. Through a valve which is opened simultaneously with the shutter of the Li beam the gas enters the tube. The flux density at the collimated hole structure can be controlled by a needle valve Iocated before the main valve. For the first measurements in TEXTOR a relatively high total helium atom flux of 4.0 x lO'*/s wasderived from the pressure decay in the reservoir. This high flux was needed to overcome the problems of a nonlinear baseline due to electrical pickup during the first operation period of this system. It seems possible to reduce the injected He flux by a factor of 100, when the electrical pick-up is minimized and other observation wavelengths are chosen. For the recording of the emitted spatial intensity profile I,(r) a 128 Si-diode array camera * * (f= 1.4 or 0.95 and an integration time of ~=0.5 ms per readout) is used in comb~ation with appropriate interference filters f T = 50%). In the case of injected lithium atoms we observe the resonance line (2pzPo-2s2S) at 6708 A with a transition probability (m = 1) of A,, = 3.7 X 10’s_l. The value for (ulzoe) can be taken either from the formula of van Regemorter [lo] or from an experimental dete~nation [ll], the values of which agree well for the accuracy needed. The rate coefficient (atiu,) was taken from Bell et al. [12]. The spatial resolution is 1.2 mm in radial and 12 mm in toroidal
** Reticon array camera, G128/10.
577
et al. / Recent developments for plasma edge diagnostics
OPTICAL LlGHTGUlOE
Fig. 3. Setup of the laser ablation system on TEXTOR.
direction due to the rectangular size of each Si element. The emission profiles could be recorded in a data acquisition system with a maximum repetition rate of 2 kHz. The HeI-line, which is used for the temperature determination in the case of the collimated helium gas stream is at h = 5016 A (3ptPia--2~‘~) and has a transition probability of 1.34 x 10’ s-l. The values for (ut,,,~~) and (u,Q) are taken from ref. [13] and ref. [12], respectively.
In order to increase the penetration depth of the injected particles one has to increase their velocities. In the eV range this is achieved on TEXTOR by the laser blow off technique. The focussed light of a ruby laser (1 J output energy) ablates small portions (+ = 1-4 mm) from the rear side of a glass substrate. We have used a carbon layer of 5000 A for the T, measurement and a LiF layer of 500 A thickness on top for the n, measurement. The ablated atoms travel 110 cm to the plasma boundary with a velocity of ijh = 8 x lo5cm/s, which results in a penetration depth of about 1 x 10” cm-‘. ne m~s~ements and T,me~~ern~ts are possible up to several centimeters beyond the main limiter edge. Also, a change of the atomic species injected is accomplished easily by changing the coatings of the glass substrate. One disadvantage of this technique is the short duration of the particle pulse permitting the deter~~tion of T,(r) and n,(r) only at a single time
578
A. Pospieszczyk
et al. / Recent developments for plasma edge diagnostics
Analyzing
Neutraltsatlon
Magnet
Cell
Faraday
Cup
.i
n
I~
I1T!!i i-
”/~--/
Magnet
‘,
‘,
Ion’Dump
Vessel
Fig. 4. Setup of the high energetic lithium injection system on TEXTOR. during a discharge. The observation system in the case of the ablation is in principle identical with that described in section 3.1) with the further simplification that only a recording over the length of the injection pulse is needed. Because of the larger penetration depth we had to make use of a different camera objective so that the spatial resolution is now 1.5 mm in radial and 15 mm in toroidal direction. For CI we have used a line at h = 9095 A (3p3P2-3s3P:) with a transition probability of 1.9 X 10’ by electron S -l, the upper level of which is only excited collisions. The values for (u~,,,u,) and (aIu,) were again taken from ref. [13] and ref. [12], respectively. In
--
order to determine the beam velocities of the injected lithium and carbon atoms, two photomultipliers are used with interference filters in front, which observe the same volume as the camera and measure the time of flight of the atoms between the blow off process and the beginning of light emission in the plasma boundary. More details concerning the determination of T,(r) by this technique can be found in ref. [14]. 3.3. High energetic
sources
(fig. 4)
The system for the determination centration profiles n,(r) consisted Li I light He I light
of impurity conof a high energy
int.
int.
i’ ,
/
,
i
40
52
44
line? plasma
radius
[cm1
Fig. 5. Spatial emission profiles for Li and He measured with 100 Hz repetition rate for a single scan and the derived TINand profiles. (I,, = 340 kA, central ?ie = 3.6X lOI cm-‘).
7”
519
A. Pospieszczyk et al. / Recent developments jar plasma edge diagnostics
Li-beam source, which for the experiments reported here was run with a voltage of 15 keV, an equivalent Li* current of about l-10 mA at the plasma edge and a repetition frequency of 500 Hz. The emitted radiation was detected by a large-aperture scanning system followed by interference filters and photomultipliers. By using a beam splitter radiation at two wavelengths could be observed simultaneously. The one was the Li I resonance line already mentioned, the other corresponded to the 6H-71 transition of CV at 4946 A excited in the charge exchange process: C5+ + Li* + C4+* + Li+ and used for the determination of C5+ concentration profiles. In particular, the C5+ concentration (n=s+/n,) can be determined from the ratio of the CV signals and Li I signals and the correspon~ng excitation rate coefficients without knowledge of the absolute Li* flux. More details concerning this technique can be found in ref. [15].
-<
16%
! /
\
al
80-l 4. Rem&a Some representative results obtained with the three diagnostic systems described above shall illustrate their capabilities. For the thermal sources the spatial emission profiles for Li and He measured with 100 Hz repetition rate for a single scan are shown in fig. 5. Because He has a much higher ionization energy (24.48 eV) than Li (5.37 eV) it is penetrating further into the plasma. The electron density profile derived from the lithium emission profile by using eq. (4) and some values of the electron temperature in the range of 46 to 48 cm derived from the helium emission profile by using eq. (5) are plotted in the same figure. Although the temperature values evaluated from the helium line emission seem to be very reasonable, they have still to be treated with caution, because the cross sections for the mechanisms, which populate the excited level 3p”Pf are only partly known. A full analysis in the whole range of overlap of the helium and Lithium signals (from 52 to 42 cm) can first be made, when in the future the He-emission signals will be measured with a higher precision (see section 3). For the present analysis we estimate a probable error of +2X% for the T, values. A demonstration of the accessible range and a representative example for the evaluation of an electron density and temperature profile obtained by the laser ablation technique is shown in fig. 6. An obvious feature are two clearly distinct e-folding lengths for T,, i.e. about 2 cm in the boundary and 10 cm between 47 and 49 cm in the scrape-off layer. Also included in these
0 40
44
48
52
rkml
Fig. 6. Electron density (a) and electron temperature profiles (b) measured by laser ablation and a movable Langmuir probe (probe data: boxes). (Zp = 340 kA, central ii, = 3.7 x lOI cm-3).
plots are points, which were obtained by a movable Langmuir probe in the outer scrape off layer [16]. The agreement there is good. Small deviations are due to the different poloidal locations of the two systems. There are basically two factors, which affect the accuracy of the determined density profiles, besides the possible systematic errors concerning atomic cross sections, positioning etc.: the signal to noise ratio and the background subtraction from the camera signal. We found that major errors of about 20% can only be expected for densities above 8 x 1012 cmm3. For densities lower than 6 X 10” cmp3, the errors concerning faulty background subtraction and noise on the signal can practically be neglected Which also accounts for larger errors at higher densities is the fact that the velocity dist~bution of the injected atom beam is
580
lG5
Time [ set] Fig. 7. The CSt density-
re lated photomultiplier output
106
1.07
0
signal at 4946 A as a function of discharge time together with the Lie-beam at a radial position of 40 cm. (I, = 350 kA, central fie = 1.21 x 1013 cm- ‘).
changed during its attenuation. Therefore all thermal and suprathermal beams penetrate somewhat further into the plasma than one would predict from their original velocity distributions. This effect, which is analytically treated in ref. [17], will be included in the evaluation process in the future. We have also tried to estimate the accuracy of our T, determination. An unavoidable error is given by the inaccuracy of the ionization and excitation functions and possibly by density dependent effects. This could lead to a systematic deviation (possibly about 20%), but the general behavior would remain unchanged. The evaluation procedure itself is mainly influenced by the choice of the background subtraction, both for the lithium and the carbon intensity profile. Therefore the errors are larger at both ends of the temperature profile. First measurements involving the high-energetic Li beam have been performed with an injection energy of 15 keV and an equivalent Lie current at the plasma edge of about 1 mA. In fig. 7 the C5+ density-related photomultiplier signal at 4946 ,& is plotted versus discharge time together with the Lie-beam monitor output for a radial position of 40 cm. The correlation between monitor and both multiplier signals is clearly visible. Although the charge exchange signal amounts to only 5% of the background radiation level, it can be evaluated making use of the Lie-beam modulation. It turned out that this value corresponded to a C5+ concentration of about 5% determined as described in section 3. For this discharge the enhancement in the soft X-ray continuum yields a central carbon concentration of l-2%, but it may very well be higher at the plasma edge. With an injected Lie-beam current of more than 10 mA at 30
monitor
keV the signal to noise ratio will improve significantly. also due to the increase in the charge exchange emission cross sections with beam energy [8]. Hence, it is expected that radial concentration profiles of several ionic states of the light impurities carbon, oxygen, and helium can be measured with Li-activated charge exchange spectroscopy.
5. Conclusions We have shown that TEXTOR is equipped with boundary layer diagnostics which can widely fulfill the needs for data important for the investigation of hydrogen and impurity transport and release mechanisms. Especially in the case of a toroidal limiter configuration (ALT-II), density and temperature profiles can be obtained also at different pdoidal locations. As all diagnostic systems are based on visible spectroscopic means, their interference with the plasma itself can be kept to a minimum. They even allow investigations some centimeters beyond the limiter radius in the main plasma. However, the accuracy of the method is always strongly dependent on the accuracy of atomic data (excitation and ionization rates) used. Therefore better model calculations are desirable taking especially density dependent effects into account.
References [I] K. Kadota, Tsuchida L260.
K. Matsunaga, H. Igushi, M. Fujiwara, K. and J. Fujita, Jpn. J. Appl. Phys. 21 (1982)
A. Pospieszczyk et al. / Recent developments for plasma edge diagnostics [2] K. McCormick and the ASDEX Team, Rev. Sci. Instr. 56 (1985) 1063. [3] P. Bogen, H. Hartwig, E. Hintz, K. Hothker, Y.T. Lie, A. Pospieszczyk, U. Samm and W. Bieger, J. Nucl. Mater. 128 & 129 (1984) 157. [4] P. Bogen and E. Hintz, in: Physics of Plasma-Wall Interactions in Controlled Fusion, Eds. D.E. Post and R. Behrisch (Plenum Press, New York, 1986) p. 211. [5] A. Pospieszczyk and G.G. Ross, Rev. Sci. Instr. 59 (1988) 605. [6] N. Brenning, J. Quant. Spect. Radiat. Tranf. 24 (1980) 298. [7] F. Aumayr et al, in preparation. [8] A. Brazuk et al., Phys. Lett. 1OlA (1984) 139. [9] F. Aumayr and H. Winter, Ann. Phys. 42 (1985) 228. lo] H. van Regemorter, Astrophys. J. 136 (1962) 906. ‘111 D. Leep and A. Gallagher, Phys. Rev. A10 (1974) 1082. j12] K.L. Bell, H.B. Gilbody, J.G. Hughes, A.E. Kinston and F.L. Smith, J. Phys. Chem. Ref. Data 12 (1983) 891.
581
[13] 1.1. Sobelman, L.A. Vainshtein and E.A. Yukov, in: Excitation of Atoms and Broadening of Spectral Lines, Springer Series in Chemical Physics, Vol. 7 (1981). [14] A. Pospieszczyk and G.G. Ross, in: Proc. 14th Europ. Conf. on Controlled Fusion Plasma Physics, Madrid, Spain, June 1987, p. 1280. [15] H.L. Bay, E. Hintz, P. Leismann, D. Rusbtildt, F. Aumayr and H. Winter, Proc. 14th Europ. Conf. on Controlled Fusion Plasma Physics, Madrid, Spain, June 1987, p. 1276; Berichte der KemforschungsanIage Jtilich, No. Jtil-2139 (July 1987). [16] W.J. Corbett et al., in: these Proc. (PSI-8) J. Nucl. Mater. 162-164 (1989) 221. [17] K. Guenther et al., in: these Proc. (PSI-8) J. Nucl. Mater. 162-164 (1989) 562.