New developments in plasma edge diagnostics

Vacuum/volume37/numbers 1/2/pages 47 to 51/1987

0042-207X/87$3.00+ .00 Pergamon Journals Ltd

Printed in Great Britain

New developments in plasma edge diagnostics H a n n s p e t e r W i n t e r , Institut for Allgemeine Physik, TU Wien Karlsplatz 13, A-1040 Wien, Austria

The significant role of the boundary region of magnetically confined hot plasmas for their production and maintenance is pointed out to underline the importance of plasma edge diagnostics. We present a short review on novel plasma edge diagnostic techniques, including laser fluorescence spectroscopy, neutral-beam-activated photon and corpuscular spectroscopies and a combination of these methods. Special emphasis is given to Liactivated photon spectroscopy as a very useful method for measuring various quantities in connection with electrons, hydrogen ions and impurity ions in the plasma edge region, without the need for extensive plasma modelling.

1. Significance of the edge region of magnetically confined plasmas As a short term goal, thermonuclear fusion research attempts production of hydrogen plasmas with sufficiently high temperature and Lawson product (i.e. plasma density n times energy confinement time rE) to obtain the so-called physical demonstration of controlled fusion (kT> 10 keV, n x rE> 1020 s m-3). Probably, this goal can already be reached with present-day large tokamak experiments ~n. In connection with these activities, one of the most difficult problems is caused by plasma-wallinteraction (PWI) 3. Although confined in a strong magnetic field, a hot plasma is in intimate contact with the walls of its vacuum vessel. In order to control the influence of PWI, measures are taken as shown in Figure 1. The tokamak plasma boundary can

be defined by means of a limiter (Figure l(a)), or a specific magnetic topology can scrape off the outermost plasma particles and guide them into a rapidly pumped chamber (divertor principle, cf. Figure l(b)). In both cases, a scrape-off layer plasma (SOL plasma) is produced, in which the temperature and density of ions and electrons rapidly decrease toward the wall (cf. Figure 2). The typical decay length of these quantities is a few cm only. It is now generally accepted that the properties of the SOL plasma have a great influence on density, temperature, stability and energy confinement time of magnetically confined hot plasmas. The SOL plasma electron temperature controls the plasma sheath potential and, therefore, the extent of wall erosion processes. The gradient of this temperature is probably decisive for the development of a recently explored new discharge regime of neutral beam-heated tokamak plasmas (so-called H-modeS). However, this favourable regime has so far only been observed in divertor discharges. The SOL plasma parameters depend critically on the amount and species of plasma impurities, which govern the partition of plasma energy losses into radiation and efflux of fast particles,

200 150 -

300

" ~

n(r)

15

_ptasma 200 X io0

~

I0

T(r) I00

Divertor

, %Lirniter

5o -

O-

0 35

b

I 40

I 45

~ l

5

50

0

r (cm)

Figure 1. Schematical view of a limiter plasma (a) and a divertor plasma (b), showing the region of the scrape-off layer (SOL).

Figure 2. Radial distribution of electron temperature Te, ion temperature and plasma density n, for a typical limiter SOL-plasma (after ref 4). 47

Hannspeter Winter." New developments in plasma edge diagnostics

respectively (see Figure 3). Again, this has important consequences for all PWI processes. Moreover, there is evidence for a strong dependence of efficiency of plasma heating methods on the SOL plasma properties. In Figure 3 a schematic demonstration of the inter-relations between PWI and SOL plasma properties has been given, showing typical characteristics of a complex feed-back system. From this short discussion it is already clear, that a good understanding of the role of SOL plasmas is necessary, which also calls for sufficiently detailed and accurate diagnostic investigations.

II

/

/

I

\\ PLasma En, T, r E]

"~1\\\

\x. [Transport

"[

/

,

i• /

j/// I I Losses

I

Phot°ns I

x \

~

de%°~it°in n

l

] J

Par tictes

-

Figure 3. Systematicsof plasma-walMnteractionand plasma edge physics, simplified. 2. Limits of standard diagnostic techniques in SOL plasma investigation Primarily due to the inhomogeneous SOL structure, standard diagnostic techniques are of limited use only for observation of SOL plasmas. This applies, for example, to microwave interferometry and Thomson scattering for electron density measurement, or standard photon spectroscopy for investigation of ion densities and temperatures. For illustration, let us discuss the measurement of the density of a given impurity species Z q+ (Z and q are atomic number and charge state, respectively). By means of vacuum ultra violet (vuv) spectroscopy, one observes the intensity lz~ of a suitable characteristic emission line,

l z , = n z q n e S e m ( T e ) + I~ x + I ~ c + . . .

(1)

In equation (1), Sem(Te) is the electron impact excitation-rate coefficient, and the other contributions are due to charge exchange (cx), recombination (rec), etc. Even in rather simple cases a plasma model has to be adopted to extract the quantity of interest nzo from the spectroscopic data. Such models involve, for example, the principal plasma parameters (n, T. . . . ), assumptions about the particle transport and data on the relevant atomic collision processes. All these inputs sometimes involve considerably large uncertainties, which, therefore, add to experimental errors of the diagnostic results. Apart from the standard diagnostic techniques just described solid probes 7 are also quite common for investigating plasma properties in the edge region. Plasma density and temperature can be measured with electrical probes and total fluxes of various 48

3. Novel diagnostic techniques for the edge plasma region As a result of the above discussion, we can specify some desirable properties of SOL plasma diagnostic methods: plasma model-independent data evaluation; good temporal (~< 10 ms) and spatial (~< 1 cm) resolution; real time performance; no disturbance of the plasma. In addition, if such methods are capable of yielding several different sets of information at once (e.g. density and temperature of ions, data on different ion species, etc.), this will be especially useful. In recent years two methods have been devised, which provide a basis of SOL plasma diagnostics of the kind described above. We refer to laser fluorescence spectroscopy (LFSI and neutral beam (NB) activated photon and corpuscular spectroscopies. We will describe both techniques in Sections 3.l and 3.2, and then deal specifically with Li-activated photon spectroscopy and its combination with LFS.

Heating fueLLing current drive, etc.

/

atomic species can be determined by means of collecting probes, which also enable time-resolved observations. Unfortunately, both maximum permissible heat load on such probes and possible disturbance of the SOL plasma restrict these methods to the outermost part of the plasma edge. However, SOL investigations should be continued toward smaller plasma radii, to obtain a complete picture of the processes taking place in the SOL plasma.

3.1. Laser fluorescence spectroscopy (LFS). Figure 4 demonstrates in very simple form the application of LFS for the purpose of studying PWI processes. Particles of interest can be selectively excited by means of a suitable dye laser beam, with the resulting fluorescence radiation being recorded. Use of the Doppler shift when scanning the laser wavelength also permits evaluation of particle temperatures and velocities. With LFS being well established for detection of atomic particles, its use for in situ-investigations in fusion plasmas s'9 has occurred only recently. We mention, for example, detection of metal atoms as AI in EBT I°, Ti at the divertor plate of ASDEX 11 and Fe from a steel reference limiter in TEXTOR 12. In the majority of such experiments pulsed lasers have been used, which permit a detection limit of typically 1012 particles in 3 and an achievable velocity resolution corresponding to about 0.5 km s 1. This can be considerably improved, if cw lasers are used, Absolute densities are obtained by intensity calibration of Loser beam II

,,~ i

iI

i

-- Observation of - fluorescence

Reference Limiter

Figure 4. Principal layout of a laser fluorescencespectroscopy (LFS} setup for detection of metal atoms eroded from a reference limiter.

Hannspeter Winter: New developments in plasma edge diagnostics

the observation set-up via Rayleigh scattering of the involved laser light, or by comparison with fluorescence light intensity obtained from LFS with sputtered or vaporized atoms, which are observed under the same geometrical conditions. LFS has also been applied for neutral hydrogen detection in plasmas. This was performed by applying either broad band laser excitation a t Ha 13 or excitation of H(1 s) at L,, the latter being produced by frequency tripling in gas cells ~4. Intense laser light in the vuv is very difficult to produce, a factor which still impedes the application of LFS to, for example, C, N, O and their ions, as well as for multi-charged ions of medium-Z and high-Z impurities. However, in the near future a further extension of LFS toward smaller wavelengths can be expected, using, for example, frequency multiplication or multi-photon excitation.

3.2. Neutral-beam-activated spectroscopies. 3.2.1. Survey. The common passive plasma spectroscopies detect photons (Figure 5(a)) or neutral particles (Figure 5(b)) emitted from a plasma without stimulation from the outside. Passive photon spectroscopy involves radiation from collisioninduced processes in the plasma, and passive corpuscular spectroscopy utilizes charge exchange between fast plasma ions and the (highly inhomogeneous) neutral background. Therefore, in both cases modelling procedures are necessary for data evaluation, with the disadvantages already explained. In general, spatial resolution is achievable by observation along different chords and use of Abel inversion techniques. In a different approach, excitation and exchange processes can be induced by injection of relatively weak beams of neutral atoms into the plasma to be observed, cf. Figures 5(c) and 5(d). Such beams do not disturb the plasma properties and automatically yield good spatial resolution. Neutral beam (NB) activated photon spectroscopy involves radiative processes induced in either of two ways. (a) Excitation of injected atoms by collisions with plasma particles, primarily electrons (NB-activated impact excitation spectroscopy/IXS) X ° + e p ~ X * + e ~ -' .

(2)

Neutrot porticte spectroscopy

Photon spectroscopy ~iiiii iiii!ii i!ih==

Possive

Aiilii iI~ PS .~i!iiiii iiiiiiiiit h~!I~ , - r - ~ - - ~i!iiiiiiii i P [ i i i i i i ~ ~--- \ i i iii iiilii llii i i i i ii.L.ii~iiiilll ii:

,iiiiiiiiiiiiiiiiiii~

~iiiiiiiiiiiiiliiiiliii~

(b)

(o)

NB

NB Neutrot beornoctivoted

~iiii iiiiiiiii~ ~iiiiliiiiii!iiiiiiii~, ~iiiiiii iiiii!iiii!!~ ~ ~

!iiiliHiilP iiii~ ~iili!iiliiiiiiiiiiii!I .i ~!iiiii!iii!iiiiii!!!l i

='~iii!i~"

(cl

EA

=iiiiiiiiiiiii~ii H°!. l-lla[~'-, iiiiiiiiiiiiiii,.ii!i!iiiiiiii!.--f'~..........

5

,

PS

.~i%ii!iiiiliii !ii~

E A

~i iiiiiiiliii ~ / / ~ iiiiiiiiiiii i !~ Ii= yilliiiii!ii!iiiiiiihit

,, P

~. \.~[

(d)

Figure 5. Comparison of passive (a, b) and neutral beam (NB) activated plasma spectroscopies (c,d). PS--photon spectrometer. EA--particle energy analyser.

In (2) and the following relations, the underlined species is the object of observation. (b) Excitation of atomic particles by charge exchange (NBactivated charge exchange spectroscopy/CXS) X°+H+~X

+ +H*

X° + Z q + - - > X + + Z ~q-u+*

(3) (4)

H + stands for any hydrogen isotope ion and Z q+ for any kind of ionized plasma impurity with 1 ~
(5)

Relation (5) can be used for proton temperature measurements (see, for example, ref 15). In a similar way, double electron capture reactions might be utilized for measuring the energy distribution of slowing-down alpha particles produced in D - T plasmas t6 X ° + He 2 + -*X 2 + + H e °.

(6)

In the following, we deal only with NB-activated photon spectroscopies involving relations (2)-(4) which, with suitable neutral beams, show interesting applications for investigation of SOL plasmas.

3.2.2. Li-activated photon spectroscopy. So far, NB-activated CXS has only been applied in connection with H ° beams, for the purpose of detecting fully stripped low-Z impurities in tokamak plasmas. Both heating or much weaker probing H ° beams have been applied (for a recent review cf. refs 17, 18). However, for SOL plasma the activation by means of H ° injection is not suitable, because of the efficient charge exchange of ions to be detected with the dense neutral hydrogen background. As discussed in more detail elsewhere 19'2°, in the plasma edge region Li beams are very well suited for activation of both IXS and CXS. (a) Li-activated IXS--plasma electrons excite injected Li atoms quite efficiently. In addition, at high injection energy, excitation by proton impact has also to be taken into account 21. Since the electron impact excitation rate for electron temperatures between 10 and 100 eV depends only weakly on Te (cf. Figure 6), the LiI resonance line intensity yields the plasma density 22'23, if beam attenuation and LiI lifetime are corrected for. From the LiI line fine structure the local magnetic field strength in a plasma can be obtained 24'25, which for tokamak plasmas, for example, permits determination of the important quantities of local fl-values and plasma current densities. (b) Li-activated CXS--compared with charge exchange with H °, for a given ion the charge exchange with Li ° is much more efficient, because of the smaller ionization potential of Li(2 s) 19. Consequently, resulting excited states involve higher lying levels, background problems due to electron impact and charge exchange with H ° are reduced, and the resulting characteristic line emission is systematically shifted toward longer wavelengths. Especially from an experimental point of view these facts are very useful. Li-activated CXS can be applied to the detection of multicharged impurity ions 19'2°'26 as well as of hydrogen ions 2°'27 in SOL plasmas. Note that for detection of H + by means of NB-activated CXS the H~ emission cross-section resulting from Li 49

Hannspeter Winter. New developments in plasma edge diagnostics pO-6

3.3.2. Combination of NB-activation with two species and LFS. As already explained in Section 3.2.2, Li-activated IXS for electron density measurement utilizes the weak Te-dependence of electron impact excitation within the plasma edge region, cf. Figure 6. The electron impact ionization rate for Li, which at low injection energy dominates the Li beam attenuation TM, shows also weak Te-dependence. This is quite different for other atomic species such as AI or Ti (cf. Figure 6), the use of which has been proposed 32"33 for the purpose of measuring the electron temperature. Li-activated IXS combined with LFS yields the electron density as already explained in Section 3.3.1. Injection of another atomic species and LFS-probing of its attenuation thus allows Te to be found. For practical use of this method it is planned to apply laser-induced evaporation of thin metal composite films 32'33. However, if deeper layers of the SOL plasma should be probed, two fast atomic beams have to be used.

i

. . . .

i

. . . .

I .... '.

""Li(2p) I

'

'

I

I

.-,-~-I---

I0 7

b v

so, I0 8

I

/i'

"

I

I

O001

I

I I ]

I

[

I

001

I i

I

I

0 I kT e

(keY)

Figure 6. Reaction rate coefficients for electron impact excitation of Li ('Li(2 p)') and electron impact ionization of Li, AI and Ti (after refs 32, 33 and refs given therein). impact (cf. Figure 7) is about two orders of magnitude larger than for collisions with H ° (cf. refs 28, 29). Similar relations hold for impurity ions such as C 4+, see Figure 7. The electron capture from Li(2 s) proceeds with practically negligible momentum transfer. Accordingly, both ion temperatures and drift velocities can be deduced from the line profiles of charge exchange-induced emission almost in the same way as with LFS. 3.3. Combination of LFS and NB-activated spectroscopy. 3.3.1. LFS-based determination of N density. One difficulty in connection with NB-activated spectroscopies results if the injected beam is strongly attenuated, as is the case for Li(2s) atoms penetrating the inner part of a SOL plasma TM. For measurement of plasma electron (IXS) as well as ion (CXS) densities, this has to be corrected for. A direct determination of the local Li beam particle density can be made by collinear injection of a laser beam and utilization of LFS. The first experimental demonstration of such a procedure has recently been given, using slow Li beams from an oven as well as from laser-induced evaporation of thin metal films3°'31. I00

I

I

I

I

C4*+Li~CIV

I '~--

253 nm

&-C4++H~CIV

38.4

nm

0 v

..0

I0

0

As H*+Li~H~

656.3

nm

i

I

,

L

I

2

4

6

8

10

3.3.3. LFS at forbidden transitions of injected atoms. If neutral atoms reside in regions with a strong electric field, they can be excited at otherwise optically forbidden lines, due to Stark effect splitting. This can be utilized to measuring the involved electric field strength in the following way 34. Injected Li atoms are excited either by plasma electron impact or with a dye laser tuned to the Li (2 s-2 p) resonance. With a second laser tuned to the vicinity of the forbidden Li (2 p-4 f) transition, fluorescence at (2 t)--3 d) produced via cascading from (3 d~, f) is induced and observed. The resulting fluorescence intensity is a measure of the electric field strength, from which the latter can be determined. In a similar way it seems possible to measure local magnetic field strengths, utilizing the motional Stark effect of injected Li atoms 35. In both cases the laser excitation is also useful for obtaining good spatial resolution, because the quantities to be measured are in fact mean values taken over the effective excited state lifetime. While the natural lifetime of Li(2 p) is relatively long, the laser-induced excitation lasts for a much shorter time. When utilizing the motional Stark effect (see above), additional information on the magnetic field direction can be obtained by varying both direction and energy of the injected Li atoms 3s. Summary and outlook This review describes some novel diagnostics involving laser fluorescence spectroscopy, neutral beam-activated spectroscopies and combinations of both, for the purpose of measuring plasma parameters in the plasma edge region. In particular, the use of Li atoms for collisional excitation and/or photon-excitation with tunable laser light offers very interesting possibilities for modelindependent SOL plasma diagnostics. Some of the methods described have already been experimentally demonstrated and the others are in the process of verification. Using such methods, the ambiguous process of plasma modelling can be avoided and very good spatial and temporal resolution in spectroscopic observations are possible. Since the interest in accurate SOL plasma diagnostics is rapidly growing, methods such as those described are rather useful and further progress in their development and application can be expected.

,~_.z

Acknowledgement

E (keV/omu)

Figure 7. Emission cross-sections for excitation by electron capture from Li(2 s) into H + and C4+ ions, and in C4+ H(I s) collisions, respeclively (after ref20 and refs thereint. 50

This work has been supported by the Kommission zur Koordination der Kernfusionsforschung at the Austrian Academy of Sciences.

Hannspeter Winter: New developments in plasma edge diagnostics

References 1 Plasma Physics and Controlled Nuclear Fusion Research 1984, Vol 1, Nuclear Fusion Suppl (1985). 2 L Pocs and A Montvai (eds) Proc 12 Europ Confon Controlled Fusion and Plasma Physics, September 2 4 , 1985, Budapest, contributed papers part 1, Section A (1985). 3 Lectures of NATO Adv Study Inst on the Physics of Plasma-WallInteractions in Controlled Fusion, July 30-August 10, 1984, Val-Morin, Quebec]Canada. 4 G Waidmann et al, ref 1, p 193. F Wagner et al, Phys Rev Letts, 49, 1408 (1982); 53, 1443 (1984). 6 N C Luhmann and W A Peebles, Rev Sci lnstrum, 55, 279 (1984). 7 G McCracken, in ref 3. s p Bogen and E Hintz, Comments Plasma Phys Contr Fusion, 4, 115 (•978). 9 W Husinsky, J Vac Sci Technol, 18, 1054 (1981). 10 E Dullni et al, J Nucl Mat, 111]112, 61 (1982). 11 B Schweer et al, J Nucl Mat, 1111112, 71 (1982). 12 H L Bay and B Schweer, J Nucl Mat, 128/129, 257 (1984). 13 G T Razdobarin et al, Nucl Fusion, 19, 1439 (1979). C H Muller, D R Eames and K H Burrell, J Vac Sci Technol, A1, 822 (1983). 14 p Bogen et al, J Nucl Mat, 1111112, 75 (1982). 15 V V Afrosimov, M P Petrov and V A Sadovnikov, Zh Eksp Teor Fiz Pis Red, 18, 510 (1973) (JETP Letts, 18, 300 (1973).

16 D E Post et al, JFusion Energy, 1, 129 (1981). t7 R J Fonck, Rev Sci lnstrum, 56, 885 (1985). is R C Isler, Nucl lnstrum Meth Phys Res, Bg, 673 0985). t9 H Winter, Comments At Mol Phys, 12, 165 (1982). 20 F Aumayr and H Winter, Ann Phys, 42, 228 (1985). 21 F Aumayr, M Fehringer and H Winter, J Phys B: At Mol Phys, 17, 4185 (1984). 22 K Kadota et al, Plasma Phys, 20, 1011 (1978). 23 K McCormick et al, Rev Sci Instrum, 56, 1063 (1985). 2, K McCormick, M Kick and J Olivain, Proc 8th Europ Conf Contr Fusion and Plasma Physics, Prague, 1977, p 140 (1977). 25 K Kadota et al, Rev Sci Instrum, 56, 857 (1985). 26 A Brazuk et al, Nucl Instrum Meth Phys Res, 139, 442 (1985). 27 F Aumayr, M Fehringer and H Winter, J Phys B: At Mol Phys, 17, 4201 (1984). zs I D Williams, J Geddes and H B Gilbody, J Phys B: At Mol Phys, 15, 1377 (1982). 29 G A Cottrell, Nucl Fusion, 23, 1689 (1983). 30 K Kadota et al, J Nucl Mat, 128]129, 960 (1984). 31 K Kadota et al, Rev Sci Instrum, 56, 1036 (1985). 32 K Kadota et al, Report Jill-1812, KFA Jfilich (1982). 33 E Hintz and P Bogen, J Nucl Mat, 128]129, 229 (1984). 34 U Rebhan, N J Wiegart and H-J Kunze, Phys Letts, 85A, 1228 (1981). 35 N J Wiegart, U Rebhan and H-J Kunze, Phys Letts, 90A, 190 (1982).

51