The Thomson scattering method was applied to determine the temperature and density profiles of plasma electrons in the NOVA II tokamak. To minimize the intensity of stray light, use was made of a viewing damp, which was an array of blue-glass knife edges. The radial profile of electron density has been found to be approximately parabolic, while the temperature profile appeared more peaky and can be represented by the square of the parabolic expression. The electron energy confinement time determined by this method is in good agreement with that predicted by the Alcator scaling law

FUKAO, Masayuki

MASAMUNE, Sadao

NISHIHARA, Hiroshi

YOKOYAMA, Issei

English

Kyoto University Research Information Repository

RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Electron TemperatureMeasurement by ThomsonScattering in Tokamak NOVA IIMASAMUNE, Sadao; YOKOYAMA, Issei; FUKAO,Masayuki; NISHIHARA, HiroshiMASAMUNE, Sadao ...[et al]. Electron Temperature Measurement by Thomson Scatteringin Tokamak NOVA II. Memoirs of the Faculty of Engineering, Kyoto University 1982, 44(4):451-4591982-12-28http://hdl.handle.net/2433/281225451 Electron Temperature Measurement by Thomson Scattering in Tokamak NOV A II By Sadao MASAMUNE*, Issei YOKOYAMA**, Masayuki FuKAO and Hiroshi NISHIHARA (Received May 6, 1982) Abstract The Thomson scattering method was applied to determine the temperature and density profiles of plasma electrons in the NOV A II tokamak. To minimize the inten-sity of stray light, use was made of a viewing damp, which was an array of blue-glass knife edges. The radial profile of electron density has been found to be approximately parabolic, while the temperature profile appeared more peaky and can be represented by the square of the parabolic expression. The electron energy confinement time determined by this method is in good agreement with that predicted by the Alcator scaling law. § I. Introduction The temperature and density of electrons in a high temperature plasma can be determined by means of the Thomson scattering of laser light.1- 5l This tech-nique provides time- and space-resolved distributions with a sufficient accuracy, on the condition that the apparatus is designed properly. In the case of tokamak plasma, the electron density is low and, what is still worse, the intensity of the background radiation is high. Therefore, the Thomson scattering apparatus for a tokamak machine has to be designed carefully, so that there will be no unfa-vourable experimental conditions. We can alternatively determine either the electron temperat\,\re T, or the ion temperature T; from an analysis of the spectrum of the Thomson scattering. The alternative is mainly controlled by the parameter a=[4irADsin (0/2)r1AL, where (J is the angle between the incident and scattered beams, AD the Debye length and AL the wave length of the incident laser light. For a::SO.l and for a~l, the scattering process is incoherent and coherent, respectively. The scattering of Department of Nuclear Engineering • Present address: Department of Electrical Engineering, Kyoto Technical University. ** Present address: Horiba Co., Ltd. 452 Sadao MASAMUNE, Issei YOKOYAMA, Masayuki FUKAo and Hiroshi NISHIHARA the visible laser radiation is incoherent when observed at 0=90°. For electron temperatures below 1 keV, the spectrum of the Thomson scattering is approximate-ly Gaussian, and in a particular case of the Ruby laser, the half width is .dA (FWHM) = 32.5-v' T,, ( 1 ) where .JA is in A and T, in eV. The electron temperature T, is directly deter-mined from Eq. ( 1) by using the observed width of the scattering spectrum. In the case of incoherent scattering, the total intensity of the scattered light is given by I= Io n S l ~ .J!J S ' d!J ' ( 2) where I0 is the intensity of incidence, S the cross section of the incident beam, l the path length of the beam in the plasma column, da/d!J the differential cross section for the Thomson scattering, .J!J the solid angle of the collecting lens, re-spectively. In designing the apparatus, the F-numbers must be matched between the monochromator and the collecting lens. The matching condition is given by ( 3) where h is the height of the monochromator slit, F1 and F0 are the F-numbers for the monochromator and the collecting lens, respectively. By using this match-ing condition in Eq. (2), it yields TC da h I= - I0 n, - -- . 4 d!J F0F1 ( 4) In this expression, the ratio h/(F0F1) is the light collecting capacity of the appa-ratus. We have chosen h= 1 cm and F0 =F1=4.5. This report describes the characteristics of the Thomson scattering apparatus for the NOV A II tokamak6l and the obtained results. §2. Apparatus Figure 1 shows a schematic layout of the Thomson scattering apparatus built recently for determining the electron temperature and density in the NOV A II. The giant pulse Ruby laser consists of an oscillator, a Q-switch and an amplifier. Helical Xenon flash lamps are used in the oscillator and the amplifier, and a KD* P pockel cell in the Q-switch. A stable pulsed beam is fired with a 20 ns pulse width, and the maximum energy is 12 J. The beam divergence is less than 3 mrad. The horizontal laser beam is sent vertically up into the machine through Electron Temperature Measurement by Thomson Scattering 458 20cm Photo Multiplier Ruby Laser { 600 ~. 20ns) Fig. l. Schematic drawing of the Thomson scattering apparatus. a rectangular prism. The lens L1 with a 40 cm focal length focusses the beam to a spot, 2 mm in diameter. The plasma is scanned vertically by moving the spot. The window for the beam incidence is fixed at the Brewster angle, with respect to the beam axis, in order to avoid excessive energy concentration on the window surface. The beam damp consists of two blue-glass plates, one of which is again fixed at the Brewster angle. The scattered light is collected by the lens L2 having a 56 mm diameter. To minimize the background due to stray light, a viewing damp is attached to the inner wall of the discharge chamber so that it covers that part of the wall where the aperture of the monochromator is visible. We have tested three types of view-ing damps; i) stainless steel knife edges, 0.5 mm thick, separated by 0.5 mm from each other, ii) stainless steel knife edges arranged closely, and iii) an array of blue-glass knife edges, having a 2 mm thickness, fixed closely. The reflection coefficient was measured with the instrumentation shown in Fig. 2 (a). The viewing damp was exposed to a helium-neon laser beam incident at an angle (} to the optical axis of the collector photo-multiplier. The relative intensity of reflection is shown in Fig. 2 (b) as a function of the angle 8. This figure clearly shows that the blue-glass viewing damp is the best among those tested. The knife edges are ex-posed to scrape-off layer plasmas whose typical parameters are n,= l 012 cm - 3 and T,=20 eV. Nevertheless, no appreciable damage of the edges was detected. The scattered light is collected by the lens L2, 56 mm in diameter, and is detected by a photo-multiplier tube (HTV R-943). The monochromator is a 25 cm Czerney-Turner mount which has a 52 mm X 52 mm grating with a dispersion of 30 A/mm. Both the entrance and exit slits are IO mm high and 1 mm wide. Therefore, the channel width is 30 A per channel. Single-channel spectrum analy-454 Sadao MASAMUNE, Issei YOKOYAMA, Masayuki FuKAo and Hiroshi NISHIHARA A~-; (al ~ Q _.,,, lw-N. LASER PHOTOMULTIPLIER ::i 5.0 ~ !'\ (b) ... 5 ::I § . J. , ~ 1.0 \ A •✓•A ...... ...... ti 11.J • •-•--•\. ...... a✓ a: I!! •-.✓'(\ 'J\ I:: i '---o-o /0 ... O'O 0 ~ 'o 0.1 -60 -30 0 30 60 9 ( DEGREE) Fig. 2. (a) Instrumentation for the measurement of angular distribution of reflection factors for three types of viewing damps. (b) Angular distribution of reflection factors. Triangle: stainless steel knife edges arranged separately at a distance of 0.5 mm; Filled circle: stain-less steel knife edges arranged closely; Open circle: blue-glass knife edges arranged closely. ses were performed for central wave lengths of 6813, 6750, 6720, 6660, 6610 and 6520 A. The photo-multiplier tube has a square photo-cathode (10 mm X 10 mm) of GaAs with a high quantum efficiency in the red and near infra-red regions. The tube is shielded from the strong magnetic field of the tokamak machine by means of a 3 mm soft iron housing plus a µ-metal shield, 1 mm thick. §3. Experiments The measurement of the electron temperature and density by the Thomson scattering method has been performed on the plasma of the NOV A II tokamak, whose major and minor radii, Rand a, are 30 and 6 cm, respectively. The experi-mental arrangement is schematically shown in Fig. 3. The impurity ions are sup-pressed and the gas recycling is controlled by the techniques of both titanium Electron Temperature Measurement by Thomson Scattering I Monoctvomalor I .9-0 Tl Sublimator & Fast Acting Yalv• ~~lRi'"-~-o-, ~ ~ to Pump Fig. 3. Experimental arrangement. 9JJN GAS PUFFING FAST GAS PUFFING 10 20 t(ms) Fig. 4. Time sequence of gas puffing. to Pump 30 455 gettering and gas puffing. The time sequence of the gas puffing is digramatically shown in Fig. 4. A slow gas injector is started about IO ms before the ignition of the discharge. In addition to this, a fast iajector is worked 2 ms before the igni-tion. The plasma density is maintained during the discharge by slow gas puffing. Fast gas puffing reduces the production of runaway electrons, and suppresses the interaction between the plasma and the limiter in the current rise stage. Fast gas puffing immediately followed by ignition is effectual for obtaining a stable high-density plasma.8 •9> The toroidal field intensity was typically 1.0 T throughout the 456 Sadao MASAMUNE, Issei YoKOYAMA, Masayuki FuKAo and Hiroshi NISHIHARA experiment. :j oi ~ ....I 0 1.0 9 ffi t ... ~ 05 9 ~ f > ... ~ £ uJ ... ~ 0.1 0 5 10 15 20 ( 6). )2 ( I 04 A2 ) Fig. 5. Typical spectrum of the scattered Ruby laser light. <( ... 15 _0.10 > 5 t Teo ~ ,..,,,,.----Fi;.,-----...... ,,,..., ..... ,I \ ,,,,., "¥-...... ,......... ....., V 5 10 t ( ms) '?E u "" -~ 2 le°' >«; 0 ~ ... i0 Fig. 6. Time behaviros of the loop voltage V1, the plasma current Ip, the average density n., and the central electron tem-perature T,(O). An example of the observed spectra of the Thomson scattering method is shown in Fig. 5. The spectrum measurement was performed in the quasi-steady state at 6 ms after the ignition. Each of the open circles is the average of 5 to IO obser-vations, while the bars show the standard deviations. The electron temperature T, was obtained by the least-square fit of the Gaussian distributions to the ob-served spectra. To establish a spectrum and its temperature, 40 to 60 shots of the discharge were tried. The electron density n, was simultaneously determined by means of the measurement of the total intensity of Thomson scattering and the microwave interferometry. Figure 6 shows the typical time behaviors of the loop voltage Vi, the plasma current I,,, the line-averaged electron density fl, by a 6-mm microwave inter-ferometer, and the central electron temperature T,(0). In this figure, the fast Electron Temperature Measurement by Thomson Scatteingr 457 7e (a) l= 3 ms V 3 ... 3 --(b)t:6 ms -o a, t=::::~ C: 2 =t 0 ~i, ~ t,! 1 ... 0 0 2 4 6 t Q I 0 2 4 6 wall r (cm) r (cm) wall Fig. 7. Typical radial profiles of electron temperature and density measured at t=3 and 6 ms after the start of discharge. initial rise of the initial density is followed by a rapid decay. The peak value indicates that the initial filling gas is ionized by half. The rapid decay of electron density is stopped, because the gas pressure is recovered by slow gas puffing, and the average density reaches a wide second peak. Figure 7 shows the radial profiles of the electron temperature T,(r) and den-sity n,(r) at 3 and 6 ms after the ignition of the discharge. Near the plasma bound-ary, both the temperature and density were measured by the method of electro-static probing. Reasonable expressions of the observed temperature and density profiles are as follows : T,(r) = [T,(O)-T,(a)][l-(r/a)2] 2+ T,(a) , 200 0 ;:i, 100 f 0 o~ ....... ..__...._...._~~,--J'--L-.,___...__.., 10__.___.__, lp(kA) Fig. 8. The central electron temperature T,(O) versus the plateau value of the plasma current l /J• ( 5) 458 Sadao MASAMUNE, Issei YOKOYAMA, Masayuki FUKAO and Hiroshi NISHIHARA n,(r) = [n,(0)-n,(a)] [l-(r/a)2]+n,(a). ( 6) Figures 6 and 7 show that the temperature profile is established in the earlier stage of discharge, and remains almost unchanged for several milliseconds. Figure 8 shows the central electron temperature T,(O) in the quasi-steady state for the plateau values of the plasma current Ip from 5 to 11 kA. The temperature increases almost linearly up to 250 eV with an increased plasma current. By using the radial profiles of the electron temperature and density, we can estimate the global electron energy confinement time -rE defined by , 4ir2R! ~ n,T,rdr _ 411:2R! ~ n,T,rdr 't" E = __ .;c_;;"----- - __ .;c_;;"-----p OH V,Ip ( 7) where P OH is the power of Ohmic heating. In the quasi-steady state, P oH is ap-proximately equal to the plasma current IP multiplied by the loop voltage V, at the position of the conducting shell. Figure 9 shows the confinement time versus the average density fl,. The average density varied from I x 1013 to 2.4 x 1013 cm -3, while the current was kept around 10 kA. The confinement time -rE increases with an increased electron density. This is because the electron temperature is mainly determined by the plasma current. By assuming T,/Ip=constant in Eq. (7), it immediately yields the relation -rEocfl,. The Alcator scaling law10> is ( 8) where a is the minor radius in m, and n14 the line-averaged density in 1014 cm-3• The coefficient a takes on a value between 0.6 and 1.0. In Fig. 9, the extreme cases of this scaling law for a=0.6 and a= 1.0 are shown for comparison. It -;;; 0.5 E "tfl 2 ne (1013 cni3 ) -cE.AI Fig. 9. The global electron energy confinement time , 8 versus the average density ii,. •J!J,.A.I is the energy confinement time expected from the so called Alcator scaling law for the two cases of the numericl coefficients of 0.6 and 1.0. Electron Temperature Measurement by Thomson Scattering 459 should be noted that Fig. 9 indicates the consistency of the NOVA II confinement time with the established Alcator scaling law for tokamak plasmas. §4. Summary The Thomson scattering method was applied to determine the temperature and density profiles of plasma electrons in the NOV A II tokamak. The results obtained are summarized as follows: I) Use was made of a viewing damp, which was an array of blue-glass knife edges. It was the most effective as regards minimizing the intensity of stray light among the three types of viewing damps tested, including two types of widely used viewing damps of stainless steel knife edges. 2) The radial profile of the electron temperature has been found to be approxi-mately the square of the parabolic expression, while the density profile appeared somewhat broader and can be represented by the parabolic expression. 3) The global electron energy confinement time determined from the tempera-ture and density profiles is in good agreement with that predicted from the Alcator scaling law. Acknowledgment The authors wish to thank Drs. H. Zushi and K. Kondo of the Plasma Phy-sics Laboratory, Kyoto University, for their valuable information and useful dis-cussions. References I) M.J. Forrest, N.J. Peacock, D. C. Robinson, V. V. Sannikov and P. D. Wilcock: Culham Laboratory Report, CLM-R 107 (1970). 2) J. S. Culver and M. Murakami: Proc. 5th S.vmP, Eng. Problems of Fuswn Res., Princeton Uni-versity (1973). 3) M. Charet, L. Durney, C. DeMichelis and P. Platz: Association Euratom-CEA, Fontenay-aux-Roses Report, EUR-CEA-FC 759 (1975). 4) T. Matoba, A. Funahashi and K. Ando: Japan Atomic Energy Research Institute Report, JAERI-M 5515 (1973). 5) K. Kawahata, Y. Hamada, A. Yasuda and K. Miyamoto: Jpn. J. Appl. Phys. 18 627 (1979). 6) M. Fukao, Y. Fujiwara, H. Zushi, H. Sumeitsu et al.: Mem. Fae. Eng., Kyoto Univ. 39 431 (1977). 7) J. Sheffield: Plasma Scattering of Electromagnetic Radiation, (Academic Press, London, 1975). 8) R.J. Hawryluk, K. Bo!, N. Bretz, D. Dimock et al.: Nucl. Fusion 19 1307 (1979). 9) R. S. Granetz, I. H. Hutchinson and D. 0. Overskei: Nucl. Fusion 19 1587 (1979). 10) W. Pfeiffer and R. E. Waltz: Nucl. Fusion 19 51 (1979). 

Electron Temperature Measurement by Thomson Scattering in Tokamak NOVA II

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/281225/1/mfeku_44_4_451.pdf

Electron Temperature Measurement by Thomson Scattering in Tokamak NOVA II

Abstract

Similar works

Full text

Available Versions

Kyoto University Research Information Repository