KFKI-1930-119
’Hungarian ’ücademy o f Sciences
CENTRAL RESEARCH
INSTITUTE FOR PHYSICS
BUDAPEST
J . S . BAKOS
PLASMA DIAGNOSTICS WITH LASERS
J.S. Bakos
Central Research Institute for Physics H-1525 Budapest 114, P.O.B. 49, Hungary
HU ISSN 0368 5330 ISBN 963 371 762 0
Different methods of laser plasma diagnostics is reviewed and the latest results are discussed.
АННОТАЦИЯ
Обсуждаются методы лазерной диагностики плазмы и дискутируются получен
ные самые новые результаты.
KI VONAT
A lézeres plazmadiagnosztika módszereit ismertetjük és a legújabb ered
ményeket diszkutáljuk.
L aser p l a s m a d i a g n o s t i c s has b e c o m e the la r g e s t b r a n c h a m o n g the p l a s m a d i a g n o s t i c s met h o d s b e c a u s e of the r a p i d d e v e l o p m e n t of laser t e c h n i q u e s d u r i n g the last t w e n t y y e a r s . Bot h the d e v e l o p ment of laser p h y s i c s itself and the a s s o c i a t e d t e c h n i q u e s
hav e i n i t i a t e d n e w d i a g n o s t i c m e t h o d s a n d the n e e d for n e w d i a g n ostics has, in turn, started the i n v e s t i g a t i o n of n e w shorts of lasers, detectors, and opt i c a l c o n f i g u r a t i o n s . The r e s e a r c h a c t i v i t y in the f i e l d of c o n t r o l l e d t h e r m o n u c l e a r fusion p l a y s a deci s i v e role in this respect.
A l m o s t all l a s e r methods can e q u a l l y be us e d for the d i a g n ostics of both l o w t e m p e r a t u r e and h i g h t e m p e r a t u r e p l a s m a s but the needs of t h e r m o n u c l e a r p l a s m a d i a g n o s t i c s d e t e r m i n e s t h e rate of development. L o w t e m p e r a t u r e p l a s m a d i a g n o s t i c s e x p e r i m e n t s tend to be only m o d e l e x p e r i m e n t s p r i o r to a p p l y i n g the n e w m e t h o d in t h e r m o n u c l e a r research. In v i e w of this, e m p h a s i s is p l a c e d on the h i g h t e m p e r a t u r e a p p l i c a t i o n s in this p r e s e n t r e port. The b o u n d a r i e s of laser p l a s m a d i a g n o s t i c s h a v e w i d e n e d v e r y m u c h and fro m time to time r e v i e w h a v e s u m m a r i z e d the r e sults in w i d e l y v a r y i n g s u b f ields [1-14]. In this p a p e r c h i e f l y the r esults p u b l i s h e d after the last r e v i e w p apers / d e t a i l e d above/ are e m p h a s i z e d besides a brief s u m m a r y of the e s s e n c e of the d i f f e r e n t l a ser dia g n o s t i c methods.
2 . WHAT DOES A LASER PLASMA D IA G N O S TIC S S E T -U P LOOK L IK E ?
The general a r r a n g e m e n t of a p l a s m a d i a g n o s t i c s m e a s u r i n g set-up can be seen in Fig. 1 b y means o f w h i c h the p l a s m a c r e ated by the p lasma device can be d i a gnosed. The m e a s u r i n g o p t i c a l c o n f i g u r a t i o n d e p e n d s on the p r o p e r t y o f the p l a s m a to be
measured. F o r instance, in the c a s e of p l a s m a d e n s i t y m e a s u r e ments, by m e a s u r i n g the index o f r e f r a c t i o n of the p l a s m a this p a r t i c u l a r o p t i c a l c o n f i g u r a t i o n acts as a sort of i n t e r f e r o m e ter /see S e c t i o n 6/. By m e a s u r i n g the e l e c t r o n t e m p e r a t u r e by T h o m s o n s c a t t e r i n g the m e a s u r i n g o p t i c a l c o n f i g u r a t i o n is given b y the s c a t t e r i n g g e o m e t r y /see S e c t i o n 3/. D e p e n d i n g on the p l a s m a to be m e a s u r e d this o p t i c a l c o n f i g u r a t i o n c a n be r e a l i z e d in m a n y ways. The d e v e l o p m e n t of m e a s u r i n g o ptical c o n f i g u r a t i o n s re s u l t s in n e w p lasma d i a g n o s t i c s m e t h o d s /for i n s t a n c e d i f f e r e n t i n t e r f e r o m e t e r s , s c a t t e r i n g ge o m e t r y / . A n y given m e a s u r i n g o p tical c o n f i g u r a t i o n is d e t e r m i n e d , naturally, also b y the type of i n t e r a c t i o n wit h the plasma /for i n s t a n c e local scattering, p h a s e a c c e l e r a t i o n d u r i n g the p a t h of the p r o p a g a t i o n / .
Lasers a r e s e l e c t e d a c c o r d i n g to the p l a s m a p r o perties, the p l a s m a p r o p e r t y to be m e a s u r e d , the type of i n t eraction, the type of d e t e c t i o n and m e a s u r i n g o p t i c a l c o n f i g u r a t i o n . In a c c o r d ance w i t h the g iven r e q u i r e m e n t s t h e laser w a v e l e n g t h varies
fro m 100 n m to 1 mm. F u r t h e r m o r e the laser can be p u l s e d or c o n t i n u o u s w a v e a n d the p o w e r r e q u i r e m e n t s are some lOO M W in the p u l s e d case a n d from som e 100 m W in the C W case. T h e p u l s e d u r a tio n is s o m e t i m e s r e q u i r e d to be some tens of picose c o n d s ;
o t h e r w i s e a l o n g e r p u l s e length in the yse c range is m o r e a d v a n tag e o u s .
The type of d e t e c t o r c h a n g e s a c c o r d i n g to the w a v e l e n g t h of the laser a n d many t i mes g r e a t e r s e n s i t i v i t y and faster r e s p o n s e are a l s o required. Recen t l y , spe c i a l T V c a m e r a s and s t r e a k - c a m e r a s have b e g u n to s e r v e as detectors.
The d a t a a c q u i s i t i o n and d i s p l a y s y s t e m is c o n t r o l l e d by c o m p u t e r s in m o d e r n d i a g n o s t i c s s y s t e m s of t h e r m o n u c l e a r rese a r c h S o m e t i m e s this da t a a c q u i s i t i o n s y s t e m is simply an osc i l l o s c o p e .
P l a s m a d e v i c e s do n o t b e l o n g to the d i a g n o s t i c s s y s t e m but such dev i c e s d o give c o n t r o l s i g n a l s to the lasers a n d the data a c q u i s i t i o n system. In m o r e s o p h i s t i c a t e d m e a s u r i n g o p t i c a l c o n figu r a t i o n s t h e data a c q u i s i t i o n s y s t e m c o n t r o l s the o p t i c s and the lasers too.
The use of the p l a s m a d i a g n o s t i c s m e t h o d enables the f o l l o w i n g p l a s m a pa r a m e t e r s to be m e a s ured.
1/ E l e c t r o n den s i t y /n /
by T h o m s o n s c a t t e r i n g /Section 3/, i n t e r f e r o m e t r y /Section 6/ and n o n linear s c a t t e r i n g /Section 5/;
2/ Sp a t i a l and temporal e l e c t r o n d e n s i t y d i s t r u b u t i o n n 0 /r,t/
by T h o m s o n s c a t t e r i n g /Section 3/, a n d i n t e r f e r o m e t r y /Section 6/;
3/ E l e c t r o n t e m p e r a t u r e /Т /
by T h o m s o n s c a t t e r i n g /Sect i o n 3/, and n o n l i n e a r s c a t t e r i n g /Section 5/;
4/ Sp a t i a l and tempo r a l e l e c t r o n t e m p e r a t u r e d i s t r i b u t i o n /Т / r ft//
by T h o m s o n s c a t t e r i n g /Sect i o n 3 / ; 5/ Ion t e m p e r a t u r e /IV/
by T h o m s o n s c a t t e r i n g /Section 3/, n o n l i n e a r s c a t t e r i n g /Section 5/, a n d s c a t t e r i n g on b o u n d e l e c t r o n s /Sect i o n 4/?
6/ Local c u r r e n t d e n s i t y /j /
by T h o m s o n s c a t t e r i n g /Sect i o n 7/;
7/ D e t e r m i n a t i o n of the d e n s i t y of the d i f f e r e n t sp e c i e s of p l a s m a / i m p urity atoms, ions in the g r o u n d and e xcited states, b a c k g r o u n d atoms in the g r o u n d and e x c i t e d state/
by s c a t t e r i n g o n b o u n d e l e c t r o n s /Section 4 / .
8/ Local m a g n e t i c f i eld /Н /
by T h o m s o n s c a t t e r i n g /Section 3/ a n d F a r a d a y r o t a t i o n /Section 7/.
3 . SCATTERING BY FREE ELECTRONS
L a s e r r a d i a t i o n w i t h p r o p a g a t i o n v e c t o r and f r e q u e n c y o)^ is sent t h r o u g h the plasma. The b y the p l a s m a s c a t t e r e d light with p r o p a g a t i o n v e c t o r к and f r e q u e n c y ш с is observed. The s p e c t r u m of the s c a t t e r e d light can be fitted by the t h e o r e t i c a l func t i o n S/w, к; T , T . , n / given in ref. [14] w h e r e w = w . - w c and k = k .- k ~ 2 k .s i n 0 /2 where 0 is the s c a t t e r i n g angle. The pa r -
l b 1
ameters are the e l e c t r o n t e m p e r a t u r e /T e /f the ion t e m p e r a t u r e /T^/ a n d the e l e c t r o n d e n s i t y /n / , all of w h i c h can be d e t e r m i n e d as a r e s u l t of the f i t t i n g pro c e d u r e .
If k>>k = -r— w h e r e X is the D e b y e length, the f o r m of
D Aq D
func t i o n S is the same as the e l e c t r o n v e l o c i t y d i s t r i b u t i o n function. In the cas e of M a x w e l l i a n d i s t r i b u t i o n the a m p l i t u d e of the s p e c t r u m gives the d e n s i t y of t h e electron, the w i d t h
ЗнТ 1/2
/ A w / d e termines the e l e c t r o n t e m p e r a t u r e Aw = k ( ---- ) . и is e
B o l t z m a n n ' s const a n t , and m is the e l e c t r o n mass.
e
T h e ion t e m p e r a t u r e does not i n f l u e n c e the func t i o n S in that c a s e at all.
The m e a s u r e m e n t is a local one d e t e r m i n i n g n and T at
e e
the p o i n t of i n t e r a c t i o n к . and к .
i s
In the m o s t r e c e n t i n v e s t i g a t i o n s of t o k a m a k p l a s m a [15]
the s p e c t r u m is m e a s u r e d u s i n g a s p e c t r o g r a p h w i t h image i n t e n s i f i c a t i o n a n d v i d e o c a m e r a d e t e ction. The d i f f e r e n t spatial points a m o n g the pat h of the light t h r o u g h the p l a s m a are s i m u l t a n e o u s l y o b s e r v e d u s i n g fibre o p t i c s to p r o j e c t the image of the l i g h t path to the i nput slit of the spectrograph. In this spatial d i s t r i b u t i o n of n and T c a n b e d e t e r m i n e d s i m u l t a -
e e
neously. In this case, a g a i n t p u l s e r u b y laser is use d as the light source.
In ano t h e r e x p e r i m e n t [16,17] the rub y l a ser is m o d u l a t e d and so the l a ser gives a series of l i g h t p ulses w i t h a b out 30 ysec s e p a r a t i o n b e t w e e n the pulses. U s i n g p o l y c h r o m a t o r
with s i m u l t a n e o u s d e t e c t i o n in the c h a n n e l s the temp o r a l e v o l u tion /т е / n e / of the t o k a m a k d i s c h a r g e is m e a s u r e d in the v i c i n ity of m a j o r d i s r u p t i o n s .
If k < < k D a n d w>>Wp /the p l a s m a frequency/ the f u n c t i o n S c a n be r e p l a c e d by S' i.e.
S(o),k;Te ,Ti ,ne ) ~S' (w,k;T1)
and the S' f u n c t i o n c o n t a i n s o n l y T^ as the para m e t e r s , c o n sequently. T h e ion t e m p e r a t u r e /T^/ c a n be d e t e r m i n e d f r o m the m e a s u r e d s c a t t e r e d spectrum. The w i d t h of the s p e c t r u m
1 ЗиТ± 1/2 Г715
w h e r e пк is t h e ion mass.
There are two ways to s a t i s f y the i n e q u a l i t y k < < k Q . One w a y is to d e c r e a s e the s c a t t e r i n g angle, the s econd is to i n c r e a s e the w a v e l e n g t h of the laser.
In the f i r s t case the s c a t t e r e d r a d i a t i o n c a n n o t be s e p a r ated from the inci d e n t beam. The s i m u l t a n e o u s d e t e c t i o n of the i n c i d e n t and s c a t t e r e d b e a m causes a b e a t i n g signal in the
detector. Due t o the s m a l l n e s s of Док the s p e c t r u m of the b e a t ing can be m e a s u r e d e l e c t r o n i c a l l y i n s t e a d of u s i n g a p o l y c h r o - mator. This h o m o d y n e d e t e c t i o n is u s e d in m a n y e x p e r i m e n t s
[18-25]. The d r a w b a c k is the poor s p a t i a l r e s o l u t i o n d u e to the small s c a t t e r i n g angle.
In the s e c o n d case, lon g w a v e l e n g t h h i g h pea k i n t e n s i t y laser and s e n s i t i v e d e t e c t o r s are needed. The c o n s e q u e n c e of this is that e x t e n s i v e r e s e a r c h in b e i n g c a r r i e d out to d e v e l op p ulsed n a r r o w b a n d w i d t h s u b m i l l i m e t e r lasers [26-28] and
s e n s i t i v e det e c t o r s .
4 . SCATTERING BY BOUND ELECTRONS
Plasma u s u a l l y c o n s i s t s of d i f f e r e n t a t o m i c species; this is a l s o true if the o r i g i n a l gas w a s v e r y pur e and the i o n i z a tion grade is n o m i n a l l y 100% /high t e m p e r a t u r e plasma/. The n e u t r a l gas b a c k g r o u n d d e n s i t y is u s u a l l y m a n y o r d e r s of m a g
n i t u d e lower t h a n the e l e c t r o n d e n s i t y in high t e m p e r a t u r e h y d r o g e n t o k a m a k plasmas, b u t this low d e n s i t y b a c k g r o u n d plays an i m p o r t a n t ro l e in p a r t i c l e t r a n s p o r t p r o c e s s e s to t h e wall and ba c k [29]. The wal l i n t e r a c t i o n w i t h h i g h e n e r g y n e u t r a l a t oms emits h e a v y impu r i t y a t oms int o the p l a s m a t h e r e b y c h a n g ing the c o n t e n t and p r i n c i p a l l y c h a n g i n g the state of the p lasma This m eans t h a t the d i a g n o s t i c s c o n c e r n i n g the d e n s i t i e s of the d i f f e r e n t p l a s m a species is of c r u c i a l importance.
The d e t e r m i n a t i o n of the p l a s m a c o n t e n t is n a t u r a l l y an i m p o rtant t a s k also for the d i a g n o s t i c s of pla s m a s of l o w t e m perature, c o n s e q u e n t l y of low i o n i z a t i o n grade.
The l a s e r m e t h o d for l o c a l l y d e t e r m i n i n g the dens i t y , t e m p e r a t u r e and d r i f t v e l o c i t y of t h e d i f f e r e n t p l a s m a spe c i e s is r e s o n a n c e f l u o r e s c e n c e i nduced b y t u n a b l e laser light. The
m e t h o d has b e c o m e a p p l i c a b l e o n l y since the d e v e l o p m e n t of t u n able lasers. T h e r e s o n a n c e light of the laser u s u a l l y s a t u rates the t r a n s i t i o n due to the hig h intensity. In this c a s e the p o p u l a t i o n i n c r e a s e AN^ in the u p p e r level - c o n s e q u e n t l y also the i n t e n s i t y of the r e s o n a n c e f l u o r e s c e n c e /G / - d e p e n d s only on the p o p u l a t i o n of the lower l e vel /N^/
G = t *a *N2.
If the a t o m i c p a r ameters, s p o n t a n e o u s r e l a x a t i o n rates and the r e l a x a t i o n rates ind u c e d by e l e c t r o n s are known, a c a n be c a l c u l a t e d [30,31] and the d e n s i t y of the atoms in the lower level can be d e t e r m i n e d a c c o r d i n g to the a b o v e formula, w h ere t is the d u r a t i o n time of the l a s e r pulse.
In a r e c e n t e x p e r i m e n t the d e n s t i y of the h y d r o g e n atoms in the first e x c i t e d s t ate was d e t e r m i n e d in tok a m a k p l a s m a [32]
W i t h the k n o w l e d g e of the d e n s i t y of the e x c i t e d a t o m / ^ / the a t o m i c d e n s i t y in the n o rmal s t ate /N^/ c a n be c a l c u l a t e d u s i n g an a c c e p t a b l e p l a s m a model. The d e n s i t y d i s t r i b u t i o n are also d e t e r m i n e d at d i f f e r e n t d i s c h a r g e times.
The d e n s i t y of d i f f e r e n t p l a s m a s pecies has b e e n d e t e r m i n e d in l o w t e m p e r a t u r e pla s m a s in m a n y r e c e n t e x p e r i m e n t s [33-38]
u s i n g the r e s o n a n c e f l u o r e s c e n c e m ethod. U s i n g this m e t h o d of laser p l a s m a d i a g n o s t i c s it is also p o s s i b l e to m e a s u r e the d i f fusion of the i m p u r i t i e s in the plasma. In t h e e x p e r i m e n t [39]
the d i f f u s i o n of a l u m i n i u m atoms w a s m e a s u r e d in h y d r o g e n plasma.
The a l u m i n i u m a t o m s are i n j e c t e d int o the p l a s m a by a s h o t from a CC>2 T E A laser to the s u r f a c e of a l u m i n i u m in the plasma. The time t aken for t h e d i f f u s i o n to d e v e l o p is m e a s u r e d b y a timed, tuned laser and b y o b s e r v i n g the r e s o n a n c e fluores c e n c e . The p o t e n t i a l a p p l i c a b i l i t y of the m e t h o d for i n v e s t i g a t i n g the p l a s m a - w a l l i n t e r a c t i o n s in t okamak d i s c h a r g e is of g r e a t s i g n i f i c a n c e .
5 . NONLINEAR SCATTERING BY PLASMA MODES
In the p r e c e d i n g d i a g n o s t i c s m e t h o d s the absence o f p e r t u r b a tion of the p l a s m a by the l a ser b e a m is r e q u i r e d /no h e a t i n g / . But sometimes t h e s c a t t e r e d signal - e s p e c i a l l y in t h e case of s c a t t e r i n g b y free e l e c t r o n s - t u r n s out to be too w e a k /the T h o m s o n s c a t t e r i n g c r o s s - s e c t i o n is t o o small/. C o n s e q u e n t l y the laser b e a m m u s t t h e n be v e r y s t r o n g a n d the p l a s m a is perturbed.
The l a ser b e a m d o e s not h e a t the p l a s m a e s s e n t i a l l y b u t induces p l a s m a m o des b y n o n l i n e a r i n t e r a c t i o n and the b e a m s c a t t e r s on these induced mode. One of t hese n o n l i n e a r s c a t t e r i n g p r o c e s s e s is the four w a v e scattering. Two c r o s s i n g laser beams i n d u c e
p l a s m a m o des so th a t the f i rst b e a m forces the e l e c t r o n to o s c i l late a n d the o s c i l l a t i n g e l e c t r o n of h i g h v e l o c i t y i n t e r a c t s w i t h the m a g n e t i c field of the s e c o n d b e a m of d i f f e r e n t f r e
quency. The r e s u l t s is a p o n d e r o m o t i v e force a c t i n g o n the e l e c trons w i t h the f r e q u e n c y d i f f e r e n c e ÍÍ = - ш 2 of t h e two
p u m p i n g b e a m a n d w i t h the w a v e v e c t o r к = k^ - k 2 . The force a c t i n g on the e l e c t r o n s is p a r a l l e l to the e l e c t r i c field. C o n sequently, a p l a s m a wave is i n d u c e d if к and ÍÍ s atisfy t h e d i s p e r s i o n r e l a t i o n in the plasma. The u s e of a t hird b e a m it p r o duces s c a t t e r i n g on the i n d u c e d p l a s m a wav e g i v i n g ri s e to a
fourth s c a t t e r e d wave. The c r o s s - s e c t i o n of t h e s c a t t e r i n g de-
pends on the p r o d u c t of the i n t e n s i t i e s of the p u m p i n g beams and it is of m a n y o r d e r s larger t h a n the T h o m s o n s c a t t e r i n g c r o s s - s e c t i o n [40-44].
The p l a s m a properties, the d e n s i t y and the t e m p e r a t u r e can be d e t e r m i n e d b y m e a s u r i n g the s p e c t r a l p r o p e r t i e s of the s c a t t ered light o r b y d e t e r m i n a t i o n of the r e s o n a n c e c o n d i t i o n c h a n g ing the f r e q u e n c y of the p u m p i n g beams.
The v e r y f i rst e x p e r i m e n t s h a v e been p e r f o r m e d [45,46] s h o w ing the r e s u l t to be in a g r e e m e n t w i t h theo r e t i c a l e x p e c t a t i o n s
[42]. in t h e s e e x p e r i m e n t s two d y e laser b e a m s of different frequency pump the plasma nodes in a free burning arc and the ruby l a ser p u l s e is s c a t t e r e d o n the i n d u c e d f luctuations. T h e incr e a s e in the i n t e n s i t y of t h e s c a t t e r e d light r e l a t i v e to the T h o m s o n s c a t t e r i n g is b e t w e e n one a n d two o r d e r s of magnitude.
A s ingle laser b e a m can a l s o e x c i t e p l a s m a m o d e s if the i n t e n s i t y is h i g h e n o u g h and the b e a m itself is s c a t t e r e d on the ind u c e d f l u c t u a t i o n /for i n s t a n c e B r i l l o u i n scattering/. T h e s c a t t e r e d l i g h t can be u s e d to d e t e r m i n e p l a s m a p a r a m e t e r s such as ion t e m p e r a t u r e etc. see for i n s t a n c e [47,48].
M a n y o t h e r n o n l i n e a r p r o c e s s e s are b e i n g i n v e s t i g a t e d w i t h a v i e w to u s i n g the m for p l a s m a d i a g n o s t i c s p u r p o s e s [49,50].
6 . MEASUREMENT OF THE INDEX REFRACTION OF THE PLASMA
The m o s t p o p u l a r m e t h o d for d e t e r m i n i n g p lasma d e n s i t y is to m e a s u r e the index of r e f r a c t i o n o f the p l a s m a u s i n g i n t e r f e r o meters w i t h laser light source; viz. the phase a c c e i e ”a tion of the light b e a m c a u s e d b y i s o t r o p i c p l a s m a w i t h o u t a m a g n e t i c field.
Д cp = - 4 , 4 6 * 1 0 ~ 14 ’ X ’ n *£
e
w h e r e Л is t h e w a v e l e n g t h , 1 is t h e length of the plasma. As can be seen t h e longer the w a v e l e n g t h and the length of the p l a s m a the s m a l l e r the d e n s i t y w h i c h can be d e t ermined. F u r t h e r more, the s m a l l e r the p h a s e a c c e l e r a t i o n w h i c h can b e m e a s u r e d
the s maller the m e a s u r a b l e density. This has §iven ri s e to r e search w o r k and p a p e r s d e a l i n g w i t h the e n l a r g e m e n t of 1 u s ing a m u l t i b e a m i n t e r f e r o m e t e r [51], and the m e a s u r e m e n t of small phase a c c e l e r a t i o n [52]. In addition, a g r eat deal of effort has als o bee n m a d e to d e v e l o p r e l i a b l e far infr a r e d C W lasers
[53-58].
The cor r e c t c h o i c e of the w a v e l e n g t h d epends on two d i s t u r b ing effects: the m e c h a n i c a l v i b r a t i o n of the parts of the i n t e r f e r o m e t e r and the r e f r a c t i o n of the b e a m on the p l a s m a d e n sity gradient. To a v oid the f i rst effect a longer w a v e l e n g t h , to avoid the s e c o n d e ffect a s h o r t e r w a v e l e n g t h has to be used.
Thus the cor r e c t w a v e l e n g t h to be chosen is the r e s u l t of a trade off b e t w e e n the two effects.
In actual p r a c t i c e a w i d e v a r i e t y of i n t e r f e r o m e t e r s are use d to m e a s u r e p h a s e a c c e l e r a t i o n t a king into account, among the o t h e r requirements, the s p e e d of the m e a s u r e m e n t n e e d e d in the case of fast p l a s m a events. T h ese i n t e r f e r o m e t e r s are the f o l l o w i n g :
1/ C l a s s i c a l two b e a m i n t e r f e r o m e t e r s w i t h r e f e r e n c e b e a m of the same f r e q u e n c y as the scene b e a m /Michelson, M a c h - Zender, Jamin, etc./. The d r a w b a c k of these i n t e r f e r o m e t e r s is th a t the m e a s u r e m e n t of t h e phase a c c e l e r a t i o n is d i s t u r b e d by the i n s t a b i l i t y of the laser intensity, the in c r e a s e of p h ase a c c e l e r a t i o n cannot be d i s t i n g u i s h e d from the d e c r e a s e of the phase a c celeration.
2/ H e t e r o d y n e interfe r o m e t e r s . The p l a s m a is set in t o the r e son a t o r of the laser. T h e p h a s e c hange c aused b y the p lasma a ppears as a f r e q u e n c y c h a n g e of the laser. T h e f r e q u e n c y c hange is m e a s u r e d by h e t e r o d y i n g w i t h the b e a m of the
s econd laser [59,60]. The u s e of this m e t h o d a n a b l e s a high d e g r e e of s e n s i t i v i t y to be achieved. Great l a s e r frequency s t a b i l i t y is needed. The s m a l l e s t m e a s u r e d d e n s i t y is about 1 0 12 c m - 3 .
3/ A s h b y - J e p h s c o t t interferometer. T h e p l a s m a is set into the F a b r y - P e r o t i n t e r f e r o m e t e r w h i c h is in series wit h the laser resonator. The i n t e n s i t y of the laser is m o d u l a t e d a c c o r d i n g to the r e s o nance c a u s e d b y the p l a s m a in the c o m p o u n d interferometer. The sign of the p l a s m a d e n s i t y c h a n g e is a m b i g u o u s in such m e a s u r e m e n t s [61].
4/ M o d u l a t e d i n t e r ferometer. The p a t h l e n g t h is artificially modulated b esides the pathlength change caused by the plasma. The display is on the oscilloscope. The у direction is proportional to the modulation. The interference fringes modulate the b r i g h t n e s s of the display.
In the x d i r e c t i o n the time is d i s p l a y e d /Zebra type display/.
The sign of t h e den s i t y c hange is u n a m b i g u o u s [62,63].
5/ L i ght b e a t i n g i n t e r f e r o m e t e r s [64,58,65]. The f r e q u e n c y of the light in the r e f e r e n c e a r m d i f f e r s from the f r e q u e n c y of the scene beam. The p h a s e s hift of the b e a t i n g signal is d e t e c t e d r e l a t i v e to the p h a s e of the b e a t i n g signal w h i c h a rises f r o m b e a t i n g the s c e n e b e a m w i t h the r e f e r e n c e b e a m b efore the scene b e a m enters the plasma. The p h a s e
d i f f e r e n c e is m e a s u r e d as a time d i f f e r e n c e b e t w e e n the zero c r o s s i n g of t h e signals fr o m the t w o detectors. T h ere is no sign a m b i g u i t y in the phase change, good temporal r e s o l u tion can be achieved. Da t a a c q u i s i t i o n is c o m p u t e r c o n t r o l led. The i n t e r f e r o m e t e r is i n s e n s i t i v e to the laser l i g h t i n t e n s i t y v a r i a t i o n s and frequency.
6/ D o u b l e interferometer. This is the u sual c l a s s i c a l typ e of i n t e r f e r o m e t e r but two w a v e l e n g t h s are us e d simult a n e o u s l y . O w i n g to the d i f f e r e n t d e p e n d e n c e of the p h ase c h a n g e d u e to the v i b r a t i o n and the p l a s m a e l e c t r o n s on the w a v e l e n g t h the v i b r a t i o n can be s e p a r a t e d [66,52]. This a d v a n t a g e is v e r y useful in b i g p l a s m a d e v i c e s suc h as b i g tokamaks. If two w a v e l e n g t h s are used the n e u t r a l a t o m c o n t r i b u t i o n can al s o be s e p a r a t e d in p a r t i a l l y i o n i z e d p l a s m a [67].
7/ H o l o g r a p h i e interfer o m e t e r s . The i n t e r f e r e n c e is r e g i s t e r e d in the w h o l e v o l u m e of t h e o b j e c t plasma. As a rule, the d o uble e x p r o s u r e [68] m e t h o d is used. The d r a w b a c k is the o f f - l i n e dat a acquisition, i.e. the h o l o g r a m s a r e reco r d e d
in fil m or o t h e r light s e n s i t i v e plate. This m e a n s that data b e c o m e a v a i lable o n l y a f t e r d e v e l o p m e n t u s i n g a l a b o r i ous da t a a c q u i s i t i o n procedure.
8/ Q u a d r a t u r e interferometer. The i n t e r f e r o m e t r i c m e a s u r e m e n t is t a ken in the two p o l a r i z a t i o n d i r e c t i o n s s i m u l t a n e o u s l y [69]. This type of i n t e r f e r o m e t e r is i n s e n s i t i v e to the ray r e f r a c t i o n on d e n s i t y gradients.
Nowadays, light b e a t i n g i n t e r f e r o m e t e r s s e e m to be the mo s t r e l i a b l e for t h e r m o n u c l e a r research. T h e r e f o r e a l m o s t all the larger t o k a m a k s are e q u i p p e d w i t h this type of i n t e r f e r o m e t e r for p l a s m a d e n s i t y d i s t r i b u t i o n measure m e n t s .
7 . FARADAY ROTATION MEASUREMENTS
In c u r r e n t c a r r y i n g p l a s m a the s patial d i s t r i b u t i o n of the cur r e n t d e n s i t y is also an i m p o r t a n t p a r a m e t e r of t h e discharge.
This is e s p e c i a l l y true in t o k a m a k devices. The c u r r e n t den s i t y can be d e t e r m i n e d b y s c a t t e r i n g on free e l e c t r o n s too, namely the c entre of the s c a t tered s p e c t r u m is s hifted r e l a t i v e to the line c entre of the incident r a d i a t i o n due to the e l e c t r o n d r ift v e l o c i t y [70,71]
A further m e t h o d for c u r r e n t d e n s i t y d e t e r m i n a t i o n is the Fa r a d a y r o t a t i o n m e asurement. N a m e l y there is m a g n e t i c field around the c u r r e n t and the i n d e x of r e f r a c t i o n of the plasma d epends on the m a g n e t i c field. B e c a u s e of the d i f f e r e n t index of r e f r a c t i o n for left and r i g h t c i r c u l a r l y p o l a r i z e d light p r o p a g a t i n g in the d i r e c t i o n of the m a g n e t i c field t h e plane of p o l a r i z a t i o n of the l i n e a r l y p o l a r i z e d light is rotated.
The rotation a n g l e is given by Э
L 2,63*10~14•X2 *
4
О
n 0 ( r ) * B ( r ) - di
w h e r e В is in К G a u s s units. A f ter m e a s u r i n g the n e (r) d e n s i t y d i s t r i b u t i o n b y i n t e r f e r o m e t r y and t h e p o l a r i z a t i o n r o t a t i o n by p o l a r i m e t r y the l o n g i t u d i n a l m a g n e t i c field B(r) c o n s e q u e n t l y the cu r r e n t d e n s i t y can be determined.
The first e x p e r i m e n t [72,73] h a s s u c c e s s f u l l y b e e n p e r f o r m ed o n the T F R 600 tokamak. On the b a s i s of the r e s u l t the d i r e c tion to be t a k e n for further r e f i n e m e n t of the m e a s u r e m e n t is determined. N e w p o l a r i m é t e r a r r a n g e m e n t s wer e r e c e n t l y p u b l i s h e d
[74] d e s i g n e d s p e c i a l l y for F a r a d a y r o t a t i o n m e a s u r e m e n t s in t o k a m a k p l a s m a s .
8 . SCHLIEREN METHOD
This m e t h o d enables p l a s m a i n h o m o g e n e i t y to be m e a s u r e d by d e t e c t i n g onl y t h e light r e f r a c t e d b y the p l a s m a [1-14].
ACKNOWLEDGEMENTS
M a n y t hanks are due to m y c o - w o r k e r s Dr. Zsuzsa S ö r l e i and Dr. P. ignácz for their h e l p in c o l l e c t i n g the m a t e r i a l for this paper, and to Miss Á n g e s Havas for the e d i t o r i a l work.
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Fig. 1.
I
Kiadja a Központi Fizikai Kutató Intézet Felelős kiadó: Szegő Károly
Szakmai lektor: Szentpétery Imre Nyelvi lektor: Harvey Shenker Gépelte: Beron Péterné
Példányszám: 310 Törzsszám: 80-699 Készült a KFKI sokszorosító üzemében Felelős vezető: Nagy Károly
Budapest, 1980. november hó
