MULTIPLE-PASS LASER HEATING OF A SHORT PLASMA COLUMN

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Submitted on 1 Jan 1979

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MULTIPLE-PASS LASER HEATING OF A SHORT PLASMA COLUMN

R. Brooks, Z. Pietrzyk, G. Vlases

To cite this version:

R. Brooks, Z. Pietrzyk, G. Vlases. MULTIPLE-PASS LASER HEATING OF A SHORT PLASMA COLUMN. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-751-C7-752.

�10.1051/jphyscol:19797363�. �jpa-00219359�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, J u i Z Z e t 1979, page ~ 7 - 751

MULTIPLE-PASS LASER HEATING OF A SHORT PLASMA COLUMN

R.D. Brooks, Z.A. Pietrzyk and G.C. Vlases.

U n i v e r s i t y o f Washington, S e a t t l e , Washington, 98195, U. S. A .

The h e a t i n g o f 1 in e a r m a g n e t i c a l l y confined plasmas w i t h pulsed C02 l a s e r s was proposed as a f u s i o n r e a c t o r scheme some t i m e ago.(') Two f a c - t o r s o f major concern f o r such a device, end losses and the absorption l e n g t h f o r t h e 1 0 . 6 ~ r a d i a t i o n , have l e d t o r e a c t o r designs on t h e order o f a k i l o - meter i n length. More r e c e n t l y , work has progres- sed on redu'cing end losses by several means.(2) The m u l t i p l e pass experiment r e p o r t e d here, which i s s i - m i l a r t o one suggested by ~ u m p h r i e s , ( 3 ) explores a c o n f i g u r a t i o n which o f f e r s the p o s s i b i l i t y o f mat- ching the e f f e c t i v e l a s e r absorption l e n g t h t o a v a r i e t y o f proposed s h o r t e r r e a c t o r s .

F i g u r e 1 shows a diagram o f t h e mu1 t i p l e pass experimental system. The o p t i c a l c a v i t y and t h e t a pinch a r e t h e same as p r e v i o u s l y described(4) ex- cept t h a t t h e r e i s a 6" Germanium (30% r e f ) o u t p u t coupler on the TEA l a s e r , producing 100 Joules o f o u t p u t energy i n about one usec.

Diagnostics i n c l u d e a x i a l viewing holographic i n t e r f e r o m e t r y , 90' r u b y l a s e r s c a t t e r i n g , photon drag C02 r a d i a t i o n detectors, c a l o r i m e t r y , and an i n f r a r e d spectrometer used t o analyze t h e backscat- t e r e d r a d i a t i o n . (52

The performance o f t h e non-laser heated t h e t a pinch plasma has been analyzed using double p u l s e holographic i n t e r f e r o m e t r y . Figure 2 shows a rep-

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FIG. 1 . Mu1 t i p l e - p a s s experimental apparatus.

r e s e n t a t i v e hologram and d e n s i t y p r o f i l e s f o r the o p e r a t i n g c o n d i t i o n s chosen, assuming a 20 cm l o n g a x i a l u n i f o r m plasma. I t can be seen t h a t a den- s i t y minimum on a x i s e x i s t s a t times between t h e bounces o f t h e plasma sheath. The numbers wi thqn t h e p r o f i l e s a r e t h e values o f b and t h e c o r r e l a - t i o n c o e f f i c i e n t s when t h e c e n t r a l regions o f t h e p r o f i l e s a r e f i t t e d t o t h e equation: ne ;neo + b r 2

,

where ne and neo a r e i n e l e c t r o n s per cm and r i s i n cm. A l o n g plasma column w i t h a ,parabolic den- s i t y p r o f i l e p e r i o d i c a l l y re-focuses a l l r a y s t h a t have gone through one focus. For o u r c o n d i t i o n s t h e d i s t a n c e between f o c i associated w i t h t h e ob- served r a d i a l d e n s i t y g r a d i e n t i s on t h e o r d e r of 50 cm. Thus a second focus i s n o t r e a l i z e d ; i n - stead t h e plasma a c t s as a l e n s by re-imaging t h e i n c i d e n t f o c a l spot. The rays e x i t t h e plasma from n e a r l v t h e e n t i r e area w i t h i n t h e d e n s i t v w e l l and

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FIG. 2. Hologram and d e n s i t y p r o f i l e s f o r t h e unheated plasma. The parameter b and t h e c o r r e l a - t i o n c o e f f i c i e n t a r e g i v e n w i t h i n t h e p r o f i l e s f o r a b e s t f i t o f t h e c e n t r a l r e g i o n t o n e = neo + b r 2

.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797363

b (ern-'

FIG. 3. Image p o s i t i o n versus b ( f i t t o parabolic d e n s i t y p r o f i l e ) f o r various i n p u t focus l o c a t i o n s (cm). Locations a r e with r e s p e c t t o t h e a x i s i n Figure 1 .

appear t o be coming from an image l o c a t i o n near t h e c e n t e r of t h e plasma column. Figure 3 shows t h e p o s i t i o n o f the focal image r e l a t i v e t o t h e a x i s i n Figure 1 versus t h e parameter b f o r various posi- t i o n s of t h e i n c i d e n t focus s p o t (shown beside t h e c u r v e s ) , f o r a 20 cm column. I t should be pointed out t h a t n e i t h e r of t h e s e s p o t s i s where t h e rays actnal!:l focus but where t h s i n c i d e n t rays :.auld focus and where the e x i t i n g rays appear t o diverge from. I t can be seen t h a t f o r t h e i n c i d e n t focus a t X = -2 cm, t h e p o s i t i o n of t h e focal image i s r e l a t i v e l y c o n s t a n t over t h e range of d e n s i t y pro- f i l e s observed i n t h e unheated pinch. For t h i s reason i t was decided t o p o s i t i o n t h e i n c i d e n t fo- cus here and the back mirror ( f a = .5 m) such t h a t i t s c e n t e r of curvature i s a t X = +2 cm. This should r e t u r n t h e rays e x i t i n g t h e plasma back upon themselves t o t h e plasma and along t h e same path through t h e plasma t o t h e l a s e r a s long a s t h e den-

FIG. 4. Electron temperature, a s determined by l a s e r s c a t t e r i n g , v e r s u s time i n t o t h e pinch cycle.

Also shown a r e t h e averaged temperatures f o r times g r e a t e r than .7 us.

s i t y prof.ile does not appreciably change i n t h e 6 ns t h e rays a r e out of t h e plasma.

When t h i s modified c o n c e n t r i c o p t i c a l c a v i t y ( i .e., incorporating t h e r e t u r n m i r r o r ) i s used, t h e TEA l a s e r output c o n t a i n s a second "spike" which i s not p r e s e n t without a r e t u r n . This second s p i k e can be l a r g e r than t h e f i r s t when t h e f i r s t s p i k e occurs a t around 400 ns i n t o t h e pinch c y c l e . For t h e f i r s t s p i k e occurring a t times between 350 and 450 ns, t h e temperature of t h e c e n t r a l portion o f t h e plasma, measured by l a s e r s c a t t e r i n g , i s shown a s a function o f time i n Figure 4. Each p o i n t i s a computer b e s t f i t t o t h e average o f a number (given b e s i d e t h e p o i n t ) o f s h o t s . Also shown here i s a r e p r e s e n t a t i v e s i n g l e pass ( i . e . , no r e t u r n m i r r o r ) l a s e r output. I t can be seen t h a t t h e plasma dyna- mics remain' important u n t i l about 700 ns, a f t e r which time t h e temperature becomes f a i r l y c o n s t a n t . Since f o r i n v e r s e Bremmstrahlung absorption a t con- s t a n t d e n s i t y

T ~

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where Tf and To a r e t h e f i n a l and i n i t i a l tempera- t u r e s , t h e s i n g l e pass ensrgy waiild have t o pas5 through t h e plasma a t l e a s t twice t o achieve 1 . 3 times t h e temperature i n c r e a s e . In t h i s manner t h e number of e q u i v a l e n t passes t o achieve t h e m u l t i p l e pass temperatures f o r t > .7 us i s c a l c u l a t e d t o be 4.1, w i t h a standard d e v i a t i o n of 1.3. T h i s i s con- s i s t e n t with what can be expected due t o r e f l e c t i v e l o s s e s a t t h e o p t i c a l s u r f a c e s of t h e c a v i t y .

In conclusion, an o p t i c a l c a v i t y including t h e plasma as an a c t i v e o p t i c a l element was chosen us- ing holography. Mu1 t i p l e pass heating e q u i v a l e n t t o 4.1 f 1 . 3 passes of t h e s i n g l e pass energy was determined by ruby l a s e r s c a t t e r i n g .

This work was supported by t h e National Science Foundation.

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