BEAT HEATING IN PLASMAS USING CO2 LASERS

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Submitted on 1 Jan 1979

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BEAT HEATING IN PLASMAS USING CO2 LASERS

E. Chu, R. Druce, M. Kristiansen, M. Hagler, R. Bengtson

To cite this version:

E. Chu, R. Druce, M. Kristiansen, M. Hagler, R. Bengtson. BEAT HEATING IN PLAS- MAS USING CO2 LASERS. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-747-C7-748.

�10.1051/jphyscol:19797361�. �jpa-00219357�

JOURNAL DE PHYSIQUE CoZZoque C7, s u p p l e ' m e n t a u n 0 7 , Tome 40, J u i Z Z e t 1979, page C7- 747

E. Chu, R. Druce, M. Kristiansen, M. Hagler and R. ~ e n ~ t s o n * .

c e p a r t m e n t E Z e c t r i c a Z E n g i n e e r i n g , T e x a s T e c h . U n i v e r s i t y , Lubbock, T e x a s , U. S . A. 79409.

D e p a r t m e n t o f P h y s i c s , U n i v e r s i t y o f T e x a s a t A u s t i n , A u s t i n , T e x a s U.S.A. 78712.

I n t r o d u c t i o n

There has been considerable i n t e r e s t i n beat h e a t i n g as a method of h e a t i n g underdense plasmas y2y3. The scheme uses two d i f f e r e n t - f r e q u e n c y ,

high-power l a s e r beams t o e x c i t e ; p a r a m e t r i c a l l ! f , plasma waves i n t h e underdense plasmas. The sub- sequent damping o f t h e waves i s expected t o heat t h e plasma. I n order t o have e f f i c i e n t c o u p l i n g o f l a s e r energy i n t o t h e plasma wave energy, t h e phase matching c o n d i t i o n must be s a t i s f i e d :

where wO,u1 a r e t h e l a s e r frequencies, w2 t h e plasma wave frequency, and

lo, El,

and

a2

a r e t h e corresponding wave vectors.

An experiment u s i n g t h e 9.56 um and 10.28 um r a d i a t i o n s o f a C02 l a s e r and a helium plasma i s reported. Evidence o f beat h e a t i n g has been ob- served. However, beat h e a t i n g i s found t o he small compared t o i n v e r s e bremsstrahlung heating. Poss- i b l e explanations of t h i s observation a r e given.

Experimental Arrangement

A c o l d cathode, electron-beam c o n t r o l l e d C02 l a s e r designed f o r double-sided o p e r a t i o n i s used f o r t h e beat h e a t i n g i n v e s t i g a t i o n s . The pulsed l a s e r i s capable of l a s i n g a t 10.6 um o r simultan- eously a t 9.56 pm and 10.28 um by t h e i n s e r t i o n o f an a b s o r p t i o n c e l l f i l l e d w i t h SF6 i n t h e o s c i l l a - t o r c a v i t y . When r u n a t 70.6 vm t h e l a s e r produces 50 ns FWHM pulses w i t h about 50 J per pulse. I n t h e double-frequency mode, t h e energy per pulse i s reduced by approximately 10%.

The plasma i s produced by a l i n e a r discharge between two s p l i t - r i n g e l e c t r o d e s w i t h an a x i a l magnetic f i e l d o f 2.8 T provided by a f i f t e e n t u r n c o i l around t h e plasma tube, as shown i n Fig. 1.

The r e s u l t i n g plasma column la approximatelv 0.1 m l o n g and 0.05 m i n diameter w i t h a d e n s i t y minimum ---

*

T h i s work was supported by AFOSR Grant 74-2639.

on a x i s . A t r a c e o f t h e plasma c u r r e n t i s shown i n Fig. 2. The c u r r e n t has a peak value o f 125 kA and l a s e r i s focused i n t o t h e plasma a t approximately 5 us a f t e r t h e i n i t i a t i o n o f t h e plasma c u r r e n t . P r i o r t o t h e f i r i n g o f t h e l a s e r , t h e helium plasma has a temperature o f about 4.5 eV and d e n s i t i e s o f 3.9 x

loz2

-6.5 x 10 22 m-3 as t h e f i l l i n g pressure i s v a r i e d from 1.0

-

1.9 T o r r .

The l a s e r i s focused by a 1 m f o c a l l e n g t h copper m i r r o r t o a spot s i z e of approximately 0.0025 m i n diameter i n t h e c e n t e r o f t h e plasma column. A 0.5 m f o c a l l e n g t h m i r r o r a t t h e e x i t end o f t h e plasma source refocuses t h e t r a n s m i t t e d l a s e r beam back t o t h e same focal spot. As a r e - s u l t , opposite wave vectors a r e present a t t h e f o - c a l spot t o a l l o w a n t i p a r a l l e l beam beat h e a t i n g i n v e s t i g a t i o n s .

A diamagnetic l o o p l o c a t e d a t t h e f o c a l spot i s used t o measure t h e temperature change due t o h e a t i n g by t h e l a s e r . The i n t e n s i t i e s o f helium s a t e l l i t e s a r e used t o measure t h e amplitude o f plasma waves i n t h e plasma as a r e s u l t o f o p t i c a l mixing.

Results

The temperature increase as measured by t h e diamagnetic l o o p under v a r i o u s c o n d i t i o n s i s sum- marized i n t h e f o l l o w i n g t a b l e :

F i l l i n g Temp increase Temp increase presslire w i t h s i n g l e w i t h double

frequency beam frequency beam

( T o r r ) (ev) ( e v )

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797361

The listed figures represent the average of two or more numbers obtained under the same set of con-

.

ditions. The indicated errors are the 50% confi- dence limits obtained by assuming the data follows the "Students" t-distribution

6

. The temperature increase with the single frequency beam which has more energy per pulse is consistently higher than that with the double frequency beam at all filling pressures except at 1.5 Torr. On applying the paired t-test to the data, this observed departure is signigicantly different from the rest of the measurements with a better than 99% confidence 1 eve1 .

As the filling pressure is varied, no signi- ficant satellite emissions associated with the

1 1 1 1

2

P

- 3 P transition and the 2 S - 3 D transition are observed, indicating that no large amplitude plasma waves are present.

Discussion

The density of plasma corresponding to 1.5 Torr filling pressure is 5.3 + ^{.45 x}

^{10 22}

^{m-3 prior}

to the laser pulse. As a density of 5.9 x m-3 is required for resonance with the incident 9.56 um and 10.28 um radiations, it is likely that the ob- served additional heating at 1.5 Torr when the laser is running in the double frequency mode is

a

result of beat heating. However, this observed additional heating is insignificant compared to inverse bremsstrahlung, which may appear to be in conflict with theoretical predictions. The appar- ent contradiction may be resolved by the following explanation. At the resonance zone, which has a physical volume of the order of 1 mm3, the elec- trons will start to stream out of the interaction volume as soon as they are heated as a result of beat heating. Therefore, the density at the reson- ance zone drops and detuning from resonance may result.

Conclusion

An experiment investigating beat heating has been conducted. Additional heating at 1.5 Torr filling pressure is observed when the laser beam has both 9.56 vm and 10.28 vm radiation components.

However, this additional heating is insignificant when compared to inverse bremsstrahlung heating.

Absence of any significant satellite emmissions in- dicates that no large amplitude plasma waves are present. This agrees with the small additional

References

1. Capjack, C.E., James, C.R., Can. J. Phys. 48, 1386 (1970).

2. Cohen, B.I., Kaufman, A.M., Watson, K.M., Phys.

Rev. Lett. 2, 581, (1972).

3. Rosenbluth, M.M., Liu, C.S., Phys. Rev. Lett.

29, 701, (1 972).

-

4. Jasper, J., Master Thesis, Dept. of EE, Texas Tech University, 1977.

5. Baranger, M., Mozer, B., Phys. Rev. 123, 25 (1961).

6. Spiegel, M.R., Theory and Problems of Statis- tics, McGraw-Hill Book Company, (1961

) .

Fig. 1.

Experimental Arrangement.

heating observed. Fig.

2.

Plasma

C u r r e n t Trace

(Vert. - 31 kA/div, Horz.- 2 vs/div).