HAL Id: jpa-00229378

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HAL Id: jpa-00229378

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PRELIMINARY ELECTRON CYCLOTRON EMISSION MEASUREMENTS FROM AN ECRIS

PLASMA

G. Melin, F. Bourg, P. Briand, R. Geller

To cite this version:

G. Melin, F. Bourg, P. Briand, R. Geller. PRELIMINARY ELECTRON CYCLOTRON EMISSION

MEASUREMENTS FROM AN ECRIS PLASMA. Journal de Physique Colloques, 1989, 50 (C1),

pp.C1-727-C1-737. �10.1051/jphyscol:1989177�. �jpa-00229378�

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JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n0l, Tome 50, janvier 1989

PRELIMINARY ELECTRON CYCLOTRON EMISSION MEASUREMENTS FROM AN ECRIS PLASMA

G. MELIN, F. BOURG, P. BRIAND and R. GELLER

CEA

-

CENG

-

DRF/PADSI, 8 5 X , F-38041 Grenoble Cedex, France

R6sum6 : On decrit les premieres mesures d'bmission cyclotronique Blectronique (ECE) d'un plasma de source ECRIS MINIMAFIOS 16.6 G H z . Pour l'instant le comportement general de 11intensit6 totale intdgree de la puissance ECE est present6 en fonction des paramstres principaux de la source, puissance HF, densit6 de neutres. Les projets ?I court terme de mesure de 1'6nergie des Blectronss partir du spectre de frequence ECE sont aussi abord6s.

Abstract : The preliminary investigations of the electron cyclotron emission(^^^) from an ECRIS plasma on a 16.6 G H z MINIMAFIOS source have been carried out. So far the general behavior of the total integrated ECE power against the main parameters of the source, RF power, neutral density, is presented. Future plans to measure the electron temperature or energy from the ECE frequency spectrum are also discussed.

1. Introduction

The electron cyclotron emission (ECE) from an ECRIS plasma, i. e. the synchrotron radiation from low energy and midly relativistic/relativistic electrons /1/ has just started being investigated on a 16.6 G H z MINIMAFIOS ion source /2/. The thermal ECE is widely used in laboratory plasmas to diagnose the electron temperature /3, 4, 5/.

In such thermal regimes, the plasma is optically thick for low order harmonics of the ECE

(A

= n 4 2 ) , which means that locally the ECE is in equilibrium with the

W C

plasma, i. e. the ECE is just the source function of the plasma, the black body emission at the electron temperature for maxwellian electron distributions. Non thermal emission has been more recently investigated in fusion plasmas, both in tokamaks /6/ and in tandem mirror devices / 7 / , the latter being closer to the ECRTS situation because of the magnetic configuration. Such a non thermal emission caused by high energy electrons is generally optically thin for high order harmonics (n b 3 ) which means that the absorption of the ECE by the plasma itself is negligible, and therefore that a theoretical spectrum calculated from the single particle ECE theory /1/ may be used to compare with an experimental spectrum. In ECRIS plasmas the electrons have likely energy components such that El/ << E J.

.

As the perpendicular energy EL is the origin of the ECE, the ECE spectrum is expected to provide valuable information about the electron energy E l

,

and possibly the density inside the ion source. This basically motivates the present preliminary work on ECE.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989177

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C1-728 JOURNAL DE PHYSIQUE

However the rather complicated magnetic structure of today ECRIS (hexapolar field) together with the compactness of such sources ( z 10 cm diameter, z 0.5 m long) make it difficult to carry out, such a comparison between theoretically calculated and experimental ECE spectra in these configurations.

It is likely that the ordinary (0) and extraordinary (x) plasma waves carrying the ECE signal within the plasma will undergo a severe polarization scrambling in so small a structure. But as El>> El/, the x mode will predominantly radiate and the ECE radiation escaping from a port of the ion source is likely to be representative of this mode, at least for the frequency spectrum. Thus it does not seem unreasonnable to see,what the radiation does look like, i. e. intensity, frequency spectrum.

2. The experimental set-up

We used the experimental set-up sketched in Fig. 1. The radiation is observed across the extraction volume (the diameter of the extraction electrode hole is 1 cm) along the main axis of the source, through a wedged crystal quartz window and a circular copper waveguide (5 cm diameter) and is detected by a liquid helium cooled InSb detector. The measured radiation lies in the 30 Ghz (-detector cut-off frequency) to several 100 GHz frequency range. So far this experimental set-up does not allow us to look at the frequency spectrum. We concentrated our analysis to the general behavior of the total power emitted (in arbitrary units as the system is not yet calibrated, neither absolutely norfnfrequency) against some parameters of the source. By using a Fabry-Perot filter we see that the radiation extends beyond 200 GHz. The MINIMAFIOS 16.6 GHz source is running in a pulsed mode operation. A typical ECE signal is shown in Fig. 2 in Argon gas.

3. Preliminary results and main behavior

Owing to the numerous parameters of the source (cw or pulsed mode, wall coating, magnetic field profile, neutral injected pressure, RF power etc

...

⁾ ^sometimes

difficult to control, the signal measured may be quiescent or quite noisy. When varying a parameter (neutral pressure or RF power) various different regimes of operation may be encoutered, which make it difficult to characterize. Thus we numerically average the signals after data acquisition. In the present situation, when looking across the extraction electrodes, the signal is, to some extent, influenced by the extraction voltage. Backward electrons (secondary from accelerated ions impinging on the electrodes) likely penetrate into the source plasma and might cause the excitation of beam-plasma instabilities near w = w (plasma).

pe

At high RF power (>, 500 W) this is not very important as shown by the signal traces in Fig. 2a and 2b. At low RF power this phenomenon becomes more important, see Fig.

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3a and 3b, a s t h e i n s t a b i l i t y r e p e t i t i v e l y b u r s t s . We e s s e n t i a l l y g o t d a t a from argon d i s c h a r g e s , we scan over t h e n e u t r a l i n j e c t e d p r e s s u r e and t h e i n p u t RF power.

F i g . 4 shows t h e dipendence of t h e ECE s i g n a l with t h e RF power. No s a t u r a t i o n i s observed. A s t h e s i g n a l depends upon t h e e l e c t r o n d e n s i t y and t h e e l e c t r o n p e r p e n d i c u l a r energy, t h i s would i n d i c a t e t h a t s t i l l a t l e a s t one of t h e s e parameters keeps i n c r e a s i n g w i t h t h e FF power.

F i g . 5a and 5b g i v e , a t d i f f e r e n t RF powers, t h e dependences of t h e ECE s i g n a l and correspondingly t h e A r l l + e x t r a c t e d c u r r e n t with t h e n e u t r a l p r e s s u r e . A s t h e ECE s i g n a l remains f l a t a g a i n s t t h e p r e s s u r e ( e x c e p t a t high p r e s s u r e ) We may c o n s i d e r t h e r e is a compensation between two e f f e c t s . ( i ) A t low p r e s s u r e , t h e e l e c t r o n d e n s i t y i s low and t h e ~ r l l ' c u r r e n t d r o p s d e s p i t e a high e l e c t r o n energy, ( i i ) a t high p r e s s u r e t h e d e n s i t y i n c r e a s e s b u t t h e energy d e c r e a s e s , c a u s i n g a g a i n t h e ~ r l l + t o drop and f i n a l l y a l s o t h e decrease o f t h e ECE s i g n a l . This i n t e r p r e t a t i o n i s confirmed i n a n o t h e r s e r i e s o f d a t a , which was more q u i e s c e n t , where both A r l l ' and Ar9+ c u r r e n t s were recorded. A s t h e ECE s i g n a l remains f l a t a g a i n s t t h e p r e s s u r e ( F i g . 6 a ) t h e maximum i n t h e A r l l + c u r r e n t ( F i g . 6 b ) t a k e s p l a c e a t a lower p r e s s u r e t h a n t h a t o f t h e Ar9+ c u r r e n t ( F i g . 6 c ) .

A few d a t a were recorded i n Tantalum d i s c h a r g e s . F i g 7a shows a t y p i c a l time e v o l u t i o n o f t h e ECE s i g n a l . F i g 7b g i v e s a few d a t a about t h e dependence of t h e ECE s i g n a l with t h e RF power. A s t h e minimum R F power r e q u i r e d t o v a p o r i z e t h e Ta f i l a m e n t t h a t f u e l s t h e d i s c h a r g e i s r a t h e r high (- 1000 W ) t h e d a t a a r e scanned over a l i m i t e d range.

4. F u t u r e p l a n s

I n o r d e r t o g e t t h e ECE frequency Spectrum, we p l a n t o b u i l d a Martin-Puplett t y p e Michelson i n t e r f e r o m e t e r u s i n g r o o f t o p m i r r o r s /4/, /8/ ; t h i s k i n d o f i n t e r f e r o m e t e r being used on p r e s e n t l a r g e f u s i o n d e v i c e s . Therefore our f u t u r e p l a n s i n c l u d e t h e a b s o l u t e c a l i b r a t i o n and t h e frequency c a l i b r a t i o n of t h e whole system between t h e s o u r c e and t h e d e t e c t o r . We then hope t o t a c k l e t h e measurement of t h e p e r p e n d i c u l a r e l e c t r o n energy, s o f a r a s p o s s i b l e , both by comparing experimental s p e c t r a w i t h t h e o r e t i c a l ones, and from t h e p o s i t i o n i n frequency of t h e h i g h o r d e r ECE harmonics, if d i s c r e t e , according t o t h e r e f e r e n c e / 7 / .

Aknowledgments

The a u t h o r s would l i k e t o knowledge D r . L. Laurent and h i s team on ECE d i a g n o s t i c s f o r TORE SUPRA, D r . M. Gregoire and D r . J . Tachon o f t h e DRF'C a t Cadarache f o r t h e i r h e l p i n l o a n i n g s p a r t s f o r t h i s experiment a s w e l l a s t h e i r encouragements.

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References

-

L G. Bekefi

-

Radiation Processes in Plasmas, J. Wiley and sons, New York (1966).

/%/ R. Geller et al, Proceedings of the Inter. Conf. on ECR ion sources and their applications, NSCL Report MSUCP-47 East Lansing (1987) pl.

/3/ A. E. Costley, R. J. Hastie, 3 . W. Paul and J. Chamberlain, Phys.

Rev. Lett.

33,

(1974) 758.

/4/ A. E. Costley, E. A. M. Baker et al. Proceedings of the 4th Int. Workshop on ECE and ECRH (1984) pl.

/5/ L. Laurent

-

Thesis

-

Universitg de Paris Sud

-

Orsay, June 1983.

/6/ A. Girard

-

^Thesis

-

USMG, Grenoble, Oct. 1986.

/7/ C. M. Celata, Nuclear Fusion

25

⁽¹⁹⁸⁵⁾^35.

/8/ D. H. Martin. Infrared and millimeter waves, edited by K. J. Button.

Volume 6 , Academic Press, New York (1982) p 66.

ECE

From MINlMAFlOS

Fig. 1 Experimental set-up

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f

OTBL INTEGRATED ECE SIGNAL , Prf = i s 4kW, WITH WR.VOLTBGE EGE

&

RF eiynals

i 1, I I I I I 1 I

TOTAL INTEGRfiTED ECE

$1 GNAL,

Pi.! = I . 4kW, WITttOlPT EXTR. UOLTAGE ECE

&

RF

sigi1als

i.BBE+Bi r

^I ^I

^,

^I ^I

ⁱ

I j

( b ) i

^I

1 1

_i

I

', ⁱ

I

!

I

I I

/ i

I

P - _ _ U _ n (

-7 J

Fig. 2 ECE signals (a.u.1 in Argon gas at Prf = 1.4 kW, w i t h RF signals (a) with extraction voltage, ( b ) without extraction voltage.

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SIGNALS WITH EXTRACTIOW UCLTAGE , P.2.E-4

t o m

, ?rf=8.25 ki4 ECEf as!, ArE+128~AIill

I I , I %

/

^I

(a)

^!

i

i i

j

i

I I

j

SIGNALS WITHOLR M2ACT:TIOW tiUI,TAGE,P=i. E-4toi.r, Prt'=~.Z5kbi ECE signal

( a . u . 1

I.BBE+Sl/

^r'

1

^I

i/

^' ^!

1

! I Ill

1

!

!!]Ii ⁴¹ ( b )

^J^!

1 j 4 :

ⁱ

I

~j

I

. BBE+BB

I I J I

1 . BBEtB B

TinE

ims! I.OBE+Eii

F i g . 3 ECX s i g n a l s ( a - u . ) i n Argon g a s a t P r f = 0.25 kW with ~ r 8 + s i g n a l

,

( a ) with e x t r a c t i o n v o l t a g e , ( b ) w i t h o u t e x t r a c t i o n v o l t a g e .

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RF BOWER (kW)

Fig. 4 Total integrated ECE signal (a.u.,lin scale) in k g o n gas as a function of the P3 power at 16.6 GHz.

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TOTAL INTEGRATED ECE SlGNlL a t different RF powers ECE signal i a . u . 1

1.88EtBB

^I,i ⁱ

1.5 kW

I i ^%\.~

(a) 1

I I

I

i

I .O kW I

I;,)oE-O~

h

0.5 kW

i

1. BEE-02

1. RBE-85 PRESSURE ( t o w 1.00E-63

.0BE+00 I

^I ^I

1.00E-85 PRESSURE (t0r.r

1

I . 0BE-83

Fig. 5 ( a ) Total i n t e g r a t e d ECE s i g n a l ( a - u . , l o g s c a l e ) i n Argon gas f o r 3 d i f f e r e n t RF powers a s a function of t h e i n j e c t e d pressure i n t o the source ( t o r r , l o g s c a l e )

( b ) Corresponding average A r l l C c u r r e n t s ( p A , l i n . s c a l e )

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TOTAL INTEGIIATED ICE SIGNAL at different 1F pouers

Fig. 6 (a) Same as Fig 4 for a more quiescent regime

ECE signal

(a.u.

( b ) Corresponding average ~rll' currents ( A, lin. scald) at 1.5 ,and 1.0 kW RF powers

1.BBEtRB

^II

I (a)

I i ¹

I 1 5 k W

^I ^.

--

1.0 k W

I

.1,r~sF-o2

i

0.5kW

I

1. BBE-B3 !

- _I

i

I

i I

I . 04E-85 PRESSURE (torrf 1.80E-83

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JOURNAL

DE

PHYSIQUE

AVERAGE Ar9 CURRENT at Prf=1.5 kW I Ar9 (14)

Fig. 6 ( c ) Corresponding average ~ r 9 + current ( TJ A , lin. scale) at 0.5 kW.

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TOTAL

IW'EGBfiTEB ECE SIGtlAL fa. u. 1, I Ta24+

11S~A/li.~is)

ECE % I Ta24+ signal

I I I I I

F i g . 7 ( a ) ECE s i g n a l (a.u.) i n tantalum d i s c h a r g e a t Prf = 1.0 kW with e x t r a c t e d Ta 24+ c u r r e n t as a f u n c t i o n of time.

( b ) T o t a l i n t e g r a t e d ECE s i g n a l i n tantalum d i s c h a r g e a s a f u n c t i o n of t h e R F power a t 16.6 GHz.

HAL Id: jpa-00229378