A FULLY AUTOMATIC CONTINUOUS ELECTRON TEMPERATURE MEASUREMENT SYSTEM USING A DOUBLE LANGMUIR PROBE

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Submitted on 1 Jan 1979

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A FULLY AUTOMATIC CONTINUOUS ELECTRON TEMPERATURE MEASUREMENT SYSTEM USING

A DOUBLE LANGMUIR PROBE

T. Todd

To cite this version:

T. Todd. A FULLY AUTOMATIC CONTINUOUS ELECTRON TEMPERATURE MEASURE-

MENT SYSTEM USING A DOUBLE LANGMUIR PROBE. Journal de Physique Colloques, 1979,

40 (C7), pp.C7-841-C7-842. �10.1051/jphyscol:19797406�. �jpa-00219406�

JOURNAL DE PHYSIEUE CoZZoque C7, suppZ6ment au no?, Tome 40, JuiZZet 1979, page

C7-

841

A

FULLY AUTOMATIC CONTINUOUS ELECTRON TEMPERATURE MEASUREMENT SYSTEM USING A DOUBLE LANGMUIR PROBE

T.N. Todd.

CuZham Laboratory, Abingdon, Oxon. OX14 3DB, U . K., (EURATOM/liKAl?A Fusion Association)

A novel system f o r continuous e l e c t r o n tempera- t u r e measurement h a s been developed a s a plasma d i a g n o s t i c on t h e Culham Superconducting Levitron.

This system has been used t o measure plasma tempera- t u r e s (T ) of between 0 . 1 and 20 eV and d e n s i t i e s

(ne) i n R e range 5 x 10' ⁺ 5 x lol'/cc, with a r e s - ponse time of a p p r o x i ~ a t e l y 300

us.

The double probe c h a r a c t e r i s t i c , from which e l e c t r o n temperature and d e n s i t y a r e d e r i v e d , i s b e s t o b t a i n e d by sweeping t h e probe w i t h an AC v o l t a g e which o n l y j u s t produces c u r r e n t s a t u r a t i o n . This i s because t h e t r a n s f o r m e r coupling which e l e c t r i -

,

tally f l o a t s t h e probe causes an i n d e f i n i t e l y main- t a i n e d c u r r e n t s a t u r a t i o n ( i e . a DC c u r r e n t ) i n t h e probe windings t o appear a s e x p o n e n t i a l l y d e c r e a s i n g i n t h e current-sensing secondary winding. Thus t h e c u r r e n t s i g n a l i s d i s t o r t e d i f t h e probe i s swept much beyond t h e "knees" of t h e c h a r a c t e r i s t i c . T h i s problem a r i s e s with f i x e d v o l t a g e amplitude systems when t h e plasma temperature i s low, b u t i s overcome by t h i s system which continuously a d j u s t s t h e v o l t a g e amplitude s o a s t o j u s t reach s a t u r a t i o n . Since t h e o n s e t of s a t u r a t i o n o c c u r s a t a v o l t a g e p r o p o r t i o n a l t o t h e e l e c t r o n temperature, p r e c i s i o n r e c t i f i c a t i o n and smoothing of t h e d r i v i n g v o l t a g e t h e n p r o v i d e s a c o n t i n u o u s l y v a r y i n g DC v o l t a g e p r o p o r t i o n a l t o t h e e l e c t r o n temperature. When t h e c u r r e n t s i g n a l i s s i m i l a r l y converted, continuous d e n s i t y measurements a r e a l s o o b t a i n e d , using t h e e x p r e s s i o n n = c o n s t x I

/<.

The system pre-

SAT

s e n t e d h e r e achieves t h i s e f f e c t by sweeping t h e probe s i n u s o i d a l l y and maintaining a c o n s t a n t pro- p o r t i o n of t h i r d harmonic i n t h e r e t u r n e d c u r r e n t s i g n a l ( s e e Fig.1). This p r o p o r t i o n i s o b t a i n e d by using an automatic g a i n - c o n t r o l l e d a m p l i f i e r (AGC) t o normalise t h e c u r r e n t s i g n a l t o a c o n s t a n t ampli- tude and t h e n f i l t e r i n g o u t t h e t h i r d harmonic. The f i l t e r o l ~ t p u t i s i n t u r n h e l d c o n s t a n t by u s i n g it a s t h e c o n t r o l i n p u t of t h e sweep v o l t s AGf

.

harmonic

wave band - pass

oscillator

inputs

amplifier

preamp

'--I---- -I---

To double probe

Fig.1 Schematic of system loop by d r i v i n g t h e probe a c r o s s t h e plasma, s o t h a t c o r r e c t l y averaged comparisons can b e made w i t h t h e ' l i n e - o f - s i g h t d i a g n o s t i c s . I n t h i s way average temperatures d e r i v e d from t h e a b s o l u t e i n t e n s i t y of

0

t h e He1 5876 A l i n e and d e n s i t i e s from 4 mm micro- wave i n t e r f e r o m e t r y a r e found t o be i n c l o s e agree- ment with t h o s e d e r i v e d from t h e probe over a wide range of plasma parameters and d e s p i t e magnetic f i e l d s of a few k i l o g a u s s .

High r e s o l u t i o n e q u i l i b r i u m p r o f i l e s of ?em- The systsm u s u a l l y o p e r a t e s w i t h a sweep f r e - p e r a t u r e and d e n s i t y (eg. Fig.2) a r e e a s i l y o b t a i n e d quency of 10 k ~ z , which p e r m i t s an AC-amplitude

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797406

I 0.5 1.0 1.5 2.0 2 5 3.0 3.5 4.0

distance from ring (cm) Fig.2 Plasma p z o f i l e s i n helium a t v a r i o u s

powers of 16 GHz microwave h e a t i n g response time of approximately 200 u s , lengthened t o about 400

us

by r e c t i f i c a t i o n and f i l t e r i n g L . These response times can be improved by i n c r e a s i n g t h e sweep frequency, b u t with a s a c r i f i c e i n t h e dynamic r a n g e of t h e system. With 10kHz t h i s range i s about 10,000:l i n c u r r e n t s i g n a l and 350:l i n v o l t a g e ( i e . e l e c t r o n t e m p e r a t u r e ) , p r o v i d i n g a system capable of t r a c k i n g f a r i n t o plasma decays.

Fig.3 i s an example of a plasma decay t o which 50 Hz ohmic h e a t i n g ( c a u s i n g a 100 Hz temperature modulation) has been added where shown.

6

-.

t Ohmic Heating

A simple dynamic s i m u l a t o r of double probe c h a r a c t e r i s t i c s has a l s o been developed, which i m i t a t e s t h e non-linear plasma impedance seen by t h e probe. I t s i m u l a t e s plasmas of temperature 0.5 + 30 eV and d e h s i t y 2 x 101°

-

1 0 /cc, w i t h a 12 maximum modulation frequency o f a b o u t 30 kHz. This u n i t p r o v i d e s a v a l u a b l e a i d i n o p t i m i s i n g and checking t h e temperature measurement system.

Although i n i t i a l l y developed t o f o l l o w t h e time dependence of L e v i t r o n plasma decays, t h e con- t i n u o u s e l e c t r o n temperature measurement system h a s become a primary d i a g n o s t i c used e x t e n s i v e l y f o r o b t a i n i n g e q u i l i b r i u m p r o f i l e s of i n t e r e s t i n our s t u d i e s of plasma h e a t i n g , containment and f l u c t u a t i p n s .

The a u t h o r acknowledges c o n s i d e r a b l e c o n t r i - b u t i o n s made t o t h i s work by D E T F Ashby and W H W F l e t c h e r .

Fig.3 Microwave plasma decay w i t h 50 Hz ohmic h e a t i n g