ECR AS A DIAGNOSTIC FOR MIRROR TRAP PLASMA

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Submitted on 1 Jan 1979

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ECR AS A DIAGNOSTIC FOR MIRROR TRAP PLASMA

A. Skovoroda, V. Ziltsov

To cite this version:

A. Skovoroda, V. Ziltsov. ECR AS A DIAGNOSTIC FOR MIRROR TRAP PLASMA. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-665-C7-666. �10.1051/jphyscol:19797323�. �jpa-00219315�

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JOUI3IAL DE PHYSIQUE CoZZoque C7, suppzgment au n07, Tome 40, JuiZZet 1979, page C7- 665

ECR AS A DIAGNOSTIC FOR MIRROR TRAP PLASMA

A.A. Skovoroda and V.A. Ziltsov.

Kurchatov I n s t i t u t e of Atomic Energy, Moscow, U.S.S. R.

When t h e electromagnetic (E.M.) wave pro- pagates along the Z a x i s of a mirror t r a p and the condition a c e maxm'Wce min i s s a t i s f i e d , where b

-

the o s c i l l a t i o n frequency, O ce max,min

-

m a x i m a l and m i - nimal e l e c t r o n cyclotron frequencies,the electron cyclotron resonance (ECR) takes place i n fixed points Zr,where W =qe(Zr).

It i s known t h a t the transmission coeffi- c i e n t of right-handed polarized E.M. wave (the e l e c t r i c vector of the f i e l d r o t a t e s i n the same direction as t h e electron i n t h e magnetic fie1d)depende on t h e electron dencity n(Z) and t h e magnetic f i e l d gradient a t %he point Z r /*I/.

It i s c l e a r t h a t the measurement of the transmission wave power oan be used f o r t h e determination of t h e Z l o c a l electron density.The l o c a l i t y of the measurement i n the transverse direction(norma1 t o the

magnetic f i e l d d i r e c t i o n ) i s done by emp- loying of directional antennae.

Measurements of electron density by the method above were carried out on the mir- r o r t r a p with mirrimrua B "Ogra-3B",The p l a s ma with density l e s s than 2r1010cm-3 w a s produced by the i n j e c t i o n of 20 keV neut- r a l hydrogen beam with the current ^0,5^A during 1 sec.The steady magnetic f i e l d i n the t r a p centre w a s l e s s then 17 kG,the longitudinal mirror r a t i o was 2.The charge exchange life-time was 0,3

-

¹^sec.

!Cwo rectangular horn antennae f o r 8 mm

&.Me waves were laced along Z axis (100 cm between horn85 .1n t h i s measmement che transverse l o c a l i t y wats l e s s Lnan 3 cm.The longitudinal l o c a l i t y i s equal t o the d i - mension of the resonance zone ~@e/zr)"/'

(G ?,here uce(Z)

=03ce

^nnin(l+Z

pe -

electron larmor radius.We used a plane

-

polarized wave ,which f s t h e superposition of o s c i l l a t i o n s with c i r o u l a r polarization The left-handed polarized E.M. wave have no absorption a t ECR.It i s necessary take i n t o account t h i s f a c t i n the experiment.

A t the wave propagation through a magnetic t r a p t h e r e are two resonance points.The presence of several resonance points can lead t o t h e t'translucencet'/3/ .We note t h a t t h e experiments have revealed no "translu cencett i n our conditions,when L X ^L

-

the distace betweep the

_h _-

the wave 1ength)and t h e small fluc-

resonant ^points,

tuations makes the wave phase stochastical.

Therefore the electron density was calcu- l a t e d from r e l a t i o n

~ g e % t

=

esp (-2%),

P

⁼

&-c--

⁹

where t

-

the transmission power coeffioi- e n t of right c i r c u l a r l y polarized wave,

CL\

-

the electron plasma frequency,c

-

liBt

^vilocitY, $=(ds/&)-lBz=z*

Factor two i n the exponent i s - t h e r e s u l t of the presence of two resonance points with the same + I n our experiment we have measured power P3W-W ,where W

-

^the

wave power,

At.

the t r d 6 i s s i o n .wave power.

I n t h e experimental conditions ^C1 and theref ore P n .me continuous me asurem- ent of P i s the continuous measurement of n(T),here T

-

^time.

Fig.1 shows the example of electron den- s i t y measurement by t h i s method in the t r a p ~ e n t r e . 1 ~ and I, are the i n j e c t i o n current and the flux-of cold ions along

z

axis c0naiguently.1~ is proportional t o plasma ^andbackground gas d e n s i t i e s .One can conclude from fig.1 t h a t the sharp chanae of the f l u x I- a f t e r the inSection switzhing i s due t o fhe improvemen6 of vacuum conditions.Thus t h e measurement of P allows 'to determine not only the density,time ^andcharacter of plasma build-up, but the change 09 the pressure of the background gas during %he injection.

By changing of the s p a t i a l a t t i t u d e of resonance point Z r we obtain the d i s t r i b u t i - on n(z),which can be used f o r the determination of the ion v i l o c i t y d i s t r i b u t i o n function.The d i s t r i b u t i o n n(e-) was measu- red i n the experiments conl;inuously by means of t h r e e generators with the ^fixed frequencies,i.e.of the three points Z r .

!the o s c i l l a t i o n amplitude of these genera- t o r s was modulated a t three various frequencies (several kEiz).We used one waveguide f o r the probing microwave signals.These measuremenfa have shown t h a t the distribu- t i o n n(z) does not depend noticeably on presence of the ion cyclotron i n s t a b i l i t y , i.e.ions d i f f u s e i n the t r a v e r s e d i r e c t i - on.In addition the measurement of n(z) makes it possible t o draw a conclusion

about the electron diffusion i n % h e mirror t r a p a t a instabiiity.Fig.2 shows the change or electron density n on the plasma peripherg.We note t h a t th3 density mag-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797323

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nitude on f i g 1 i s 10 times a s large a s on fig.2.Fig.2 shows a l s o t h e amplitude of ion cyclotron o s c i l l a t i o n s A h f . I t i e seen t h a t the e l e c t r o n s appear

on

^{the pe-}

riphery when t h e h f o s c i l l a t i o n s are aD-

sent , A t t h e same time t h e plasma p o t e n t i a l decreases up %o the c l a s s i c a l 1evel.The

density o s c i l l a t i o n s l e a d t o the modula- t i o n of P .Fig.J shows t h e spectrum of lowfrequency o s c i l l a t i o n s of e l e c t r o n den- s i t y n i n t g e t r a p center,which was measured from P .The time r e s o l u t i o n of spectrum measurement was 10 msec,It i s seen t h a t i n t h e beginning of the i n j e c t i o n t h e o s c i l l a t i o n frequency increases with increase of the e l e c t r o n density.The experiments have shown t h a t such f l u c t u a t i o n s

corresponde t o the e l e c t r o n branch of t h e lowfrequency o s c i l l a t i o n s ( s e e report of

the same group). The simultaneous me asu- rement of P and

P

can be used f o r t h e de- tsrminaAion of the density modulation

P/P

-

^N/N.The simultaneous me asuremen-t:

i n the t h r e e p o i n t s Z r can be used f o r t h e determination of t h e phase v i l o c i t y along

the Z axis.

%hen the frequency of E.U. wave is l e a s then t h e minimum e l e c t r o n cyclotron frequency t h e dependence P( A W ) ( t h e cyclotron absorption l i n e )can be measure4/4/ .Here

A W = W,, - - ,in- 0 . .In the experimental conffitions t h e dopler broadening of t h e l i n e took place.This can be used f o r t h e determination of e l e c t r o n temperature Te and t h e e l e c t r o n d i s t r i b u t i o n function/2/.

Fig.4 shows t h e continuous measurement of Te by means of the t h r e e generators i n our experiment 8,

Thus t h e measurement of t h e transmission of E.M, wave a t EGR allows t o determine the many parameters of t h e mirror t r a p plasma.The method s e n s i t i v i t y i s enough f o r t h e measurement of t h e small e l e c t r o n density (10?c,-3 i n our experiments).

. . ~.-

where t h e using of t h e microwave i n t e r f e r o meters i s more complicated.Moreover, t h e s e

measurements a r e l o c a l and more simple, ECR allows t o determinethe l o c a l p r o p e r t i - e s of lowfrequency o s c i l l a t i o n s i n plasma.

The e l e c t r o n temperature i s measured i n the broad ran@;e(more then 10 eV i n our experiments).The e l e c t r o n d i s t r i b u t i o n function i s r e s t o r e d up t o t h e energy 4

-

⁵^Ye.

The method, described above ,can be used f o r the diagnostic of plasma under the aasnmp- t i o n of geometrical o p t i c s and. when

>>w2 .We note t h a t *he absorption

ce oe

e t a > ) l is- l a r g e and the measurements e r e d i f f i c u l t i n such condition.But always

the absorption can be decreased by changing of t h e s p a t i a l a t t i t u d e of the resonance point Z r o r t h e magnitude of A W .

/I/ Bndden K,G."Radio Waves i n %he Ionos- phere" ,Cambr ,Uaiv.Press,l %l /2/ Z i l t s o v V.A. a t a l l . V L I Int,Conf.

on P1.Phys.and Contr.Nucl.Fus.Res., Insbmck, 1978 .IAsA43?-37 .S-2.

/3/ Skovoroda A.A.,Shvilkin B.R.

Zh,Exp .Theor.LTiz,75, 898(1978) /4/ Skovoroda A.A. ,Shvilkin 8.N.

Zh.Elrp.Theor.Fiz.73,526(1977-)

ECR AS A DIAGNOSTIC FOR MIRROR TRAP PLASMA

HAL Id: jpa-00219315

https://hal.archives-ouvertes.fr/jpa-00219315

ECR AS A DIAGNOSTIC FOR MIRROR TRAP PLASMA

A. Skovoroda, V. Ziltsov

To cite this version:

-

-

-

=03ce

pe -

-

-

h -

resonant points,

=

P

&-c--

-

-

-

liBt

-

At.

-

z

on

P

-

-

_h _-

resonant ^points,