THOMSON SCATTERING WITH A HIGH BACKGROUND LEVEL OF PLASMA RADIATION

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THOMSON SCATTERING WITH A HIGH

BACKGROUND LEVEL OF PLASMA RADIATION

B. van der Sijde, T. Poorter, S. Adema, B. Pots, D. Schram

To cite this version:

B. van der Sijde, T. Poorter, S. Adema, B. Pots, D. Schram. THOMSON SCATTERING WITH A

HIGH BACKGROUND LEVEL OF PLASMA RADIATION. Journal de Physique Colloques, 1979,

40 (C7), pp.C7-851-C7-852. �10.1051/jphyscol:19797411�. �jpa-00219412�

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JOURNAL

DE

PHYSIQUE CoZZoque C7, suppZ6mnt au n07, Tome 40, JuiZZet 1979, page C7- 851

THOMSON SCATTERING WITH

A HIGH

BACKGROUND L E V U OF PLASMA RADIATlON

8.Van der Sijde, T. Poorter, S. Adema, B.F.M. Pots and D.C. Schram.

Physics Department

,

Eindhoven University o f TeehnoZogy, E<ndhouen, m e NetherZands.

Introduction: Thomson scattering is nowadays a standard technique in plasma physics. Most of the experiments are carried out by using short pulse- high power lasers (e.g. a 10 ns-100 MM Q switched ruby laser). In that case the Thomson signal is mostly relatively small and there are normally minor problems with the background of plasma radiation owing to the very short pulse duration. The aim of this paper is to describe a device with a long pulse-high energy ruby laser (1.5 ms, 50 J) appro- priate for measurements at small and medium electron densities. Correction for the plasma radiation (continuum and weak line radiation) and improvement of the signal to background ratio by adequate rejection of the stray light of strong lines is necessary in that case. We obtained a high

a wavelength shift of 5 nm

-

in combination with a four periodic 5 nm bandwidth filter

-

transmission at the top of 75% and of 2. at a shift of 5 nm

-

to diminish the stray light still furthermore. The T.S.D. exists of a ruby laser with an incident beam focussed to a waist of 2 mm. The laser light, scattered in a volume of

1 x 2 ~ 2 mm3 under go0, is focussed on the entrance slit of polychrometer with an entrance angle of 1.3 sr. Homemade perspex light guides with 60% transmission transmit the signals to photo- multipliers with a high quantum efficiency (10%).

Photon counting is carried out after amplifica- tion, and discrimination with a PDP 11/20 com- puter system. A large entrance angle, a relatively high measured transmission factor (1.6

rejection by using a grating in combination with an a large quantum efficiency and the application of optical filter. The device built up according to filters is essential to get good results.

this principle is a simply handling small volume

Improvement bf the signal to background (S/b) system at a relatively low cost. The applicability

ratio. We determined the rejection factor of the of the device is tested with a hollow cathode arc

-

grating by applying Rayleigh scattering with a (H.C.A.) as a plasma source; the electron density

20 torr argon gas sample. The rejection factor is and temperature ranges of the argon plasma used

defined here as the ratio of the counted photons were 5 . 1 0 ~ ~ - 8 . 1 0 ~ ~ and 2.5-4.5 eV,

in a Thomson channel (with a certain wavelength The Thomson scattering device (T.S.D.). Our aim is

to develop a T.S.D. for a wide range of electron densities n = 1018-1021 m-3 at low electron tem- peratures of 2.5-5 eV in the presence of a large amount of annoying plasma radiation. Laser stray light is adequately diminished by a laser- and a viewing dump. The specific problem in the case of a spectrum with a multitude of lines is to find one or more appropriate "valleys" between strong lines to place the exit channels and to clean these valleys as good as possible from the stray

<

light of plasmaradiation. Therefore, we built a six channels polychromator with a special purpose concave holographic grating

-

transmission 60%,

180b lines/m, dispersion 1.1 nm/mm, rejection factor 1.3 for a 1 nm bandwidth channel at

shift

Ah

with respect to the laser wavelength) and those in the Rayleigh channel and is as such de- pendent on the shift

AX.

These results shown in Fig.1 are valid for normalized slits equivalent to a bandwidth of 1 nm. We also indicate in the same figure the rejection factor for the combination of grating and filter for the first channel. The factor is better for the other channels. The combination improved to reject almost all stray light of strong neighbouring lines and the relative improvement in the background signals of the five channels is i factor 1.5-3.5. Additional 1 m bandfilters appear to give no further improvement.

It means that the remaining background exists of continuum and weak line radiation from the channel's wavelength itself. Application

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797411

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of a p o l a r o i d f i l t e r g i v e s a f u r t h e r improvement s t r o n g l i n e s can be found. A wide e n t r a n c e a n g l e , a of t h e S/b r a t i o w i t h a f a c t o r 1.2. This e f f e c t l a r g e quantum e f f i c i e n c y , a good t r a n s m i s s i o n , d i s p e r -

i s r a t h e r small caused by t h e f a c t t h a t t h e s i o n and r e j e c t i o n of s t r a y l i g h t a r e c r u c i a l condi- g r a t i n g has a

referential

t r a n s m i s s i o n i n t h e t i o n s .

d i r e c t i o n of t h e p o l a r i z a t i o n of t h e l a s e r l i g h t . Reference

Measurement of t h e S / b r a t i o . The S/b r a t i o h a s [ I ] B. van d e r S i j d e , B.F.M. P o t s , D.C. Schram, Proc.

been i n v e s t i g a t e d f o r more t h a n two decades of 14th ICPIG, Grenoble 1979.

n -values a t T = 3.2-4.0 eV by a p p l i c a t i o n of t h e above mentioned equipment. I n Fig.2 t h e s e r a t i o ' s a r e i n d i c a t e d . They appear t o be l e s s than

1 f o r a l l c o n d i t i o n s and channels. Values of t h e S/b r a t i o between 0.1 and 1 a r e a p p r o p r i a t e a s t o be used a s r e l i a b l e d a t a f o r n and T determina-

t i o n . The accuracy of t h e ne-values a f t e r 10 s h o t s v a r i e s from 20% a t 5 . 1 0 ~ ~ m-3 t o 6% a t 2 . 1 0 ~ ' m-3

and 20% a t 8 . 1 0 ~ ~ mW3. The c a l i b r a t i o n c a u s e s an _,10-5 a d d i t i o n a l e r r o r of 3%. The accuracy of Te i s 8-30%

a f t e r 10 s h o t s . I n Fig.3 we show t h e background r a d i a t i o n f o r t h e h i g h ne-values. I n channels 1

and 2 a 692.5 nm argon I l i n e and i n channels 4 and ,

P

5 a 690.1 urn argon I1 l i n e i s p r e s e n t . The i n f l u - ence of t h e f i r s t i s e v i d e n t a t low n -values and t h a t of t h e l a t t e r a t high n -values. For high ne- v a l u e s > 2. 1020 mv3 t h e i n c r e a s e of r a d i a t i o n i s p r o p o r t i o n a l t o n 2 . Continuum r a d i a t i o n i s pro-

I

p o r t i o n a l t o n .C z j 2 n i 5 / ~ e ' , s o t h a t the measured i n t e n s i t y seems t o be p r o p o r t i o n a l t o n

n i l o r ne ni2 ( s i n g l y and doubly i o n i z e d p a r t i c l e s ) . I n

[ I ] we p o i n t o u t t h a t t h e ni2 d e n s i t y i s n o t c l e a r a t t h i s moment. The l i n e f o r continuum r a d i a t i o n from s i n g l y i o n i z e d p a r t i c l e s i s i n d i c a t e d i n t h e Fig. and shows a f a c t o r 3 d i s c r e p a n c y w i t h t h e measured curve f o r t h e channels 1 and 2. The l i n e f o r continuum r a d i a t i o n from doubly i o n i z e d par- t i c l e s has been s h i f t e d a f a c t o r 2 i f Biberman and Gaunt f a c t o r s a r e t h e same. I f we compare t h e

- 1

-F - ^\ -%

I-<

^{' s / b}

\

- t

"

===Q

0.01 lo1'- ne(m-3) 1020 1021

I I I I

Fig.

2.

~ignat/back~round patio's as a function

o f ne.

Fig.

3.

Background countings as a function o f

Me.

q u a l i t y of t h i s system w i t h a 50J

-

1.5 ms l a s e r p u l s e w i t h a 1J

-

10 ns p u l s e system, i t appears t h a t on a s i n g l e s h o t b a s i s t h e accuracy of t h e Thomson counts i n t h e channels ar'e roughly a s f o l l o w s :

n = 5 . 1018 m-3: 50% 50 J s . ; 150% 1 J s.

ne = 5. 1019 m-3: 25% 50 J s.; 75% 1 J s . n = 5 . 1 0 ~ ~ m-3: 25% 50 J s . ; 25% 1 J s . Conclusions. We conclude t h a t f o r a s t e a d y s t a t e plasma w i t h n 5. m-3 t h e h i g h energy-long p u l s e l a s e r system i s more a p p r o p r i a t e t h a n a low energy s h o r t p u l s e l a s e r i n s p i t e of t h e complica- t i o n of background r a d i a t i o n i f a v a l l e y between

?-

4

⁵⁴

-

3 2

(m-3) , lo3 _{2 0} 1

-

^e