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Thomson scattering system of the ERASMUS tokamak
E. Desoppere, G. van Oost
To cite this version:
E. Desoppere, G. van Oost. Thomson scattering system of the ERASMUS tokamak. Re- vue de Physique Appliquée, Société française de physique / EDP, 1983, 18 (12), pp.803-808.
�10.1051/rphysap:019830018012080300�. �jpa-00245150�
Thomson scattering system of the ERASMUS tokamak
E. Desoppere (*) and G. Van Oost
Laboratoire de Physique des Plasmas, Association
«Euratom-Etat belge »,
Ecole Royale Militaire, 1040 Brussels, Belgium
(Reçu le 2 novembre 1982, révisé le Il août 1983, accepté le 29 août 1983)
Résumé.
-Un système permettant la mesure de la diffusion Thomson, adapté
auxcaractéristiques du tokamak ERASMUS,
aété réalisé. Ce système compact, complètement motorisé et facile à entretenir, est d’une grande souplesse. Le spectre de lumière diffusée est analysé à 5 longueurs d’ondes comprises entre les raies rubis et H03B1.
Tout le système optique est placé dans une seule enceinte qui est posée sur coussins d’air. Ceci permet un alignement
et une calibration aisés.
Les températures électroniques mesurées couramment sont comprises entre 20 et 350 eV pour des densités
électroniques supérieures à 8
x1011 cm-3. Les profils radiaux sont relevés
sur30 cm pour
unplasma de 39
cmde diamètre avec une résolution de 3
cm.Abstract.
-A Thomson scattering device has been designed and built, especially adapted to the characteristics of the ERASMUS tokamak. The system is characterized by its compactness, flexibility in operation, extensive
motorization and ease of maintenance.
The spectrum of the scattered light is analysed at 5 wavelengths between the ruby and the H03B1-line. All optical parts are housed in
asingle compact frame, that is mounted
on anairtrack, facilitating alignment and calibration.
Measurements of Te-values from 20 up to 350 eV with ne ~ 8
x1011 cm-3
areroutinely performed and radial profiles are taken over 30
cmof the 39
cmplasma diameter with
aresolution of 3
cm.Classification Physics Abstracts
52.70
1. Introduction.
A detailed interpretation of the plasma behaviour
in tokamaks depends critically on an accurate know- ledge of plasma temperature and density. Thomson scattering surpasses other measurement techniques [1]
by providing unambiguous local measurements of
Te and ne with excellent temporal resolution and without perturbing the plasma. Although scattering
of laser light has become a standard diagnostic technique, turning the well-known principles into an operational diagnostic adapted to a particular machine, is not straightforward. The design of a
Thomson scattering apparatus for a university type tokamak requires a considerable effort : the system should measure low ne and Te values over the plasma radius, be characterized by compactness, flexibility
in operation and ease of maintenance, at a reasonable
cost compared to the cost of the tokamak, and all
this without sacrificing accuracy.
Experimental results, obtained on the ERASMUS tokamak with such a diagnostic can be found else- where [2, 3]. In this tokamak [4], characterized by a
low aspect ratio (2.4), the basic properties of magneto-
sonic resonances and their damping were studied with
a view to heating toroidal devices in the ion cyclotron
resonance domain. The main parameters of the ERASMUS tokamak are : toroidal magnetic field
on axis BT
=3.6 kG. Plasma current Ip ~ 25 kA.
Minor radius (chamber) a
=0.25 m ; limiter
=0.195 m.
Major radius Ro
=0.5 m. Flat top ~ 10 ms. Typical plasma parameters (time-averaged on axis) are
Teo ~
=130 eV, ~ neo~ = 8
X1012 cm-3.
The paper is organized as follows. In section 2,
a description is given of the scattering system design
and the experimental apparatus. In section 3 the
alignment and calibration systems are presented.
Section 4 contains a discussion of the data acquisition
and reduction, and the uncertainties. Finally, in
section 5, the performances of the system are outlined.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019830018012080300
804
2. Experimental apparatus.
2.1 INPUT SYSTEM. - The model 2000 ruby laser
manufactured by JK lasers consists of an oscillator-
amplifier configuration and is capable of producing
10 J, 29 ns (FWHM) pulses, in a beam with divergence
less than 1 mrad. The laser energy is monitored by a photodiode, calibrated against a calorimetric energy
meter. The high power path is schematically shown
in figure 1. The laser beam is focused into the plasma by a f = 120 cm telescope ; after entering and leaving
the vacuum vessel through Brewster angle quartz windows, placed far (75 cm) from the plasma centre,
the beam is dumped on a black glass plate. The beam
diameter is less than 4 mm over a 40 cm distance centred on the focus : radial scanning is performed
without refocussing the laser.
A light baffling system has been mounted at the
input side, consisting of a series of black apertures
ground to a knife edge and protruding as far as
Fig. 1.
-Schematic of the Thomson scattering apparatus,
not drawn to scale. a) Laser system. R1 : oscillator rod ; RM, FM :
rearand front mirror of oscillator cavity ; 45°M :
450 mirrors ; CT : telescope ; R2 : amplifier rod. b) Input and output systems. T : telescope; P : steering prism; BW :
Brewster windows; stray light elimination system not shown ; D : glass beam dump. c) Collection and detection system. C : 2 plano-convex lenses ; M : mirror followed by
asystem of relay lenses, filter-mirrors (IF) and photomulti- pliers.
possible into the torus ; the output side is a straight
tube.
A viewing dump provides a black background for
the detection system.
An attempt was made to mate the effectiveness of knives with the simplicity of a flat matt-black surface
or black glass plate. The design [5] of figure 2 consists
of a series of holes drilled in an aluminium plate,
fixed inside the vacuum vessel, facing the detection system.
The surface is a thick and coarse matt-black anodized layer. On the test bench, at 450 incidence,
a reduction in stray light level of a factor 100 was
measured compared to smoked MgO.
2.2 COLLECTION AND DETECTION SYSTEMS.
-A radial resolution of 3 cm (11 points over a full profile) and
a beam diameter of 4 mm give a scattering surface
of 1.2 cm2. The geometry of the tokamak and the
requirement not to vignette in the extreme scanning position, limit the solid angle to d03A9 = 2 x 10 - 2 sterad.
(collecting lens j’
=605 mm, 0 95 mm), giving a throughput of 2.4
x10-2 cm2 sterad.
The low value of the plasma density, resulting in
a small number of scattered photons, necessitates the use of a matched dispersive system with high overall transmission ; a filter-polychromator (Fig. 3) was
chosen as a dispersive system. The cost of a custom- made interference filter and two lenses times the number of channels is, for a small number of channels,
lower than the cost of a modified monochromator with fiber optic array.
The photons scattered from the interaction zone enter the system through collecting lenses 1 and 2
that image the interaction zone on the input slit
Fig. 2.
-Cross-section of viewing dump : machined alu-
minium plate, blackened by anodization.
Fig. 3.
-Schematic outline of the filter-polychromator.
(Lenses not drawn to scale.)
(50 x 6.7 mm2 ) of the polychromator. The image magnification reduces the cone within which the
light is incident on the interference filters, as this
effect degrades the maximum transmission and band- width specifications with respect to the specifications
for a collimated beam. Lenses 4, 6, 8 and 10 reimage
the slit at positions 5, 7 and 9, where field lenses prevent widening of the beam and vignetting by
the filters (50
x50 mm2).
Additional lenses behind the filters image lens 1
on the cathode surface (position C) of each photo- multiplier (image ~ 0 25 mm), assuring uniform illu- mination. All lenses are AR coated. Stray light at the
laser wavelength or originating from the plasma light is attenuated by a polarizer P and 2 notch filters LF 1 and LF 2 in series.
Stray light scattering from the large number of optical surfaces is blocked by diaphragms mounted
on each lens and by baffles (broken lines).
Each of the three-cavity interference filters FI to F5 passes a different band of the spectrum and reflects the other wavelengths to the next filter.
In this way the filter set constitutes a five-channel
polychromator. Centre wavelength and half-width of each filter were chosen to transmit an approximately equal number of photons for Te ~ 110 eV in order to obtain the same quantum noise in each channel.
The rejection ratio of the unblocked filters was
improved at the blue side by an uncoated coloured
glass filter (Schott type RG 630) while the S20 photo- multiplier cathode serves as an infrared blocking
system.
Each spectral band is detected by EMI 9658 BM photomultipliers, selected for high quantum effi- ciency (q > 7 % at 700 nm). The PM’s have been
tested with 30 ns pulses for linearity up to 2 mA, and for stability, showing less than 0.7 % variation
in 24 h of operation. The PM’s are magnetically
shielded [6] and protected for RF interference by a Faraday cage; precautions against X-rays were not
necessary. The overall transmission of the poly-
chromator is shown in figure 4, for each channel
separately. As the beam progresses inside the poly- chromator, approximately 1 % of the photons is
lost at each glass-air interface of a lens ; 8 % is lost through the coloured glass filter and on the average 2 % is lost at each reflection on a filter. The effect of the diverging and converging beam is clearly
visible : maximum transmission (specified > 85 %) lowers, bandwidth widens and the squarer profile typical for multi-cavity filters disappears ; degrading
is worse for small-bandwidth filters. Fortunately
these effects compensate to provide a larger overall
transmission but at the penalty of increased cross-
talk.
Fig. 4.
-Measured transmission curves of the 6 channels of the polychromator. 100 % represents the photon flux inci-
dent at position 3 of figure 3. For each channel the upper
curve