• Aucun résultat trouvé

Thomson scattering system of the ERASMUS tokamak

N/A
N/A
Protected

Academic year: 2021

Partager "Thomson scattering system of the ERASMUS tokamak"

Copied!
7
0
0

Texte intégral

(1)

HAL Id: jpa-00245150

https://hal.archives-ouvertes.fr/jpa-00245150

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

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�

(2)

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

aux

caractéristiques du tokamak ERASMUS,

a

été 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

x

1011 cm-3. Les profils radiaux sont relevés

sur

30 cm pour

un

plasma de 39

cm

de 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

a

single compact frame, that is mounted

on an

airtrack, facilitating alignment and calibration.

Measurements of Te-values from 20 up to 350 eV with ne ~ 8

x

1011 cm-3

are

routinely performed and radial profiles are taken over 30

cm

of the 39

cm

plasma diameter with

a

resolution 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

X

1012 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

(3)

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 :

rear

and 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

a

system 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

x

10-2 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.

(4)

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

x

50 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

is the transmission of the polychromator up to the interference filter, the lower

curve

gives the same

measure-

ment but with the filter inserted. Normalized Gaussian spectra for Te

=

20, 110 and 350 eV

are

also shown.

3. Alignment and calibration.

3.1 MOTORIZED ALIGNMENT SYSTEM. - The mecha- nical properties of the machine, and future programs led to an involved mechanical system, consisting of a unique frame rigidly connecting laser, beam focussing optics, receiving optics, polychromator and photo- multipliers (Fig. 5). The system is compact, its ease of maintenance and its reliability enable one single

operator to run the system and perform all routine

alignment tasks; the measurement of Te and ne

(5)

806

Fig. 5.

-

Schematic of the mechanical support for the laser, the calibration set-up and the collection system.

profiles is made on a shot by shot basis and the same structure can be employed for two-dimensional scans

in future experiments.

Every item, necessary for routine alignment is fully motorized ; portable power supplies allow remote

control from any viewing position. The frame is not

fixed with respect to the tokamak, except for the

two supports A ; its many degrees of freedom allow easy correction of small displacements of the tokamak.

Base B can be tilted around axis a with motor M3 ;

4 air-floated disks C, C’ support the frame ; fixed to

it is a shaft D sliding in linear bearings c and d on

motors Ml and M2, allowing a large excursion perpendicular to and away from the machine for maintenance and positioning of the frame for two-

dimensional scans. Screws housed in disks C are

driven by motor M4 ; they tilt the structure around pivots on disks C’. A limited lateral movement

(2 cm) or a rotation is performed by actuating Ml

and M2 respectively in the same or in opposite direc-

tions. The advantage of an involved airtrack support

over a conventional wheels-on-track system lies in frictionless, fast and accurate positioning while the

track is activated; with pressurized air cut off, the system sits firmly and is not sensitive to vibration.

A profile is taken by tilting the detection platform carrying collection optics, polychromator and photo- multipliers around pivot b by means of an oleo- pneumatic piston PP. The optical axis of the collection

optics intersects axis b perpendicularly in order to

have a simple relationship between the rotation angle

of the detection system and the radial scanning

distance in the plasma.

3.2 CALIBRATION.

-

The measurement of the abso- lute value of the electron density requires the absolute calibration of the detection system. Absolute cali- bration by Rayleigh scattering was performed on a duplicate of a tokamak window section ; afterwards it was found that Rayleigh scattering was not feasible

in ERASMUS due to vacuum technical difficulties.

Instead the absolute electron density was determined by comparing the integral of the density profile in

relative units to the line average value from 4 mm

microwave interferometry. Relative calibration is

performed with a standard light source. The lower

platform of the frame carries a rail with all calibration

optics; the rail slides on shafts and can accurately

be positioned in the plane of the laser beam (between

M and P in Fig. 5). An image of the ribbon is projected

on a smoked MgO screen, inclined by 450 upwards ;

an intermediate image accessible for measurement

and control is formed at a point equivalent to the scattering volume, after which the light is directed into the collecting lens by means of a 450 mirror.

Carrying out a calibration is fast and easy, making

a secondary calibration system, e.g. a pulsed LED, superfluous. PM gain variations between dc and

pulsed mode of operation were avoided by adding a light chopper delivering short (500 ys) light pulses

with low repetition rates [7]. Allocating all photons

in one channel to its central frequency is a good approximation for the flat calibration spectrum;

for low Te values, the scattered spectrum has steep

intensity gradients in the innermost channels; the central wavelength is no longer an accurate value for

the mean scattered wavelength. Decreasing the band-

width closer to the laser wavelength limits this syste- matic error, which was estimated to be of the order of the error bars at worst.

4. Data handling.

4.1 TIME SEQUENCING AND DATA ACQUISITION. - The data acquisition system is fully automatic except for manual setting of the time of measurement, the radial scan position, and the laser energy.

The output of the PM’s feeds two data gathering

systems in parallel. The first system (Fig. 6) basically integrates plasma light and scattered light together

and plasma light only.

After ADC conversion, the computer subtracts the two numbers giving the net charge from the

scattered laser light and at the same time eliminating

zero order fluctuations.

Fig. 6.

-

Scattering signal flow and trigger logic.

(6)

Pick-up caused by the high power radio frequency heating generator, can be software eliminated by delaying the second gate with respect to the first

one by 200 ns, corresponding to one RF period.

The Thomson scattering interface, besides generating

and synchronizing the integration gates, also gathers

all information relevant to the diagnostic : time of

measurement with respect to the beginning of the plasma shot, laser output energy and inclination of the detection platform.

The second system, indispensable during the start-up

period, now serves as a visual control of all outputs.

Monitoring of pulse height and pulse shape allows early detection of any malfunction of the system like

uncoming damage of optical surfaces.

4.2 DATA TREATMENT. - The data acquisition and

treatment operations associated with the Thomson

scattering (TS) diagnostic are encompassed in a general data acquisition and playback system (DAS) [8]

which collects all the data from the tokamak expe- riment. The computer program determines the number of photons nl, corresponding to the net generated charge Pi in a channel with central wavelength Ài,

and the uncertainty 0394ni arising from Poisson noise and fluctuations on the pedestals of the integrators.

Using weights wi = (ni/0394ni)2, the points (Xi, 0y = 03BBi - 03BBL) are fitted to the following expression [9]

giving the number of scattered photons ni(03BB, S2) in a

solid angle dQ, and in a given wavelength interval d03BB

i is the channel index (1 to 5)

L the length of the observed scattering volume No number of incident photons

ne electron density in cm - 3 Te electron temperature in eV

0 angle between incident and scattered wave vectors

039403BB =03BBi - 03BBL difference between scattered and ruby wavelength in nm.

This computation furnishes the expected Te and ne

values together with their confidence intervals. When-

ever necessary, a rejection algorithm using a x2

test discards aberrant points. TS uses, for its real time outputs, a line printer and a scope.

A detailed summary of the data and data treatment is printed out showing the data values and regression

values together with all regression parameters and results. Data rejection is also mentioned whenever it occurs. A scope output is represented in figure 7.

Fig. 7.

-

Thomson scattering drum display.

For a typical measurement in the centre of the

plasma from 150 to 400 photoelectrons are detected

per channel.

5. Performance.

The accuracy of the measured parameters is deter- mined for high temperatures by the flatness of the Gaussian in the observed wavelength domain, and

for low temperatures by the requirement of having

a sufïicient number of photons in at least three channels.

For normal discharges in ERASMUS, we can

measure as low as Te N 20 eV, with ne > 8 1011 cm-3.

The high temperature limit amounts to 350 eV, the precise value being codetermined by the electron

density.

Figure 8 gives an example of a radial temperature

profile with error bars showing the good reproduci- bility of the measurement, while figure 9 gives the density profile for the same plasma conditions.

Fig. 8.

-

Radial profile of electron temperature.

Fig. 9.

-

Radial profile of electron density (arbitrary units).

(7)

808

Table 1.

-

Comparison between Thomson scattering, soft X-rays and 4 mm interferometer.

Application of RF-power increases plasma noise and favours disruptivity of the discharges, yielding larger

error

bars.

This accounts for the less perfect agreement between the diagnostics.

At the plasma periphery expression 1 is fitted to

the three innermost channels, other channels being

Acknowledgments.

rejected by means of the computer algorithm. The

resulting low errors remain unexplained ; the scatter We are grateful to Dr. R. Koch for developing the

on consecutive measurements is probably a better data treatment programs, to Drs. G. Bosia and

measure of the uncertainty. D. Pearson for the TS interface.

Table 1 shows good agreement between the results We appreciate the technical assistance of Messrs obtained by Thomson scattering, the soft X-ray E. De Meester, A. Leskens, J. Neefs, A. Paes, C. Pleur- diagnostic and the 4 mm interferometer. The error deau, G. Reunis, F. Van Goethem, W. Van Lauwe, bars are normally of the order of 10 %. F. Van Thillo and I. Waterloos.

References

[1] PEACOCK, N. J., BURGESS, D. D., Culham Laboratory Report, CLM-P612 (1980).

[2] BHATNAGAR, V. P., BOSIA, G., CALDERON, M., DARIUS, I., DESOPPERE, E., KOCH, R., MESSIAEN, A. M., PEARSON, D., PIRET, C., TELESCA, G., VANDEN-

PLAS, P. E., VAN OOST, G., WEYNANTS, R. R., Proc. of the 9th European Conference

on

Con- trolled Fusion and Plasma Physics, Oxford, 1979, Vol. 2, p. 140.

[3] BHATNAGAR, V. P., BOSIA, G., DESOPPERE, E., D’HONDT, P., KOCH, R., MESSIAEN, A. M., NOTERDAEME, J. M., PEARSON, D., POULAERT, G., PRATES- DROZAK, R. M., TELESCA, G., VANDENPLAS, P. E.,

VAN OOST, G., VAN WASSENHOVE, G., WEYNANTS, R. R., Proc. of the 8th Conference

on

Plasma

Physics and Controlled Nuclear Fusion, Brussels, 1980, Vol. II, p. 85, IAEA Vienna.

[4] BHATNAGAR, V. P., BOSIA, G., MESSIAEN, A. M., PAI- THANKAR, A. S., PIRET, C., VANDENPLAS, P. E., WEYNANTS, R. R., Proc. of the 6th Conference

on

Plasma Physics and Controlled Nuclear Fusion, Berchtesgaden, 1976, Vol. I, p. 359, IAEA Vienna.

[5] HURUP HANSEN, B., RisØ National Laboratory, Den- mark, private communication.

[6] DESOPPERE, E. and VAN OOST, G., LPP-ERM/KMS Laboratory Report 77, August 1981.

[7] LASALLE, J., PLATZ, P., Laboratory Report EUR-CEA-

FC 987 (1979).

[8] BOSIA, G., KOCH, R. and MATHIEU, J.-L., Diagnostics for Fusion Experiments, E. Sindoni, C. Wharton

editors (Pergamon Press) 1978, p. 567.

[9] SHEFFIELD, J., Plasma Scattering of Electromagnetic

Radiation (Academic Press) 1975.

Références

Documents relatifs

On each subsystem the main process has various message queue links with the EVB: an input queue for receiving control messages (also from other processes in the subsystem DAQ),

In the case of the exterior domain problem, the long time behavior of the energy is addressed in [7]: therein, the decay of the energy is obtained under additional

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des

Wich the technique of tmnsilluxnination microscopy in vivo (Knisely) we could conclude that the lymphatic system within the wound regenerated muchquicker with

Actually, the fractal dimensionality d computed by the method described in the preceding section turns out to be less than or equal to DI (3). We have computed the

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des

Design and testing of a magnetic shield for the Thomson scattering photomultiplier tubes in the stray fields of the ERASMUS

The use of consistent X-ray centring and the inclusion of all available sample information allow the calculation of optimized data collection strategies that directly translate