RF HEATING EXPERIMENTS IN THE WEGA TOKAMAK

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RF HEATING EXPERIMENTS IN THE WEGA

TOKAMAK

P. Blanc, W. Hess, G. Ichtchenko, P. Lallia, T. Nguyen, G. Tonon, C. Mahn,

W. Ohlendorf, G. Pacher, H. Pacher, et al.

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C3, suppldment au no 8, Tome 38, Aoiit 1977, page C3-165

RF HEATING EXPERIMENTS

IN

THE WEGA TOKAMAK

P. BLANC, W. HESS, G. ICHTCHENKO, P. LALLIA, T. K. NGUYEN, G. TONON Association EURATOM-CEA

DCpartement de Physique du Plasma et de la Fusion ContrGlQ

Service IGn. Centre d'Etudes Nucltaires, 85 X, 38041 Grenoble Cedex, France

C. MAHN, W. OHLENDORF, G. W. PACHER, H. D. PACHER, J. G . WEGROWE Max-Planck Institut fiir Plasmaphysik,

8046 GarchingIMiinchen, West Germany

R6sum6.

-

Une puissance H F atteignant 90 kW a 500 MHz, frkquence, voisine de la rksonance hybride infkrieure, a ktk couplke au plasma du Tokamak WEGA a l'aide de boucles plackes a I'intk- rieur de la chambre a vide. Des profils de tempkrature klectronique obtenus par diffkrentes mkthodes, il apparait que les klectrons n'absorbent pas une fraction importante de la puissance HF. Des ions rapides sont produits pendant l'impulsion HF. Leur temps de dkcroissance est de I'ordre de 100 ps. Une augmentation de la tempkrature des ions au coeur du plasma d'environ 90 eV (soit 60 %) est observke. Elle d k r o i t apres I'impulsion HF en 2 ms environ.

Abstract. - To the WEGA Tokamak plasma, up to 90 kW RF power near the lower hybrid frequency (500 MHz) have been applied using internal coupling structures. Measurements of electron temperature profiles by different methods are presented. It appears that the electrons do not absorb a significant fraction of H F power. Fast ions are produced during the H F which flux decays in about 100 ps. A bulk ion temperature increase of about 90 eV (or 60 %) is observed with a decay time after the H F is turned off of the order of 2 ms.

Introduction.

-

WEGA, a joint experiment of the CEA Grenoble (EURATOM-Assoc.) and the Max- Planck Institut fur Plasmaphysik (EURATOM- Assoc.) has been presented at the Grenoble Confe- rence [I]. Further results were reported in [2], [3] and presented at the APS Conference [4]. Here, the latest results on HF heating near the lower hybrid frequency are presented.

The main characteristics of the device and of the target Tokamak plasma are given in table I and figure 1. As can be seen there, WEGA is a Tokamak without copper shell and with iron core. Radial equilibriumA;s provided by an external vertical field fed back from the radial position measurement.

In figure 2 is shown the arrangement for coupling the H F power at 500 MHz (corresponding to the Iower hybrid frequency in deuterium plasma at 14.3 kG magnetic field and at a density of 3 x 1013 ~ m - ~ ) . Two current loops lying in the equatorial plane and separated by about 14 cm are excited to produce H F fields out of phase. Estimates of the Fourier excitation spectrum without taking into account plasma effects show that about 60

%

of the power should be

in the region of the spectrum of parallel refractive index between 1.4 (accessibility limit [S]) and 6. The power at N I I lower than 1.4 is expected

[q

to go into surface waves which should not heat the body of the

WEGA discharge parameters

- W EGA churacterisrics

Large plasma radius R, = 72 cm Minor plasma radius a, = 13-16 cm Minor wall radius a, = 22 cm Toroidal magnetic field B, = 14.3 kG

-

Plasma characteristics without R.F.

D plasma

Plasma current I, = 45-60 kA

Ohmic heating power P , = 100-130 kW Peak electron density n,(O) = 1-6 x 1013 cm-3 Peak electron tempe;ature

Peak ion temperature Impurity level

Energy confinement time - R.F. heating waveparameters

R.F. frequency Maximum R.F. power Maximum pulse duration Parallel wave index

f = 500 MHz

P,, = 100 kW

At,, = 5-15 ms

NII k l l / k o 2: 2-9

plasma, whereas the wave remaining at higher N , , can be absorbed in part by electron Landau damping, but should not reach the core of the discharge. The H F power is applied at a level of 90 kW (total ohmic heating power 130 kW) for 10 ms with less than 10 % reflected to the generator.

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P. BLANC ET AL.

WITHOUT HF WITH

W

EGA

Primary windinas /

FIG. 1. - Cross section of the WEGA Tokamak.

Waveguide doaxial

"

Top view

l#yyt$pqm"plifier Pilot Oscillator

FIG. 2. -Coupling arrangement for Lower Hybrid Heating.

1. Basic plasma parameters with and without

HF.

-

In figure 3 are shown the evolution of the basic parameters of the discharge as a function of time with and without HF. On application of the HF, the plasma current does not change, and the vertical and radial positions change little. The loop voltage and with it the ohmic heating power change by 15

%.

The line density, as measured by a 2 mm microwave inter- ferometer, increases by 20 % on application of the HF.

The density increase observed here is not always present and appears to be related to the cleanliness of the device. The density profile has been measured by Thomson scattering at the same time in the discharge with and without H F (Fig. 4). The increase in line density deduced from this profile agrees with the microwave measurement. The shape of the profile does not change significantly on application of the HF.

5 0

1

+1 la

-IB

1

j%di (fringes)

0 5 0 100 (ms) 0 5 0 lOO(ms)

Frc. 3. - Basic plasma parameters : a) plasma current I,, b) loop voltage V, c) ohmic power W, d ) radial position AR (positive

signal : displacement to outside), e ) line density

S

n, dl.

2. Effects of the

HF

on the electrons. - Four different measurements relating to the electrons have been

performed. Thomson scattering measures the tem- FIG. 4. - Density profile from Thomson scattering.

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RF HEATING EXPERIMENTS IN T H t WEGA TOKAMAK C3-167

perature using the bulk of the electron energy distri- WITHOUT HF W I T H HF

bution (up-to energies about 1.5 keV). Emission at the second harmonic of the electron cyclotron frequency

t- t-

should be proportional to the electron temperature

for a Maxwellian distribution but is expected and _I observed [T7] to be enhanced by the presence of a

suprathermal tail. Good agreement between the two

methods is shown in figure 5, which has been obtained

_I

X rays (soft) ~ r a y s ( s o n )

1

(arb.units) l(arb.units)

A

Te (ev) A THOMSON RADIOMETER 2uce 500: limiter

FIG. 5. -Comparison of electron temperature deduced from Thomson scattering with power emission at the 2nd harmonic of electron cyclotron frequency performed by varying the main

magnetic field.

from a series of discharges in which the main field was varied, and the Thomson scattering temperature was measured on the magnetic surface on which the frequency of the radiometer (73 GHz) equalled twice the electron cyclotron frequency. Although these discharges are not the same as the standard discharges discussed in the remainder of the paper, the corres- pondance between the two measurements remained, equally good. The other two methods relating to the electrons are based on the detection of X-rays emission. The soft X-rays are observed using 4 channels with different thicknesses of aluminum absorber foils which therefore measure various integrals of the Bremsstrahlung spectrum between 3 and 20 keV. The hard X-ray detector is sensitive to emission down to 25 keV (since the plasma is observed via a quartz window).

Figure 6 shows the result on application of the HF, of three of these measurements as a function of time. Although each channel of the soft X-ray measurement shows an increase of 50 % of the intensity, the tempe-:

- 1 . .

. . I l l . .

.

*

I

50 I00 (ms) 0 50 100 (ms)

FIG. 6. -Electron and ion measuremehts with and without HF :

a) cyclotron harmonic emission, b) soft X-ray signal (1 channel), c) electron temperature deduced from soft X-rays, d) hard X-rays,

e) ion temperature deduced from charge exchange analysis.

rature deduced from the ratios of the various channels decreases by 10

%.

The hard X-ray measurement also shows an increase, small in absolute value, probably the continuation of the Bremsstrahlung spectrum to 100 keV as supported by other measurements.

The emission at tiyice the electron cyclotron frequency decreases by 10

%,

i.e. no suprathermql electrons tail is observed. The electron profile measured (Fig. 7) at the same time in the discharge with and without HF by Thomson scattering shows a decrease of the electron temperature by 20

% at the center of

the discharge. Figure 8 shows for the same shots, the profile of nTe, demonstrating that the energy content of the electrons does not change significantly during application of the HF.

3. Effect of the HI? on the ions. - In the present experiments, the peak perpendicular ion temperature is deduced from the spectrum of charge exchanged neutrals. The energy spectrum is Maxwellian without

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C3- 168 P. BLANC ET AL.

0 WITHOUT HF

(m

FIG. 7. - Electron temperature profile from Thomson scattering.

g..% (lo4 e r ~ , , ~ ) 9 0 WITHOUT HF I 9 WITH HF down UP

FIG. 8. -Electron energy density profile nT, from Thomson scattering.

The signals at the various energies as a function of time are represented in figure 8b. In this measurement, the output pulses of the ion detector, proportional to the number of charge-exchanged neutrals received, is counted during a time interval of 1 ms. probably not situated at the center of the plasma [2], [3], Each point therefore represents the average during it is subtracted from the values at lower energies and this interval, but no instrumental constant appears a new Maxwellbn representing the bulk temperature as with an integrating technique. It is seen in figure 9b. fitted to the corrected points (Fig. 9a). that the signal at 7 keV disappears from one time

• experimental points

I--

-

!

; Q ~ - - - - i .5=

1

*.

El corrected points

FIG. 9. - Effect of HF on the ions, observed by charge exchange measurement. a) Ion energy distribution deduced from charge exchange during HF ; b) Ion flux at various energies as a function of time ; c) Bulk ion temperature deduced from charge exchange measurement

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RF HEATING EXPERIMENTS IN THE WEGA TOKAMAK C3- 169

point to the next, whereas the signals at low energies have a longer decay time after the H.F. is turned off.

The ion temperature calculated in the fashion described above is shown in figure 8c. It is seen that the temperature increases quickly by 60

%,

i.e. from 150 to 240 eV, when 90 kW of HF are applied. When the HF is turned off, the ion temperature decreases to the old value with a decay time constant of 2 ms.

4. Discussion. -The effect of HF near the lower

hybrid frequency on the WEGA Tokarnak plasma may be summarized as follows.

As concerns the electrons, the four measurements sensitive to various portions of the energy spectrum show no evidence of absorption of significant H F energy by the electrons. In the present experiments, the product ne Te remains constant. A small (10- 20

%)

decrease in the bulk electron temperature during the HF was observed.

Also in a different series a small increase in Te was observed, but a small decrease has been observed more consistently. Since the conditions for electron Landau damping demand a parallel refractive index greater than 6 for WEGA parameters, and the major part of the HF power is coupled at lower N,,

,

electron heating is not to be expected in the experimental situation of WEGA.

As concerns the ions, experiments previously report- ed [3], had shown an increase of the bulk ion temperature by 20 to 30

%

measured simultaneously by charge exchange and Doppler broadening of an impurity (Ow' line. The agreement between the methods at

Refer [I] BLANC, P. et al., 3rd Znt. Meeting on Theoretical and Experimental

Aspects of Heating of Toroidal Plasmas, Grenoble (1976) p. 251, vol. 11.

[2] LALLIA, P. P., PACHER, G. W., PACHER, H. D., Znt. Symposium on Plasma Heating, Varenna (1976).

[3] BLANC, P. et al., 6th Znt. Conference on Plasma Physics and

Controlled Nuclear Fusion Research, Berchtesgaden (1976) paper IAEA CN-3S/G9.

that time confirms the validity of the treatment of the charge exchange data (subtraction of the fast ion tail). At that time, the geometrical arrangement of the antennas was different. According to the preliminary calculation of the excitation spectrum without plasma effects, only 25 % of the power was coupled at parallel refractive indices between 1.4 and 6 (versus 60

%

now), and the distance between the loops and the plasma was 5 crn (versus 1 cm now). These changes are expected to lead to more power coupled at low

NII (1.4 to 6) for the present configuration.

In the present experiments, fast ions are observed

in the perpendicular direction. They are not well- confined (loss time 100 ps) for the modest parameters of WEGA (14.3 kG, 15 cm, 60

kA).

During the H F an increase of the bulk perpendicular ion temperature by 60

%,

i.e. 90 eV, is observed on the application of 90 kW of HF power.

The inference that the measured increase of the ion temperature by 90 eV represents the bulk ion temperature is supported by the measured decay time of 2 ms.

The ion temperature in this series is measured only by charge exchange without confirmation by spec- troscopic measurements. The increase of density (20 %) observed during this series could not by itself explain an increase in ion temperature greater than 10

%,

through classical electron-ion energy exchange. The measured peak ion temperature increases rapidly

(t < 1 ms) which may initially reflect a local heating

of a small number of particles before the establishment of an equilibrium ion temperature profile with HF.

[4] BLANC, P. et al., Bull. Atn. Phys. Soc. 21 (1976) 5D-2. HESS, W. et al., Bull. Am. Phys. Soc. 21 (1976) 5D-3. [5] GOLANT, V. Z., SOV. Phys. Tech. Phys. 16 (1971) 1980. [6] BARANOV, Yu. F., SHCHERBININ, 0. N., 3rd Int. Meeting on

Theoretical and Experimental Aspects of Heating of Toroidal Plasmas, Grenoble (1976), p. 131, Vol. I. [7] BOYD, D. A., STAUFFER, F. J., TRIVELPIECE, A. W., Phys. Rev.