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Cyclotron Radiation from Thermal and Non-Thermal Electrons in the Wega-Stellarator

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Cyclotron Radiation from Thermal and Non-Thermal Electrons in the Wega-Stellarator ASSOCIATIE EURATOM-FOM FOM-INSTITUUT VOOR PLASMAFYSICA RIJNHUIZEN - NIEUWEGEIN - NEDERLAND CYCLOTRON RADIATION FROM THERMAL AND NON-THERMAL ELECTRONS IN THE WEGA-STELLARATOR by H.W. Piekaar and W.R. Rutgers Rijnhuizen Report 80-128 ASSOCIATIE EURATOM-FOM November 1980 FOM-INSTITUUT VOOR PLASMAFYSICA RIJNHUIZEN - NIEUWEGEIN - NEDERLAND CYCLOTRON RADIATION FROM THERMAL AND NON-THERMAL ELECTRONS IN THE WEGA-STELLARATOR by H.W. Piekaar and W.R. Rutgers' Rijnhuizen Report 80-128 * Present address KEMA-Laboratory. Arnhem. This work was performed as part of the research programme of the association agreement of Euratom and the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver-Weten­ schappelijk Onderzoek" (ZWO)and Euratom. CYCLOTRON RADIATION FROM THERMAL AND NON-THERMAL ELECTRONS IN THE WEGA-STELLARATOR by H.W. Piekaar and W.R. Rutgers Association Euratom-FOM FOM-Instituut voor Plasmafysica Rijnhuizen, Nieuwegein, The Netherlands ABSTRACT Electron cyclotron radiation measurements on the WEGA- stellarator are reported. Emission spectra around 2to and 3oJce were measured with a far-infra-red spectrometer and InSb detectors. When the plasma loop voltage is high, runaway electrons give rise to intense broad-band emission. Runaway par­ ticles can be removed by increasing the plasma density. For low loop voltage discharges the electron temperature profile was deduced from thermal emission around 2w . In spite of the ce r low E-field, runaway particles are still created and pitch- angle scattered because "pe/w^^l. From non-thermal emission below 2u and 3u> the energy and number of non-thermal particles could be calculated, and was found to be in agreement with existing theories. I. INTRODUCTION The WEGA-device is a small-aspect-ratio medium-sized stel- larator (R = 72 cm, a = 19 cm) with £ = 2, m = 5 helical windings on the vacuum vessel. A description of the experimental set-up is given in Ref. 1. Tokamak and stellarator discharges with the same value of the rotational transform i (at the limiter) can be produced. Titanium gettering of the wall of the vacuum vessel reduces the impurity content, and so the loop voltage, considerably. First results on lower hybrid heating experiments in the stella­ rator configuration are reported in Ref. 2. -I- The stellarator is well-provided with diagnostic equipment as is shown in Fig. 1. This report deals with measurements of electron cyclotron radiation in the fre­ quency range of 70-140 GHz with a multi-channel far-infra­ red spectrometer (see photo 1). The equipment was installed and operated by a visiting team from the Institute for Plasma Physics in Nieuwegein, The Netherlands. Measurements were col­ lected during three short measuring campaigns in the winter of 78/79. The experimental set-up and some special problems of electron temperature measurements in a stellarator geometry are discussed in section II. In section III we present measure­ ments of electron cyclotron emission for different modes of machine operation as well as a temperature profile deduced from thermal emission at 2d) (ui is the electron gyrofrequency) . ce Apart from thermal emission around 2w , suprathermal radiation was detected below 2a> and 3w . In section IV the number and ce ce energy of these non-thermal electrons is inferred. Finally, in section V,these numbers are compared with theoretical ex­ pectations,followed by some concluding remarks in VI. Radiometer (4 Kanal) schneller GaseinlaB 1 Kanal \ Ho Umladungsanaiyse 15 Kanal» Positionsspuie Limiler Fig. 1. Diagnostic equipment on the WEGA-stellarator. (From: Jahresbericht 1978, Max-Planck Institut für Plasmaphysik, Garching). I I A photograph showing the far-infra-red diagnostic system linked to the WEGA-stellarator. P polychromator; D screened box with InSb detectors; L light pipes to the diagnostic port. The coaxial cables passing in front of our system connect the HF-generators to the antennas. -4- II. DIAGNOSTIC SYSTEM The WEGA-machine is operated with a toroidal magnetic field on axis of BQ = 1.44 tesla. Therefore, the electron cyclotron frequency fQ = qB/ 2™ on axis is 40.3 GHz. When re- absorption of radiation is sufficiently large, the plasma emits like a black-body which means that the intensity is directly proportional to the (electron) temperature T if the electron velocity distribution function is (close to) a Max­ well ian. For WEGA plasma parameters as given in table I the optical depth is calculated to be larger than 1 for radiation at 2 ^ce (A is around 3.5 mm) for 64 cm < R < 74 cm, where R is the distance to the torus axis. TABLE I Machine parameters Plasma parameters B = 1.44 tesla 25 kA (stellarator) o (50 kA, tokamak) 1=2, m=5 helical windi igs 2-3 volt XH.W. " 14° kA loop R =0.72 meter Z 3 o eff = a,. = 0.15 m 100 IBS lim pulse a , = 0.19 m T 450 eV wall eo T. 125 eV io n = 2.4'1019 m~3 eo Si = 12-13 cm Because of the low electron temperature, Doppler and relativistic line broadening are smaller than 1% for obser­ vation nearly perpendicular to the B-field. On the other hand, line broadening of the order of 20% occurs due to the varia­ tion of the magnetic field across the discharge tube. In the tokamak mode of operation the poloidal B-field is so small that the total B-field has roughly the 1/R dependence of the toroidal field BT. Along the line of sight (22*5° downwards) through our diagnostic port the position where the plasma is emitting 2w radiation is therefore easily calculated f rom + B(r) ^ l/(rcos22^ RQ)> where r is the distance to the minor axis of the torus. For the stellarator mode of operation the -5- poloidal field cannot be neglected and was calculated by com­ puter*. Lines of constant (B* + B2) are not at all vertical in this case as is shown in Fig. 2. The gradient in the B-field in the outer part of the torus is smaller, and in the inner part larger, than in the tokamak mode. The flux surfaces are no longer circular as is indicated in the same figure. Cyclotron radiation is collected with a quartz lens and a 2.5 cm diameter circular waveguide acting as a light-pipe. During the last experiments this lens was replaced by a fused quartz vacuum window because the lens was covered by a layer of titanium. Titanium gettering was used to reduce the impurity content of the plasma and to reduce the density increase during lower hybrid heating experiments. A grid polarizer (Cambridge Physical Sciences) could be inserted between vacuum window and entrance to the light-pipe system. The setup of the four-channel grating spectrometer and the calibration source to analyze the cyclotron emission is shown schematically in Fig. 3. An optical switch is used to compare the plasma emission with radiation from a microwave noise tube (T=11.000 K) or a klystron (f = 69 GHz). The spectrometer was calibrated over the whole frequency band with the noise tube, whereas the klystron was used for a day-to-day check of the spectrometer setting and the detector sensitivities. The klystron and the calorimeter (TRW) were also used to measure the transmission of the system. The attenuation from plasma up to the detectors turned out to reach 17 dB due to some misalignment of the light-pipes, the gaps for the vacuum window and the optical shutter, and the low trans­ mission of the spectrometer at long wavelengths. The determina­ tion of the absolute value of the plasma emissivity is not ex­ pected to be better than 3 dB. The four-channel grating spectrometer is of the Czerny- Turner type with blazed gratings (cut at an angle of 25°, A../d * 0.825) as dispersive elements. Gratings with a groove distance of 4 mm and 3.25 mm were used for the frequency band of 70-100 GHz and 90-140 GHz respectively. The frequency cali­ bration was performed by measuring radiation from a 4-mm and a 2-mm wavelength klystron in first and second order with four different gratings. The dispersion curves for the four channels are shown in Fig. 4. * The authors are indebted to Drs. G. Pacher and M. Lipa for providing us with these calculatioi5. -6- lines of constant B flu* surface vacuum vassal steilarator windings i.d. 38 cm toroidal field coil to polychromator Fig. 2. Topology of the magnetic field for standard steilarator operation. The dotted areas indicate the plasma volumes seen by the four detectors for a given setting of the grating spectrometer. klystron or noise tube hollow mirror light guides 1" diameter vacuum windows X-U/r-V U switch .i-.i quartz lens or window shutter box screened box with low noise pre-amplifiers 1 spectrometer (evacuated) Fig. 3. Schematic of our mm-wave diagnostic system. -7- The wavelength of the klystrons was measured to high accuracy with a Fabry-Perot interferometer. Therefore, the wavelength calibration of the spectrometer is better than 1%. The resolu­ tion A/A A was also measured. For the 4-mm grating the resolution is roughly proportional co the square root of the wavelength instead of proportional to the wavelength as we measured before for shorter wavelength. The reason seems to be the limited num­ ber of grooves and the large focal spot. The etendue of the spec­ trometer is Adfl=2*10~s m2 sr. The radiation is detected with InSb crystals at a temperature of 2.5 kelvin in a magnetic field of 1 tesla. The responsivity is typically 500 V/W. The noise level, referred to the preamplifier input, is 2 uV at a band­ width of 30kHz .
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