Deviation of the electron energy distribution function from maxwellian during ECR heating in the L-2M stellarator in reg
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Deviation of the Electron Energy Distribution Function from Maxwellian during ECR Heating in the L-2M Stellarator in Regimes with a High Specific Heating Power A. I. Meshcheryakov, D. K. Akulina, I. Yu. Vafin, G. A. Gladkov, and S. E. Grebenshchikov Prokhorov Institute of General Physics, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia Received July 5, 2005
Abstract—In the previous experiments on ECR heating of a low-density plasma with ne = (0.3–0.5) × 1019 m–3 in the L-2M stellarator, the electron temperature profile measured from the intensity of electron cyclotron emission was found to be asymmetric about the magnetic axis and the electron temperature measured by this diagnostics turned out to be higher than that expected from diamagnetic measurements. To find out the character of distortion of the electron energy distribution function, the soft X-ray spectrum was measured in regimes with large values of the specific heating power η (1.5 MW per 1019 particles). Under these conditions, the X-ray spectrum plotted on a semilogarithmic scale has no linear segments in the photon energy range from 1.5 to 12 keV. This indicates that the electron distribution function is non-Maxwellian over the entire energy range under study. PACS numbers: 52.50.Sw DOI: 10.1134/S1063780X06020036
1. INTRODUCTION In recent years, one of the directions of stellarator research has been the experimental study of electron cyclotron resonance (ECR) heating in regimes with a high specific heating power. These experiments are aimed at achieving the highest possible plasma temperature and are performed at either an increased heating power or a reduced average plasma density. Experiments carried out over the past two decades have revealed several interesting effects: the onset of internal transport barriers [1, 2], the distortion of the electron temperature profile measured from the electron cyclotron emission (ECE) spectra [3], and the disagreement between the central electron temperatures obtained by the ECE and Thomson scattering diagnostics [4]. Thus, in the CHS experiments [1] performed at a plasma density of ne = 0.4 × 1019 m–3 and heating power of P = 200 kW (the specific heating power η being as high as 0.6 MW/(1019 particles)), an internal transport barrier formed and the central electron temperature reached 2.5 keV. Here, the specific heating power (the heating power per electron) is defined as η = 1019P/Vne, where P is the heating power, V is the plasma volume, and ne is the line-averaged (over the central chord) plasma density. The X-ray spectrum measured during ECR heating in the LHD stellarator in regimes with a low plasma
density, ne = (0.2–0.4) × 1019 m–3, at a specific heating power of η = 0.03 MW/(1019 particles) [5] was nonMaxwellian in character, the deviation from Maxwellian being most pronounced at high photon energies, E = 70–200 keV. A comparison between the measured and calculated spectra showed that the plasma was comprised of two electron components with very different temperatures:
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