Electron Cyclotron Resonance Heating Experiments in the Wendelstein VII-A Stellarator

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Abstract
Plasma buildup and heating of ohmically heated currentless plasmas by electron cyclotron resonance heating (28 GHz, 200 kW, 40 ms) were investigated in the WENDELSTEIN VII-A stellarator. Two different kinds of wave launching were examined in detail. First, the gyrotron mode mixture, containing 50% of the total power in ordinary (O)-mode and 50% in extraordinary (X)-mode polarization, was irradiated from the low-field side. Then a linearly polarized wave in O-mode polarization was launched from the low-field side, the nonabsorbed fraction being reflected back to the plasma from the high-field side in X-mode polarization. An increase of the central electron temperature from 0.6 keV (first case) to 1.2 keV (second case) was observed, which is explained as being due to the narrow power deposition profile in the latter case rather than due to the slight increase of the heating efficiency from 40 to 50%. Strong evidence of parametric decay of the X-mode fraction is found. Transport calculations using neoclassical plateau coefficients for the electron heat conduction including ripple losses fit well with the measured profiles, if enhanced losses at the plasma edge are introduced.