ELECTRON CYCLOTRON RESONANCE HEATING ON THE W-VII-A STELLARATOR
Plasma build-up and heating of OH current free plasmas by ECR wave irradiation (28 GHz, 200 kW, 40 ms) were investigated on the WENDELSTEIN VII-A stellarator using three kinds of wave launching: direct irradiation of the gyrotron modes (mainly TE-02 mode corresponding to a 50% O mode and 50% X-mode mixture) from the low field side, or advanced wave launching (linearly polarized TE-11 or HE-11 mode) in O mode polarization from the low field side, the non-absorbed fraction being reflected back to the plasma from the high field side in X-mode polarization. An increase of the central electron temperature from 0.7 keV (TE-02 mode) to 1.2 keV (TE-11, HE-11 mode) was observed which is explained by the narrow and well centred power deposition profiles for TE-11, HE-11 modes. However, there is only a slight increase of the heating efficiency from 40 % to 50 %. The reflected X-mode fraction does not contribute to bulk plasma heating via Bernstein wave conversion and absorption as expected. The reason seems to be local absorption of the arising electron Bernstein waves due to a macroscopically turbulent structure around the upper hybrid resonance layer. Correlated with X-mode irradiation direct ion heating was observed (500 eV ion tail), possibly due to low frequency decay waves. In all ECRH experiments a toroidal plasma current was generated due to asymmetrically confined fast electrons. Optimum confinement in the shearless l + 2 configuration was achieved at most irrational values of the rotational transform with small toroidal net current. It can be concluded from a numerical 1D transport analysis that neoclassical electron confinement seems to be dominant in the hot central plasma core.
Bibliographic Reference: 1984 INTERNATIONAL CONFERENCE ON PLASMA PHYSICS, LAUSANNE (SWITZERLAND), JUNE 27-JULY 3, 1984 VOL. I, PP. 271-284 EUR 9708 EN (1985), FS, VOL. I AND VOL. II, 1155 P., BFR 3500 (BOTH VOLUMES), EUROFFICE, LUXEMBOURG, POB 1003
Record Number: 1989124055500 / Last updated on: 1987-01-01
Available languages: en