Asymmetric Wave Transmission During Electron-Cyclotron Resonant Heating

In low density plasmas in the RTP tokamak the single-pass absorption of O-mode waves at the fundamental electron cyclotron resonance is observed to be toroidally asymmetric. The absorption is highest for waves travelling in the direction opposite to the toroidal plasma current. Fokker-Planck quasi-linear code simulations show that this asymmetry is most likely to be due to a synergism between the electron cyclotron waves and the DC toroidal electric field. The electron cyclotron waves create a population of suprathermal electrons, which is localized in the region of electron cyclotron power deposition. This population is accelerated by the toroidal electric field. The ensuing asymmetry in the electron distribution function is then responsible for the asymmetry in the wave absorption. On the basis of this interpretation, a heuristic scaling of the asymmetry is derived, which is confirmed by simulations. The scaling of the experimental results with the plasma density lends further support to the proposed interpretation.