Second harmonic electron cyclotron heating in low density and overdense plasmas in Heliotron DR

Abstract

Experimental investigations of second harmonic electron cyclotron heating have been made on Heliotron DR (R = 90 cm, āp ≃ 7 cm), using a 28 GHz, 200 kW gyrotron. Effective heating of the plasma has been observed in a wide range of electron density, even above the cut-off value of the ordinary mode (overdense plasma). When the electron density is lower than the cut-off value of the extraordinary mode, the electrons are strongly heated and Teo ≃ 1.7 keV at n̄e ≃ 0.25×1013 cm−3. In Heliotron DR, the heating is found to be less sensitive to the magnetic field intensity than in Heliotron E, and it is most effective at a central magnetic field B0 lower than the resonant magnetic field Bres. These phenomena are discussed in connection with the orbits of electrons accelerated in the cavity mode of the RF field (multiple reflections of the RF field at the vessel wall are dominant). Overdense plasmas (n̄e > l×1013 cm−3) are strongly heated at B0 ⪆ Bres. Their typical parameters are n̄e = 2×1013 cm−3, Teo ≃ 330 eV, and average beta, ⟨β⟩ ≃ 0.4%. The maximum ⟨β⟩ value is limited to ≤ 0.5% by the onset of MHD instability, which agrees well with theoretical predictions for pressure driven ideal interchange instability. The possibility of heating overdense plasmas through mode conversion of injected radiofrequency waves to electron Bernstein waves is examined and is shown to be likely in the present experimental conditions with multiple wall reflections.