A Quasi-Coherent Electrostatic Mode in ECRH Plasmas on TJ-II

A new type of instability mode, related to suprathermal electrons, was found in extremely low-density plasmas, (ne = (0.2-0.5) × 1019 m−3) with electron cyclotron resonance heating and current drive (ECRH/ECCD) in the TJ-II heliac. The quasi-monochromatic density and plasma potential oscillations in the frequency range 20-120 kHz tend to have several “branches” with constant frequency shift between them. The typical amplitude of the mode induced potential oscillations was estimated to be ΔφEM ∼ 20 V. The mode branches have an individual and finite radial extent, odd low poloidal structure (m ≤ 5) and angular phase velocity of poloidal rotation of about 8 × 104 rad/s in the ion diamagnetic drift direction. The contribution of the mode to the turbulent particle flux ΓE×B for the observed wave vectors kθ < 3 cm−1 was found to be small in comparison with the contribution from broadband turbulence.