On the Possibility of Anomalous Power Absorption in Experiments On-Axis X2-Mode Electron Cyclotron Resonance Heating at the TJ-II Stellarator

An explanation of the dependence of the efficiency of electron cyclotron resonance heating (ECRH) on chord-average plasma density discovered during the analysis of experiments on-axis ECRH in the TJ-II stellarator is proposed. It is shown that the effect of decreasing heating efficiency with decreasing density can result from the excitation of a two-plasmon parametric decay instability (PDI) of the extraordinary microwave. The decay of the microwave (pump wave) occurs in two spatially close points as a result of the development of an absolute PDI leading to the excitation of electron Bernstein (EB) waves at frequencies close to half the frequency of the pump wave. The saturation of the instability is caused by secondary decays of the primary EB waves followed by the excitation of daughter wave pairs consisting of an EB and a slow lower hybrid (SLH) wave. The power deposition region of the daughter waves can differ substantially from that of the pump wave, which can explain the discovered discrepancy between the efficiency of ECR power absorption and the predictions of the standard linear theory. The daughter SLH waves, after linear transformation into ion Bernstein (IB) waves, can interact effectively with the ion component, which allows for a qualitative explanation of the appearance of superthermal ions discovered at the TJ-II installation during the ECRH.