Third-harmonic X-mode, real-time controlled top-launch ECW experiments on TCV Tokamak

In the moderate magneticfield of TCV (1.5T), the recently installed X3 system (3 gyrotrons @118 GHz, 0.45 MW/each, 2s) broadens the operational space with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system (X2 cutoff at n_e = 4.2 × 10 ¹⁹m^-3). To compensate the significantly weaker absorption coefficient compared to the absorption of X2, the top-launch injection allows to maximize the ray path along the resonance layer thus maximizing the optical depth. To maintain the maximum absorption in plasma discharges with a dynamic variation of both density (refraction) and temperature (relativistic shift) a real time control system on the poloidal injection angle has been developped and succesfully tested on TCV. With a total injected power of 1.35 MW and using the mirror realtime control, full-single pass absorption has been measured in an L-mode plasma. A significant fraction of the absorbed power is associated to the presence of suprathermal electrons generated by the X3 wave itself. In X3 heating experiments of H-mode plasmas it has been possible to enter into a different ELMy regime compared to the ohmic/low-power-heating ELMy regime. In these experiments a significant increase of the plasma energy is obtained with nearly fullsingle pass absorption. Results on the comparison of the absorbed fraction calculated with the TORAY-GA ray-tracing code and the beam-tracing code, ECWGB, which includes difiraction effects, are discussed.