Electron cyclotron heating and current drive control by means of frequency step tunable gyrotrons

Abstract

While beam steering has been used for control of the location of the ECRH power absorption in tokamaks, alternative or complementarytechniques are worthy of investigation to improve the control of power deposition and to make the launcher system more resistant against the harsh environment of a reactor. Here, frequency tuning of ECRH power for reactor scale tokamak plasma heating is investigated. Particular emphasis is put on the adjustment of the power deposition near the m/n = 2 tearing mode resonance. Mode stabilization is studied by combining the ECRH absorption and MHD stability models with transport code calculations of the plasma evolution. The positioning of the power deposition should be performed with a moderate speed of frequency tuning (1 GHz/s) over a sufficient frequency range ( ≈ ±5 GHz). This is found to be within the reach of present gyrotron technology. The inductance and the width of the power deposition set lower limits for the step duration and power as well as an upper limit for the frequency step which is required in tuning for stabilization. For further fine tuning of the power deposition and current density profiles, fast frequency tuning by a properly programmed frequency sweeping of the order of 10 ms can be effective. A review of available methods to tune the frequency of gyrotrons is provided and their suitability for performing MHD mode stabilization is discussed.