Electron-cyclotron Heating on TCV with 118 GHz Gyrotrons

Three 118GHz, 0.5MW, 2 sec. gyrotrons have been installed on TCV for the purpose of electron cyclotron heating at the third harmonic in the X-mode (X3). The radiation is vertically launched. In this way, the absorption is maximized because the electromagnetic beam traverses the length of the cyclotron resonance; a curve nearly following a vertical line in the poloidal plane. The plasma facing mirror can be steered, during a pulse, in the poloidal plane and can be shifted, radially, between pulses. This allows one to control the absorption in real time during a pulse. The resonance is very narrow and coupling of the beam to the resonant electrons can be lost due to refraction and/or plasma movement. As a result it is necessary to use real time control of the launcher mirror to maintain absorption. The real time control system will be described and results from its useage will be described. The vertical launch allows the X3 system to be used as a 'probe' of fast electrons. By moving the radial position of the launcher from shot to identical shot it is possible to selectively heat non-thermal electrons at energies commensurate with the shift toward the high field side from the cold resonance. Experiments studying this possibility will be described. Using X3 allows access to densities above the cut-off for second harmonic ECRH. In particular X3 gives access to core heated H-mode plasmas in TCV. Results from attempts to heat H-mode using X3 will be presented and in particular, the effect of X3 heating on ELMs will be presented.