Lower hybrid current ramp-up in the PLT tokamak

A study of radiofrequency current ramp-up in the PLT tokamak is reported. The plasma current was first raised to 200-300 kA by the Ohmic heating transformer, and the current in the transformer primary circuit was then held constant to remove the OH drive. After the current fell below 200 kA, up to 300 kW of the toroidally directed RF power at 800 MHz was transmitted into the PLT plasma via a 6-element phased waveguide array. Current ramp-up rates between 0 and 120 kA/s for a 0.35 s time interval ((1/2-1/3)L/R time) were measured at densities between 2 and 4x1012cm-3. It is estimated that about 20% of the RF energy introduced into the vacuum vessel was converted into poloidal magnetic field energy, LI2/2, where L approx.= 3 μH is the total inductance of the plasma current loop. This conversion ratio should depend on a variety of factors, including the percentage of RF power absorbed by resonant electrons and the magnitude of the back current induced by the changing poloidal flux LI. The high ramp-up efficiencies are predicted theoretically in the regime in which the PLT ramp-up experiments operate, i.e. where the phase velocity of the waves is approximately equal in magnitude to the runaway velocity due to the back voltage. Comparison of the raw data with theory suggests that about 1/2 to 3/4 of the incident RF power is absorbed by resonant high-velocity electrons. (author)