Second harmonic electron cyclotron heating for the extraordinary mode in lower hybrid current driven plasma has been investigated experimentally in the JFT-2M tokamak. Effective electron heating is observed at largely downshifted cyclotron frequencies as well as at the usual cyclotron resonance frequency. An absorption efficiency of 15% at f0/2fc0 = 0.76 is estimated from measurements of the Shafranov parameter and the diamagnetic flux (f0 is the frequency of the RF wave and fc0 is the cyclotron frequency at the plasma centre). From a measurement of the transmission of radiated microwaves the absorption efficiency is deduced. The dependences of the absorption efficiency on the toroidal magnetic field obtained from the two measurements are consistent with each other within experimental error. From measurements of the soft X-rays and the electron cyclotron emission it is deduced that there is selective perpendicular heating of the fast electrons which satisfies the relativistic cyclotron resonance condition. The absorption efficiency obtained is in reasonable agreement with a theoretical prediction taking mildly relativistic electrons into account. Selective cyclotron heating of 70 kW produces a 65 kA/s ramp-up of the lower hybrid driven plasma current; a resulting ramp-up efficiency of 30% is deduced.
Radiofrequency plasma breakdown in a modular stellarator