RF preionization in tokamak Thor - Effect of sub-harmonics of Omega sub 0 on the absorption of the wave

During the study of the RF preionization in Thor tokamak an interesting feature was observed: the starting of the plasma and the time behavior of its density were correlated with the presence of the resonance conditions both at the electron cyclotron frequency and at its subharmonics. A simple qualitative calculation shows that this effect is due to the radial gradient of the magnetic field typical of a toroidal geometry.