Current sustaining of a tokamak plasma by a travelling RF field

Momentum transfer from a travelling RF field to the electrons in a toroidal plasma is analysed to generate a DC current which is proportional to the RF power absorption and inversely proportional to the phase velocity of the RF field. The RF power necessary to sustain the current in a tokamak plasma is found to be high at a phase velocity in the range of electron thermal velocity. In the case where an electron transit time magnetic pumping plays a dominant role in power absorption, the optimum phase velocity which minimize the RF power with moderate strength of RF field is obtained to be considerably low. The necessary RF power to sustain the current for a medium size tokamak (R ∼ 1.5 m, B_t ∼ 4 T, I_p ∼ 1 MA T_e ∼ 5 KeV) is estimated to be several mega-watts. While for a fusion reactor (thermal output ∼ 2 GW), estimated power is some ten mega-watts. (author)