The scenario of toroidal plasma start-up with microwave initiation and heating near the electron cyclotron frequency is suggested and examined in this paper. Microwave irradiation from the high-field side and an anomalously large absorption of the extraordinary waves near the upper hybrid resonance are assumed. The dominant electron energy losses are assumed to be due to magnetic-field curvature and parallel drifts, ionization of neutrals, cooling by ions, and radiation by low-Z impurities. It is shown by particle and energy balance considerations that electron temperatures around 250 eV and densities of 1012-1013 cm−3 can be maintained, at least in a narrow region near the upper hybrid resonance, with modest microwave powers in the Impurity Study Experiment (ISX) (120 kW at 28 GHz) and The Next Step (TNS) (0.57 MW at 120 GHz). The loop voltages required for start-up from these initial plasmas are also estimated. It is shown that the loop voltage can be reduced by a factor of five to ten from that for unassisted start-up without an increase in the resistive loss in volt-seconds. If this reduction in loop voltage is verified in the ISX experiments, substantial savings in the cost of power supplies for the Ohmic heating (OH) and equilibrium field (EF) coils can be realized in future large tokamaks.
Electron-cyclotron heating of plasma in toroidal systems