Progress in Steady-State Plasma Operation Using ICRF Heating on LHD

As one of the main objectives of operation of the Large Helical Device (LHD), long-pulse plasma discharge experiments have been carried out using ion cyclotron range of frequency (ICRF) heating power (mainly using a minority heating method). Discharges with electron density n_e < 10¹⁹ m^-3 and T_e0 > 1 keV have been sustained with an ICRF heating power around [approximately]1 MW and an electron cyclotron heating power of [approximately]0.1 MW. The total injected heating energy exceeded 1.6 GJ. Many technological improvements were made before undertaking long-pulse plasma discharges, e.g., the installation of a steady-state high-rf power generator and a liquid stub tuner capable of real-time impedance matching. Over the past decade, the achieved pulse length has increased to 1 h. One of the keys to this success was dispersion of the local plasma heat load onto divertors, which was accomplished by cyclically sweeping the magnetic axis inward and outward. Eventually, the plasma terminated due to the penetration of impurities, which originated from the release of thin flakes on the divertor plates. The pulse length was extended by installing new divertor plates with better thermal conduction. A mode conversion heating scenario has been considered as an alternative to the minority ICRF heating scenario; the former may have advantages due to the lack of an ion cyclotron resonance layer in front of the antennas in the mode conversion case.