High Power Long Pulse Experiment by ICRF Heating in LHD

Long pulse discharge experiments have been conducted at LHD as one of the challenges in the quest for a fusion reactor. The plasma could be sustained for 48 min, using 1.2 MW of heating power from the ion cyclotron range of frequencies (ICRF) and electron cyclotron heating (ECH) power. The line-averaged electron density was 1.2 × 1019m-3, the ion and electron temperature was 2 keV, and the input energy reached 3.36 GJ. The ICRF antennas used were the HAS (handshake form) and the FAIT (field-aligned-impedance-transforming) type, which upgraded and replaced the existing PA (poloidal array) antennas, in addition to the existing PA antenna. These antennas had the following characteristics. The PA one had a Faraday shield on one antenna strap removed. The HAS antenna’s straps were aligned in the toroidal direction and was phase controllable in the toroidal direction. The FAIT antenna had a built-in impedance transformer to obtain high plasma loading resistance. Long pulse discharges are often terminated by an influx of impurities, which are the result of exfoliation of deposits left during the discharges. Overcoming this impurity problem is one of the challenges of a steady state operation.