Long pulse test of the KSTAR ICRF antenna with water-cooling

The KSTAR ICRF (ion cyclotron range of frequency) antenna is being developed and tested for a high-power and long pulse operation. In the previous test campaign, the standoff capability was increased to 31.2 kVp (kV peak) from 24.3 kVp for a 300 s pulse duration by applying water-cooling to the ICRF antenna, but it was limited by an overheating of the vacuum feedthrough (VF) and the transmission line of the unmatched section which did not have any cooling channel. Prior to the RF test campaign in 2005, water-cooling system for the VF and the transmission line of the unmatched section was developed to remove the dissipated RF heat load. For a cooling of the central conductors of the VF and transmission line, the joining part of the two transmission lines was carefully designed and fabricated to ensure no water leakage and to have a tight electrical contact. During the RF testing, a cooling water was fed by a coaxial tube inside the central conductors and it flowed out through the space between the center conductor and the coaxial tube. Outer conductors near the current maximum were also water-cooled by using Al cooling blocks which had a cooling channel inside them. The high voltage tests with long pulse durations were performed at a frequency of 30 MHz. A bottom half of the current strap-1 was connected to the RF source and the other three straps were shorted at the input ports. The achieved standoff voltage was 41.3 kVp for a 300 s operation, which was much higher than that of the previous campaign. The maximum standoff voltage for a 20 s pulse was 46.0 kVp. A voltage of 41.3 kVp is equivalent to a heating power of 7.4 MW in the case of a plasma loading of 6 Ω/m. To simulate a steady state operation, much longer pulse tests were also performed. As a result, we achieved standoff voltages of 35.0 kVp for 600 s and 27.9 kVp for 1000 s.