Steady-state tests of high-voltage ceramic feedthroughs and coaxial transmission line for ICRF heating system of the large helical device

Steady-state ion cyclotron range of frequency (ICRF) heating technologies have been developed to heat plasma for >30 min in the Large Helical Device (LHD). Steady-state-operation tests of high voltages up to 40 kV(Op) for >30 min were carried out on radio-frequency (rf) vacuum feedthroughs and a coaxial transmission line in a test set. Four types of ceramic feedthroughs, each having a 240-mm diameter were tested. Cone-type alumina ceramic and cylinder-type silicon nitride composite-ceramic feedthroughs produced good performances of 40 kV/30 min and 50 kV/10 s. The others had vacuum leaks when subjected to long-pulse duration. The temperature of the cone-type alumina ceramic feedthrough was measured during the ICRF operations. By using gas-cooling techniques, the temperature increase of the ceramic was substantially reduced and saturated within 20 min. Without any gas-cooling techniques, the temperature increased linearly and did not saturate. Therefore, this approach could not be used for steady-state operation. The rf dissipation on the ceramic was calculated using the ANSYS finite element computer code. It was found that damaged feedthroughs had local high heat spots, which could result in vacuum leaks. A 240-mm-diam water-cooled coaxial transmission line was designed and tested for steady-state operation. Specially designed connector components and Teflon insulator disks were tested. During the test operation, the insulation gases of nitrogen, sulfur hexafluoride, and carbon dioxide were used to compare their insulation capabilities for steady state. For the duration of a 10-s pulse, these gases performed well up to 60 kV(Op). However, for steady-state operation, carbon dioxide gas could not withstand voltages >40 kV(Op). The connector components of the transmission line performed without problems below 50 kV(Op) and 1 kA(Op) for 30-min steady-state operation. The performance of the feedthroughs and transmission line exceeded the specifications for steady-state heating in the LHD.