The first plasma of the Large Helical Device(LHD) having super conducting windings is scheduled on the March in 1998. The ICRF heating will also begin in 1998. The one of the main objectives of the LHD ICRF heating is steady state operation of over 30 minutes. Therefore the various ICRF heating components have being developed in the test stand. High power and steady state operation tests of the transmitter and the liquid stub tuners were successfully done and already reported. In this conference, we present the results of the steady state and high voltage test of the coaxial transmission line and ceramic feedthroughs and test antenna set in the vacuum chamber.

The transmission line is composed of a large outer diameter of 240mm aluminum coaxial line having the characteristic impedance of 50 ohm. Both inner and outer conductors are water cooled to remove the dissipated RF power loss. The insulation spacers are made by Teflon plates with small recess to reduce the local electric field concentration. We tested the three kinds of insulation gas filling inside at 3 atmosphere. Nitrogen, CO2 and SF6 gases are tested at high RF standing wave voltage. Test frequency is 43 MHz. For the pulsed operation of 10 second, all three cases of insulation gas can stand without any breakdown at over 60kV(0-peak) RF voltage. On the contrary, for the steady state operation, high voltage test was successfully done at 41kV/30minutes in the case of nitrogen gas and at 50kV/30minutes in the case of SF6 gas. But in the case of CO2 gas, it does not stand for over 30kV/ several minutes.

Various types of vacuum feedthrough ceramics are tested in the test stand. Four types of alumina ceramic feedthroughs and one Si3N4 composite ceramic feedthrough are tested. Among them the cone shape alumina ceramics which is cooled by circulating gas from the outside and the cylindrical shape Si3N4 composite ceramics worked very well. They can stand for the RF voltage over 40kV and 30 minutes. The other type ceramics can not stand for the high voltage and long pulse operation over 20kV/30minutes. The finite element method analysis shows the reduction of the local concentration of dielectric loss dissipation is effective to reduce the mechanical stress. The experimental data are consistent with these calculation.