NIFS-552

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Author(s):

T. Mutoh, R. Kumazawa, T. Seki, F. Simpo, G. Nomura, T. Ido and T. Watari

Title:

Steady State Tests of High Voltage Ceramic Feedthoughs and Co-Axial Transmission Line of ICRF Heating System for the Large Helical Device

Date of publication:

June 1998

Key words:

ICRF, steady state heating, LHD, ceramic feedthrough, co-axial transmission line, insulation gas

Abstract:

Steady state Ion Cyclotron Range of Frequency (ICRF) heating technologies have been developed to heat plasma for more than 30 minutes in the Large Helical Device (LHD). Steady state operation tests of high voltage up to 40 kV_0P for more than 30 minutes were carried out on the RF vacuum feedthroughs and the co-axial transmission line in the test set. Four types of ceramic feedthroughs each having a diameter of 240 mm were tested. The cone-type alumina ceramic and the cylinder-type silicon nitride composite-ceramic feedthroughs produced good performances of 40 kV/30 minutes and 50 kV/10 seconds. The others had vacuum leaks when subjected to a 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 minutes. Without any gas-cooling techniques, the temperature increased linearly and did not saturated. Therefore, this approach could not be used for steady state operation. The RF dissipation on the ceramic was calculated using the finite element computer code (ANSYS). It was found that damaged feedthroughs had local high heat spots, which could result in vacuum leaks. A water-cooled co-axial transmission line of 240 mm diameter was designed and tested for steady state operation. The 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 the capability of insulation for steady state. For the duration of a 10-second pulse, these gases performed well up to 60 kV_0P. However, for steady state operation, carbon dioxide gas could not withstand voltages above 40 kV_0P. The connector components of the transmission line performed without problems below 50 kV_0P and 1 kA_0P for a 30-minute steady state operation. The performance of the feedthroughs and transmission line exceeded the specifications for steady state heating in the LHD.

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