Development of the Molybdenum Millimeter Wave Target Tile for ECRH Antenna Alignment after the Evacuation in LHD

Abstract

Alignment of the quasi-optical millimeter wave antenna for the electron cyclotron resonance heating (ECRH) can be required even after the evacuation of the vacuum vessel in fusion oriented plasma experimental devices. We developed a molybdenum millimeter wave target tile and installed it as one of the protection tiles of the vacuum vessel wall of the Large Helical Device (LHD) on the opposite side of the ECRH antennas. Inside the target tile, a sheathed thermocouple is mounted to measure the temperature increase during the millimeter wave injection toward this tile. In magnetic field configurations where neither the first nor the second electron cyclotron resonance (ECR) exists, the measured temperature increased sufficiently to be detected by the millimeter wave injection of approximately 100 kW for 0.1 s toward the target tile. Under the assumption that the distance between the target point that brings the largest temperature increase and the target point just above the thermocouple head gives the offset of the target point to be corrected, the offset of the antenna injection angle was confirmed with an accuracy of 0.4 • with changing the target point shot by shot.