The first ICRF heating experiment in the large helical device

The first experiment of the ion cyclotron range of frequencies (ICRF) heating in the Large Helical Device (LHD) was carried out at the end of 1998. The LHD is a large superconducting heliotron device and its first plasma was produced in March 1998. During the ICRF heating experiment, a maximum 300 kW/0.2 s of ICRF power was injected into the LHD plasma by using a pair of loop antennae. This paper reports on the installation of the loop antennae, the results of antenna coupling and the first heating experiments. The antennae are designed to operate in the steady state and to change their distance from the plasma by 0-15 cm. In the experiment, the antenna resistance coupled with the plasma was measured by changing the distance between the last closed flux surface and the launcher front from 9 cm to 5 cm. The resistance was almost doubled by decreasing the distance. The target plasma was produced by the second harmonic electron cyclotron heating (ECH) of 84 GHz gyrotrons at a magnetic field of 1.5 T and a low plasma electron density of less than 1 × 1019m-3 . Therefore, the low coupling resistance limited the maximum injected power to less than 300 kW. The heating efficiency and heating species were varied by the minority ion gas-puffing rate. The heating characteristics were compared with a one-dimensional full-wave analysis code, and the experimental results were consistent with wave damping analysis. For the optimum condition of the minority hydrogen gas-puff ratio, the plasma internal energy increased from 13 kJ to 26 kJ by almost the same power as the ECH power.