The first ICRF heating experiment in the Large Helical Device

The first experiment of the ion cyclotron range of frequencies (ICRF) heati ng in the Large Helical Device (LHD) was carried out at the end of 1998. Th e LHD is a large superconducting heliotron device and its first plasma was produced in March 1998. During the ICRF heating experiment, a maximum 300 k W/0.2 s of ICRF power was injected into the LHD plasma by using a pair of l oop antennae. This paper reports on the installation of the loop antennae, the results of antenna coupling and the first heating experiments. The ante nnae are designed to operate in the steady state and to change their distan ce from the plasma by 0-15 cm. in the experiment, the antenna resistance co upled with the plasma was measured by changing the distance between the las t closed flux surface and the launcher front from 9 cm to 5 cm. The resista nce was almost doubled by decreasing the distance. The target plasma was pr oduced by the second harmonic electron cyclotron heating (ECH) of 84 GHz gy rotrons at a magnetic field of 1.5 T and a low plasma electron density of l ess than 1 x 10(19) m(-3). Therefore, the low coupling resistance limited t he maximum injected power to less than 300 kW. The heating efficiency and h eating species were varied by the minority ion gas-puffing rate. The heatin g characteristics were compared with a one-dimensional full-wave analysis c ode, and the experimental results were consistent with wave damping analysi s. For the optimum condition of the minority hydrogen gas-puff ratio, the p lasma internal energy increased from 13 kJ to 26 kJ by almost the same powe r as the ECH power.