RF experiments in LHD

Large Helical Device (LHD) has been started its operation from the end of March 1998. Second harmonic electron cyclotron heating (ECH) is used as a plasma production and heating. Three gyrotrons have been used at the frequency range of 84 GHz as power sources. Up to 420 kW power with the pulse length of 0.5 sec has been injected in LHD via long distance corrugated waveguide transmission system. The antenna system composed of four mirrors for each line focuses the microwave beam with the 15 mm in 1/e waist radius on the midplane of LHD in the radial direction that enables the local power deposition. The maximum stored energy with using only second harmonic ECH recorded so far is 36 kJ at the averaged density of ne,ave=1.5×1019 m−3. The maximum central electron temperature Te0=2 keV is achieved at ne,ave=3×1018 m−3. Power modulation experiments are carried out mainly to estimate the power deposition profile using ECE radiometer. The initial ICRF heating experiment was also carried out using a pair of loop antennas. The applied ICRF power is at the level of 300 kW for 0.2 sec with the frequency of 25.6 MHz. The increase in the plasma stored energy was up to 13 kJ using the target plasma produced by ECH. Under the optimum H minority ratio to He, significant electron heating without any appreciable increase in the density was observed.