ICRF plasma production at hydrogen minority regime in LHD

This study aim is to develop further an ion cyclotron range of frequencies (ICRF) method of plasma production in stellarators based on the minority heating. The previous studies demonstrate production of low density plasma (9.5 × 1017m−3) at low power of up to 0.2 MW. The higher ICRF heating power experiments become possible after introducing a programmable ICRF power ramp up at the front of the ICRF pulse. With this trick, all the shots went with the antenna voltage within the safe range. Increase of the ICRF power predictably results in increase of the density of produced plasma. Without pre-ionization the plasma density achieved was 6 × 1018 m−3 which is 6 times higher than in previous experiments. However, the electron temperature was not high, the light impurities were hot fully stripped, and there were no recombination peaks after termination of the ICRF pulse. Plasma density is too low to provide good conditions for efficient plasma heating. For the reference, the ICRF heating of high density cold plasma prepared by electron cyclotron resonance heating is performed. Both electrons and ions were heated to high temperatures, and this plasma state is sustained. The antenna–plasma coupling was much better which result in larger heating power with the lower antenna voltage.