Overview of ICRF plasma production and heating in gas mixtures in stellarators

The study of plasma production and heating in the ion cyclotron range of frequencies (ICRF) has a long history in fusion research. The ICRF discharges have been studied in stellarators, tokamaks and mirror devices, mainly. The possibility of efficient additional plasma heating using ICRF is one of the main aims of the studies. Plasma production using ICRF discharges is also studied, but to a lesser extent. For the purpose of wall conditioning in both pure gases and their mixtures has been used with lower RF power level. In support of the ICRF experiments for plasma production at Wendelstein 7-X, studies on the development of an ICRF start-up scenario were initiated on the Uragan-2M (U-2M) stellarator. Experiments with a controlled minority of hydrogen in helium atmosphere showed a significant increase in the resulting plasma density compared with pure helium and pure hydrogen. Then, the ICRF plasma production was demonstrated on the LHD with the scenario based on U-2M experiment. Successful experiments at U-2M and LHD showed that the ICRF start-up scenario can be scaled up to large stellarator devices, producing plasma with favorable parameters from scratch using ICRF only.