Influence of RF Driven Frequency on the Tokamak Glow Discharge Cleaning Characteristics

In order to study the influence of RF driven frequency on the glow discharge cleaning characteristics, RF frequencies of 2.1 MHz, 13.56 MHz and 40.68 MHz are employed to perform radio-frequency glow discharge cleaning (RF-GDC) during the initial discharge phase of HL-2M. Results show that the breakdown pressure of 2.1 MHz RF-GDC is 2.0×10⁻¹ Pa, while that of 13.56 MHz is 5.6×10⁻¹ Pa with a preset power of 1500 W. However, the breakdown happens at 9.2×10⁻¹ Pa only when DC bias voltage is added for the 40.68 MHz case. The impurity cleaning efficiency is also studied by monitoring the residual gas composition during the RF-GDC with different driven frequencies. Results show that, due to the more heated electrons at low frequency, the 2.1 MHz RF-GDC has the highest impurity yield. Additionally, although the 40.68 MHz RF-GDC has the lowest power coupling efficiency, its impurity yield is still higher than 13.56 MHz RF-GDC due to its high plasma density. The results indicated that the 2.1 MHz RF-GDC has the best cleaning performance. However, because the power coupling efficiency of different frequency is different, which also have an impact on the cleaning efficiency, the impedance matching problem should be solved to improve the coupling power in the next step.