Plasma impedance measurement of uncesiated RF-driven negative hydrogen ion source in CSNS

We developed and optimized an RF-driven H- source at the China Spallation Neutron Source (CSNS) to achieve the requirement of project phase-II. At the beginning of 2019, the first uncesiated H- beam was produced, but 15% RF power was reflected when the forward power was 20 kW. To decrease the reflection of the RF power, the RF power matching network was optimized based on the measurement of the plasma equivalent impedance, and the reflected power was decreased to less than 200W at the same forward power. The plasma equivalent impedance is studied with different conditions (RF power, hydrogen flow rate, and frequency). The results show that the plasma equivalent resistance increases at first and then decreases as the RF power increase. Then the transformer model is adopted to derive the plasma impedance.