Characterization of the CW starter plasma RF matching network for operating the SNS H^- ion source with lower H₂ flows

The Spallation Neutron Source H^- ion source is operated with a pulsed 2-MHz RF (50-60 kW) to produce the 1-ms long, ∼50 mA H^- beams at 60 Hz. A continuous low power (∼300 W) 13.56-MHz RF plasma, which is initially ignited with a H₂ pressure bump, serves as starter plasma for the pulsed high power 2-MHz RF discharges. To reduce the risk of plasma outages at lower H₂ flow rates which is desired for improved performance of the following radio frequency quadrupole, the 13.56-MHz RF matching network was characterized over a broad range of its two tuning capacitors. The H-α line intensity of the 13.56-MHz RF plasma and the reflected power of the 13.56-MHz RF were mapped against the capacitor settings. Optimal tunes for the maximum H-α intensity are consistent with the optimal tunes for minimum reflected power. Low limits of the H₂ flow rate not causing plasma outages were explored within the range of the map. A tune region that allows lower H₂ flow rate has been identified, which differs from the optimal tune for global minimum reflected power that was mostly used in the past.