A comparison of SNS internal and external antenna source performance with and without cesium

Experiments comparing an internal and an external antenna H^- source at the Spallation Neutron Source with and without cesium revealed key performance differences which provide insight to the source physics and will guide the development of an RF H^- source at the ISIS Neutron Source. RF power sweeps were taken for each of these cases, for which the total charge, electron to H^- ratio, and H^- extracted per kW are all studied and plotted. At around 40 kW and typical hydrogen flow and cooling parameters, cesiated sources output 35 mA square beam pulses where uncesiated sources output 15 mA. At these settings, the beam pulse for the internal source initially overshoots, while it is flat for the external source. This observation is discussed with the difference in coupling between the antenna, the plasma, and the outlet. Sweeps of pulsed RF repetition rate and the collar temperature only affected cesiated sources, which is attributed to surface processes affecting the H^- production only in the presence of cesium. Possible future experiments using a fast optical measurement and modifications to the gas flow into the external source's plasma gun and by removal of the collar assembly in uncesiated operation are discussed.