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.

Upgrading the LANSCE accelerator with a SNS RF-driven H- ion source.

The LANSCE accelerator is currently powered by a filament-driven, biased converter-type H- ion source that operates at 10%, the highest plasma duty factor for this type of source, using only ∼2.2 SCCM of H2. The ion source needs to be replaced every 4 weeks, which takes up to 4 days. The measured negative beam current of 12-16 mA falls below the desired 24 mA acceptance of the LANCSE accelerator. The SNS (Spallation Neutron Source) RF-driven, H- ion source injects ∼50 mA of H- beam into the SNS accelerator at 60 Hz with a 6% duty factor and an availability of >99.5% but requires ∼30 SCCM of H2. Up to 7 A h of H- have been produced during the 14-weeks-long source service cycles, which is unprecedented for small emittance, high-current, pulsed H- ion sources. The emittance of the SNS source is slightly smaller than the emittance of the LANSCE source. The SNS source also features unrivaled low Cs consumption and can be installed and started up in <12 h. LANSCE and SNS are working toward the use of SNS H- ion sources on the LANSCE accelerator because they could (a) fill the LANSCE accelerator to its capacity, (b) decrease the source replacement time by a factor of up to 7, and (c) increase source lifetime by a factor of about 4. This paper discusses some of the challenges that emerge when trying to match a different H- source into an existing injector with significantly different characteristics and operating regimes.