Developing reliable internal antennas and standardizing performance of H^- RF ion sources

The Spallation Neutron Source (SNS) now routinely operates near 1 MW of beam power on target with 30-40 mA peak current in the linac and an overall availability of ∼90%. H^- beam pulses (∼1 ms, 60Hz) are produced by a RF-driven, Cs-enhanced, multi-cusp ion source closely coupled to an electrostatic Low Energy Beam Transport (LEBT) which focuses the beam into an RFQ accelerator. The ion source and LEBT normally have a combined availability of ∼99%. The source plasma is generated by RF excitation (2MHz, ∼60kW) of a copper antenna which has been encased with a thickness of ∼0.7 mm of porcelain enamel and is immersed into the plasma chamber. Failure of this coating material during operations has been a long-standing problem and a cause of downtime. This report describes new antenna installation criteria which have led to trouble free antenna performance over this last year, a significant improvement over previous years. These results suggest that inclusions and defects in the antenna coating play an important role in the failure process. This report also addresses the problem of inconsistent performance among our inventory of five ostensibly identical baseline SNS ion sources. Over the last several years only source #3 has regularly provided the required beam current to the SNS by outperforming the others by 5-10 mA. To address this, all baseline sources have been characterized on the ion source test stand and small physical differences between source #3 and the others have been identified and the impact of those differences on beam production assessed.