Search

After acquiring several reliable spare targets, SNS ramped the beam power from 850 kW to 1.4 MW, which required an increase in H- beam pulse length from 0.88 to 1.0 ms at 60 Hz. This increase initially produced slow 2-MHz power ramp-ups and, after several weeks of uninterrupted operation, it produced plasma outages every time the pulse length was raised above ~0.95 ms. Similar outages were previously observed towards the end of long service cycles, which were believed to indicate that the breakdown voltage of the high purity hydrogen started to exceed the induced electric fields. In 2011 the RF was reconfigured to start with 10 cycles of 1.96 MHz, which yielded the shortest H- beam rise times and apparently eliminated those plasma outages. The new, pulse-length dependent outages were eliminated by increasing the initial frequency to 1.985 MHz. However, careful frequency studies are unable to justify this frequency. In addition, the paper discusses the issues and solutions for the electron-dump voltage, which starts to sag and become unstable after several weeks of high current operation. [ABSTRACT FROM AUTHOR]

This paper describes the performance of the SNS H- ion source and LEBT as they continue to deliver ∼50 mA H- beams at a 5.3% duty factor required for neutron production with a ∼1MW proton beam since the fall of 2009. The source continues to deliver persistent H- beams for up to 6 weeks without adding Cs after an initial dose of ∼4 mg, except when there are excessive plasma impurities. In one case the H- beam decayed due to an air leak, which is shown to be consistent with sputtering of the Cs layer, and which allows us to bracket the plasma potential. In another case, the performance of two sources degraded progressively, which appears to be consistent with a progressive deterioration of the Cs covered Mo converter. These two and other recently discovered issues are discussed in detail. [ABSTRACT FROM AUTHOR]

This paper describes the accomplishments of the SNS H- ion source, which delivers routinely ∼50 mA at a 5.4% duty factor with ∼99% availability, enabling 1 MW beams for neutron production with ∼90% availability. It discusses the need for increasing reliability and beam current. But mostly it focuses on its unexpected feature: H- beams that are apparently persistent for up to 5 weeks without adding Cs after an initial dose of less than ∼5 mg. Thermal emission and sputtering are qualitatively evaluated, and appear consistent with a negligible Cs sputter rate after the initial dose disappears from the Cs plasma. It concludes with a list of future experiments that can shed more light on this apparently unique Cs cycle. [ABSTRACT FROM AUTHOR]

The Spallation Neutron Source (SNS) now routinely operates near 1 MW of beam power on target with a highly-persistent ∼38 mA peak current in the linac and an availability of ∼90%. The ∼1 ms-long, 60 Hz, ∼50 mA H- beam pulses are extracted from a Cs-enhanced, multi-cusp, RF-driven, internal-antenna ion source. An electrostatic LEBT (Low Energy Beam Transport) focuses the 65 kV beam into the RFQ accelerator. The ion source and LEBT have normally a combined availability of ∼99%. Although much progress has been made over the last years to achieve this level of availability further improvements are desirable. Failures of the internal antenna and occasionally impaired electron dump insulators require several source replacements per year. An attempt to overcome the antenna issues with an AlN external antenna source early in 2009 had to be terminated due to availability issues. This report provides a comprehensive review of the design, experimental history, status, and description of recently updated components and future plans for this ion source. The mechanical design for improved electron dump vacuum feedthroughs is also presented, which is compatible with the baseline and both external antenna ion sources. [ABSTRACT FROM AUTHOR]

LBNL designed and built the Frontend for the Spallation Neutron Source, including its H- source and Low-Energy Beam Transport (LEBT). This paper discusses the performance of the H- source and LEBT during the commissioning of the accelerator, as well as their performance while ramping up the SNS beam power to 540 kW. Detailed discussions of major shortcomings and their mitigations are presented to illustrate the effort needed to take even a well-designed R&D ion source into operation. With these modifications, at 4% duty factor the LBNL H- source meets the essential requirements that were set at the beginning of the project. [ABSTRACT FROM AUTHOR]