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The U.S. Spallation Neutron Source (SNS) is a state-of-the-art neutron scattering facility delivering the world's most intense pulsed neutron beams to a wide array of instruments which are used to conduct investigations in many fields of science and engineering. Neutrons are produced by spallation of liquid Hg due to bombardment of short (∼1µs), intense (∼35 A) pulses of protons delivered at 60 Hz from a storage ring which is fed by a high-intensity, 1 GeV H− LINAC. This facility has operated almost continuously since 2006, with ion source performance increasing over those years, and currently providing 50-60 mA of H− ions with a duty-factor of 6% for maintenance-free runs of ∼120 days with near 100% availability. Ion source research and development at ORNL has played a key role in enabling and supporting these achievements: this report provides some historical highlights of this effort, describes our current ion source testing facilities, discusses the goals of the program and finally provides a snapshot of some of our ongoing R&D activities. In particular, we have recently simplified and improved the reliability of the plasma ignition gun for the external antenna ion source which is discussed in detail. [ABSTRACT FROM AUTHOR]

The U.S. Spallation Neutron Source (SNS) now operates with 1.2 MW of beam power on target with the nearterm goal of delivering 1.4 MW and a longer-term goal of delivering >2 MW required by the planned Proton Power Upgrade (PPU) and Second Target Station (STS) projects. In early 2018 we plan to replace the entire 2.5 MeV injector configuration which includes the ion source, the Low Energy Beam Transport (LEBT) and the Radio Frequency Quadrupole accelerator (RFQ) with one which is currently being tested on a research accelerator called the Beam Test Facility (BTF) located at the SNS facility. This report first provides a description of the new injector: the ion source and the cage enclosure, the new LEBT support structure and the new RFQ. Since fall of 2016, this system has been tested extensively with regard to output beam current, beam persistence, emittance and energy. The results of these experiments employing both internal and external antenna ion sources will be summarized here showing the system to be capable of supporting SNS 1.4 MW operations with a significant margin as well as potentially meeting facility upgrade requirements. This represents a significant performance upgrade over the current SNS front end system with the compromised RFQ and will replace that system in early 2018. [ABSTRACT FROM AUTHOR]

The U.S. Spallation Neutron Source (SNS) now operates with 1.2 MW of beam power on target with the nearterm goal of delivering 1.4 MW and a longer-term goal of delivering > 2 MW to support a planned second target station. Presently, H- beam pulses (50-60 mA, 1 ms, 60 Hz) from an RF-driven, Cs-enhanced, multi-cusp ion source are first accelerated to 2.5 MeV by a Radio Frequency Quadrupole (RFQ) accelerator, injected into a ~1 GeV linac, compressed to <1μs in an accumulator ring and ultimately delivered to a liquid mercury target for pulsed neutron production. In recent years concerns about the RFQ performance has motivated the procurement of a new RFQ and the creation of a Beam Test Facility (BTF) to allow off-line testing. The purpose of the BTF is to first validate performance of the new RFQ before installing it in place of the existing RFQ and later to serve as a stand-alone 2.5 MeV research accelerator employing the original SNS RFQ. After validating the new RFQ with respect to energy, emittance and transmission, the initial applications of the BTF will be to conduct 6D beam dynamic studies, develop & demonstrate ion sources capable of meeting the current and future requirements of the SNS, and contribute to neutron moderator development. This report provides a facility update, description of the BTF ion source systems as well as a discussion of the first LEBT and RFQ beam current measurements performed at the BTF. [ABSTRACT FROM AUTHOR]

The U.S. Spallation Neutron Source (SNS) now operates with ~1 MW of beam power to target with the near-term goal of delivering 1.4 MW. Plans are being considered to incorporate a second target station into the facility which will require ~2.8 MW of beam power. Presently, H- beam pulses (~1 ms, 60 Hz) are produced by an RF-driven, Cs-enhanced, multi-cusp ion source which injects beam into an RFQ (Radio Frequency Quadrupole) accelerator that, in turn, feeds the SNS Linac. Currently the source/RFQ system delivers ~35 mA of pulsed current to the linac which is mostly sufficient for 1.4 MW operations while ~50 mA are needed for the second target station upgrade. This paper provides a look forward for the SNS by providing (i) the present and future SNS source/RFQ beam requirements and our plans to achieve these, (ii) a description and status of the external antenna ion source being developed for the replacement of the current internal antenna ion source, and (iii) a description and status of the newly constructed RFQ test facility. [ABSTRACT FROM AUTHOR]

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]