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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]

In experiments with RF saddle antenna surface plasma sources (SPS), the efficiency of H- ion generation was increased by up to a factor of 5 by plasma electrode 'activation', without supplying additional Cs, by heating the collar to high temperature for several hours using hot air flow and plasma discharge. Without cracking or heating the cesium ampoule, but likely with Cs recovery from impurities, the achieved energy efficiency was comparable to that of conventionally cesiated SNS RF sources with an external or internal Cs supply. In the experiments, optimum cesiation was produced (without additional Cs) by the collection and trapping of traces of remnant cesium compounds from SPS surfaces. Such activation by accumulation of impurities on electrode surfaces can be a reason for H- emission enhancement in other so-called 'volume' negative 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]

© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Noble atoms are chemically inert because their full-shell electronic configuration strongly binds all electrons to the parent atom; therefore, noble atoms have ionization energies that are substantially higher than the ionization energies of non-noble atoms. This is different for ions, for which the ionization energy increases with every charge state, although the increases are much larger whenever a new shell is opened. As long as the energy of the ionizing electron is far above the ionization energy threshold, comparisons need to focus on ionization cross sections, rather than ionization energies, because only the ionization cross section takes into consideration the number of electrons that can be ionized. As long as the K-shell is filled, cross sections for direct ionization fall off approximately geometrically, meaning that every time an electron is removed, it becomes roughly 30% more difficult to directly remove the next electron, a charge state dependence that is only minimally affected by the structure of the remaining electrons. For this reason, evolution of charge states calculated with cross sections for direct ionization predict only minimal enhancements for noble ions—ions with a full-shell configuration. [ABSTRACT FROM AUTHOR]