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During the past year the Spallation Neutron Source has operated with proton beam powers up to 1.4 MW with good availability. However, after a recent premature target failure the power was reduced to 1 MW for the ~20-week service cycle of the new target, whereas the subsequent target will be used at 1.2 MW. The ion source and low-energy beam transport deliver 50 to 60 mA H- beams into the RFQ with availbilities of ~99.5%. The compromised RFQ transmission limits the LINAC beam to ~35 mA, which is sufficient for 1.4 MW. Our current injector efforts focus on improving reliability and increasing the source service cycles beyond the achived 7 A·h to reduce the venting of the RFQ and to improve the stability of the accelerator operations. In addition, extraction studies show that higher extraction fields improve the RFQ transmission, apparently due to a reduced emittance of the LEBT H- beams. [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]

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]

The emittance is an important characteristic of propagating charged particle beams, because it allows the prediction of focus size and beam losses. After introducing the concept and various definitions of the emittance and Twiss parameters, useful applications of this representation will be explored for the transverse emittances of charged particle beams. This is followed by a presentation of different measuring methods, together with their advantages and disadvantages. The second part discusses the analysis of emittance data, and possible mitigations for effects introduced by noise, bias, and other artifacts. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

For over 20 years, Allison scanners have been used to measure emittances of low-energy ion beams. We show that scanning large trajectory angles produces ghost signals caused by the sampled beamlet impacting on an electric deflection plate. The ghost signal strength is proportional to the amount of beam entering the scanner. Depending on the ions, and their velocity, the ghost signals can have the opposite or the same polarity as the main beam signals. The ghost signals cause significant errors in the emittance estimates because they appear at large trajectory angles. These ghost signals often go undetected because they partly overlap with the real signals, are mostly below the 1% level, and often hide in the noise. A simple deflection plate modification is shown to reduce the ghost signal strength by over 99%. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents the SCUBEEx rms emittance analysis, a self-consistent, unbiased elliptical exclusion method, which combines traditional data-reduction methods with statistical methods to obtain accurate estimates for the rms emittance. Rather than considering individual data, the method tracks the average current density outside a well-selected, variable boundary to separate the measured beam halo from the background. The average outside current density is assumed to be part of a uniform background and not part of the particle beam. Therefore the average outside current is subtracted from the data before evaluating the rms emittance within the boundary. As the boundary area is increased, the average outside current and the inside rms emittance form plateaus when all data containing part of the particle beam are inside the boundary. These plateaus mark the smallest acceptable exclusion boundary and provide unbiased estimates for the average background and the rms emittance. Small, trendless variations within the plateaus allow for determining the uncertainties of the estimates caused by variations of the measured background outside the smallest acceptable exclusion boundary. The robustness of the method is established with complementary variations of the exclusion boundary. This paper presents a detailed comparison between traditional data reduction methods and SCUBEEx by analyzing two complementary sets of emittance data obtained with a Lawrence Berkeley National Laboratory and an ISIS H[sup -] ion source. [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]

The High Intensity Neutrino Source (HINS) program at Fermilab (formerly the Proton Driver) aims to develop a multi-mission linear accelerator (LINAC) capable of accelerate H- ions to 8 GeV. This paper touches on the ion source requirements for the HINS and discusses long pulse length testing of three ion sources which appear to have the capability of meeting these requirements. © 2007 American Institute of Physics [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]