S N Murray Jr

Ramping up the Spallation Neutron Source beam power with the H- source using 0 mg Cs/day.

This paper describes the ramp up of the beam power for the Spallation Neutron Source by ramping up the pulse length, the repetition rate, and the beam current emerging from the H(-) source. Starting out with low repetition rates (< or = 10 Hz) and short pulse lengths (< or = 0.2 ms), the H(-) source and low-energy beam transport delivered from Lawrence Berkeley National Laboratory exceeded the requirements with almost perfect availability. This paper discusses the modifications that were required to exceed 0.2 ms pulse length and 0.2% duty factor with acceptable availability and performance. Currently, the source is supporting neutron production at 1 MW with 38 mA linac beam current at 60 Hz and 0.9 ms pulse length. The pulse length will be increased to approximately 1.1 ms to meet the requirements for neutron production with a power between 1 and 1.4 MW. A medium-energy beam transport (MEBT) beam current of 46 mA with a 5.4% duty factor has been demonstrated for 32 h. A 56 mA MEBT beam current with a 4.1% duty factor has been demonstrated for 20 min at the conclusion of a 12-day production run. This is close to the 59 mA needed for 3 MW neutron productions. Also notable is the Cs(2)CrO(4) cesium system, which dispenses approximately 10 mg of Cs during the startup of the ion source, sufficient for producing the required 38 mA for 4 weeks without significant degradation.

The continued development of the Spallation Neutron Source external antenna H- ion source

The U.S. Spallation Neutron Source (SNS) is an accelerator-based, pulsed neutron-scattering facility, currently in the process of ramping up neutron production. In order to ensure that the SNS will meet its operational commitments as well as provide for future facility upgrades with high reliability, we are developing a rf-driven, H(-) ion source based on a water-cooled, ceramic aluminum nitride (AlN) plasma chamber. To date, early versions of this source have delivered up to 42 mA to the SNS front end and unanalyzed beam currents up to approximately 100 mA (60 Hz, 1 ms) to the ion source test stand. This source was operated on the SNS accelerator from February to April 2009 and produced approximately 35 mA (beam current required by the ramp up plan) with availability of approximately 97%. During this run several ion source failures identified reliability issues, which must be addressed before the source re-enters production: plasma ignition, antenna lifetime, magnet cooling, and cooling jacket integrity. This report discusses these issues, details proposed engineering solutions, and notes progress to date.

Ramping Up the SNS Beam Power with the LBNL Baseline H−Source

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.

Recent performance of the SNS H(-) ion source and low-energy beam transport system.

Recent measurements of the H(-) beam current show that SNS is injecting about 55 mA into the RFQ compared to ∼45 mA in 2010. Since 2010, the H(-) beam exiting the RFQ dropped from ∼40 mA to ∼34 mA, which is sufficient for 1 MW of beam power. To minimize the impact of the RFQ degradation, the service cycle of the best performing source was extended to 6 weeks. The only degradation is fluctuations in the electron dump voltage towards the end of some service cycles, a problem that is being investigated. Very recently, the RFQ was retuned, which partly restored its transmission. In addition, the electrostatic low-energy beam transport system was reengineered to double its heat sinking and equipped with a thermocouple that monitors the temperature of the ground electrode between the two Einzel lenses. The recorded data show that emissions from the source at high voltage dominate the heat load. Emissions from the partly Cs-covered first lens cause the temperature to peak several hours after starting up. On rare occasions, the temperature can also peak due to corona discharges between the center ground electrode and one of the lenses.

Producing persistent, high-current, high-duty-factor H− beams for routine 1 MW operation of Spallation Neutron Source (invited)a)

Since 2009, the Spallation Neutron Source (SNS) has been producing neutrons with ion beam powers near 1 MW, which requires the extraction of ∼50 mA H(-) ions from the ion source with a ∼5% duty factor. The 50 mA are achieved after an initial dose of ∼3 mg of Cs and heating the Cs collar to ∼170 °C. The 50 mA normally persist for the entire 4-week source service cycles. Fundamental processes are reviewed to elucidate the persistence of the SNS H(-) beams without a steady feed of Cs and why the Cs collar temperature may have to be kept near 170 °C.

H- radio frequency source development at the Spallation Neutron Source.

The Spallation Neutron Source (SNS) now routinely operates nearly 1 MW of beam power on target with a highly persistent ∼38 mA peak current in the linac and an availability of ∼90%. H(-) beam pulses (∼1 ms, 60 Hz) are produced by a Cs-enhanced, multicusp ion source closely coupled with an electrostatic low energy beam transport (LEBT), which focuses the 65 kV beam into a radio frequency quadrupole accelerator. The source plasma is generated by RF excitation (2 MHz, ∼60 kW) of a copper antenna that has been encased with a thickness of ∼0.7 mm of porcelain enamel and immersed into the plasma chamber. The ion source and LEBT normally have a combined availability of ∼99%. Recent increases in duty-factor and RF power have made antenna failures a leading cause of downtime. This report first identifies the physical mechanism of antenna failure from a statistical inspection of ∼75 antennas which ran at the SNS, scanning electron microscopy studies of antenna surface, and cross sectional cuts and analysis of calorimetric heating measurements. Failure mitigation efforts are then described which include modifying the antenna geometry and our acceptance∕installation criteria. Progress and status of the development of the SNS external antenna source, a long-term solution to the internal antenna problem, are then discussed. Currently, this source is capable of delivering comparable beam currents to the baseline source to the SNS and, an earlier version, has briefly demonstrated unanalyzed currents up to ∼100 mA (1 ms, 60 Hz) on the test stand. In particular, this paper discusses plasma ignition (dc and RF plasma guns), antenna reliability, magnet overheating, and insufficient beam persistence.

Surface Plasma Source Electrode Activation by Surface Impurities

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.

Recent Advances in the Performance and Understanding of the SNS Ion Source

The ion source developed for the Spallation Neutron Source (SNS) by Lawrence Berkeley National Laboratory (LBNL), is a radio frequency, multi‐cusp source designed to produce ∼ 40 mA of H− with a normalized rms emittance of less than 0.2 pi mm mrad. To date, the source has been utilized in the commissioning of the SNS accelerator and has already demonstrated stable, satisfactory operation at beam currents of ∼30 mA with duty‐factors of ∼0.1% for operational periods of several weeks. Once the SNS is fully operational in 2008, a beam current duty‐factor of 6% (1 ms pulse length, 60 Hz repetition rate) will be required in order to inject the accelerator. To ascertain the capability of the source to deliver beams at this high duty‐factor over sustained time periods, several experimental runs have been conducted, each ∼1 week in length, in which the ion source was continuously operated on a dedicated test stand. The results of these tests are reported as well as a theory of the Cs release and transport processe...

Ion Source Development at the SNS

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 a...

Towards Understanding the Cesium Cycle of the Persistent H− Beams at SNS

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.