The Spallation Neutron Source (SNS) located at the Oak Ridge National Laboratory is an accelerator-based, pulsed neutron facility utilized for a broad range of scientific research and industrial applications. In the past 5 years, the facility has reliably been operated at its baseline design beam power of 1.4 MW, and it is currently undertaking an upgrade to double its power to 2.8 MW. A 65-keV H- injector, which comprises an rf-driven H- ion source and an electrostatic low energy beam transport, delivers the required high-current, time-structured H- beam to the accelerator. The H- injector can reliably provide 50-60 mA beam current at 6% duty-factor (1 ms, 60 Hz) for a 3-4 month service cycle. To ensure sufficient operational margins for the SNS routine operations and ongoing upgrade requirements, the injector system has been continuously improved on a R&D test stand. This paper presents the operational status and recent advancements with the injector system performance including the enhanced ion source beam output capability up to similar to 110 mA, the improved ion source plasma ignition reliability using a 27.12-MHz RF system, and the upgraded LEBT beam chopper system etc.

The introduction of this chapter explains the use of pulsed high-current H− ion sources. It then discusses the radio frequency-driven H− ion sources serving advanced accelerators starting with the award-winning SNS ion source. It explains all important characteristics in detail, including limitations and lifetimes. A table lists over 30 characteristics and performance data. Designed at LBNL and fully developed at SNS/ORNL, the success of this H− source is evidenced by the fact that since 2014, three other advanced, high-current (~40 to 60 mA) accelerators switched to this type of H− ion source and others will switch within a few years. The three new RF-driven, high-current H− sources supplying advanced accelerators are then described and compared in the table. This chapter ends with an outlook and the projects that develop new RF-driven H− sources for additional long-pulse, high-current, advanced H− accelerators.

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 from spallation of liquid Hg by bombardment of short (similar to 1 mu s), intense (similar to 35 A) pulses of protons delivered at 60 Hz by a storage ring which is fed by a high-intensity, similar to 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 several months with near 100% availability. Ion source research and development at ORNL has played a key role in enabling and supporting this success: this report provides an update on some of the ongoing ion source research and development efforts which have been undertaken since the previous Negative Ion Beams and Sources (NIBS) conference in 2020. These include significant improvements to H- beam current by extraction from a larger source outlet aperture and improvements to the electron dumping system which should eliminate the gradual loss of electrode voltage over the course of a run which has occasionally impacted SNS operations. Improvement and simplification of the plasma ignition system for the external antenna ion source, a long-standing problem, was also realized. Lastly, RF coupling efficiency was measured for both the SNS internal and external antenna ion sources.

High brightness, negative hydrogen ion sources are used extensively in scientific facilities operating worldwide. Negative hydrogen beams have become the preferred means of filling circular accelerators and storage rings as well as enabling efficient extraction from cyclotrons. Several well-known facilities now have considerable experience with operating a variety of sources such as RF-, filament-, magnetron- and Penning-type H - ion sources. These facilities include the US Spallation Neutron Source (SNS), Japan Proton Accelerator Research Complex (J-PARC), Rutherford Appleton Laboratory (RAL-ISIS), Los Alamos Neutron Science Center (LANSCE), Fermi National Accelerator Laboratory (FNAL), Brookhaven National Laboratory (BNL), numerous installations of D-Pace (licenced by TRIUMF) ion sources used mainly on cyclotrons and, most recently, the CERN-LINAC-1 injector. This report first summarizes the current performance of these ion sources in routine, daily operations with attention toward source service-periods and availability metrics. Sustainability issues encountered at each facility are also reported and categorized to identify areas of common concern and key issues. Recent ion source improvements to address these issues are also discussed as well as plans for meeting future facility upgrade requirements.

The U.S. Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is the world's most powerful accelerator-driven pulsed neutron source. An H- injector feeds the accelerator with high current (>50 mA), time structured (1ms, 60 Hz) H- beam. The injector consists of an RF-driven, Cs-enhanced H- ion source and an electrostatic low energy beam transport section. In the recent three run cycles, an H- ion source operated 4 months for each run with excellent availability near 99.9%. The presently ongoing run is also on track to finish another similar to 4 months run with similar high availability. Lately, we have tested a solid-state RF system for the ion source to replace the existing vacuum-tube type RF amplifier for the sake of improving the ease of operation and maintenance. Progress has also been made on the continued development of an external antenna RF H- ion source with a focus on improvement to its plasma ignition scheme.

The Spallation Neutron Source (SNS) accelerator system includes a 65 keV H- injector, a 2.5 MeV radio frequency quadrupole (RFQ), a 1 GeV linac chain (DTL-CCL-SCL) and an accumulator ring. The H− injector consists of an RF-driven, Cs-enhanced, multi-cusp H− ion source and a compact, two-lens electrostatic low energy beam transport (LEBT). In 2018, a new RFQ was installed in place of the original RFQ along with upgraded ion source support infrastructure. This paper presents the operational performance and some highlights of beam studies with the SNS ion source since the new RFQ was put into operation. SNS routinely operates at 1.4 MW average beam power for three run cycles per year. During the most recently finished run, FY20B, we operated a single source for the entire run spanning ~4 months with a >99.5% availability. A single dose of cesiation was conducted during the ion source startup which yielded ≥54 mA current for the entire run with just few minor adjustments of parameters.

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 engineering, physics, chemistry, material science, and biology. Neutrons are produced by spallation of liquid Hg by the bombardment of short (∼1 µs), intense (∼35 A) pulses of protons delivered at 60 Hz by an accumulator ring which is fed by a high-intensity, 1 GeV, H- LINAC (linear accelerator). This facility has operated nearly continuously since 2006 but has recently undergone a 4-month maintenance period, which featured a complete replacement of the 2.5 MeV injector feeding the LINAC. The new injector was developed at ORNL in an off-line beam test facility and consists of an ion source, low energy beam transport, and a Radio Frequency Quadrupole (RFQ). This report first describes the installed configuration of the new injector detailing the ion source system. The first beam current, RFQ transmission, emittance, and energy measurements from the injector installed on the SNS are reported. These data not only show a significant performance improvement for our existing facility but will also make accessible the higher beam current requirements for future SNS upgrade projects: the proton power upgrade and second target station.

The LANSCE accelerator is currently powered by a filament-driven, biased converter-type H- ion source that operates at 10%, the highest plasma duty factor for this type of source, using only ∼2.2 SCCM of H2. The ion source needs to be replaced every 4 weeks, which takes up to 4 days. The measured negative beam current of 12-16 mA falls below the desired 24 mA acceptance of the LANCSE accelerator. The SNS (Spallation Neutron Source) RF-driven, H- ion source injects ∼50 mA of H- beam into the SNS accelerator at 60 Hz with a 6% duty factor and an availability of >99.5% but requires ∼30 SCCM of H2. Up to 7 A h of H- have been produced during the 14-weeks-long source service cycles, which is unprecedented for small emittance, high-current, pulsed H- ion sources. The emittance of the SNS source is slightly smaller than the emittance of the LANSCE source. The SNS source also features unrivaled low Cs consumption and can be installed and started up in <12 h. LANSCE and SNS are working toward the use of SNS H- ion sources on the LANSCE accelerator because they could (a) fill the LANSCE accelerator to its capacity, (b) decrease the source replacement time by a factor of up to 7, and (c) increase source lifetime by a factor of about 4. This paper discusses some of the challenges that emerge when trying to match a different H- source into an existing injector with significantly different characteristics and operating regimes.

Experiments comparing an internal and an external antenna H- source at the Spallation Neutron Source with and without cesium revealed key performance differences which provide insight to the source physics and will guide the development of an RF H- source at the ISIS Neutron Source. RF power sweeps were taken for each of these cases, for which the total charge, electron to H- ratio, and H- extracted per kW are all studied and plotted. At around 40 kW and typical hydrogen flow and cooling parameters, cesiated sources output 35 mA square beam pulses where uncesiated sources output 15 mA. At these settings, the beam pulse for the internal source initially overshoots, while it is flat for the external source. This observation is discussed with the difference in coupling between the antenna, the plasma, and the outlet. Sweeps of pulsed RF repetition rate and the collar temperature only affected cesiated sources, which is attributed to surface processes affecting the H- production only in the presence of cesium. Possible future experiments using a fast optical measurement and modifications to the gas flow into the external source's plasma gun and by removal of the collar assembly in uncesiated operation are discussed.

The ORNL team will provide LANL with a scoping / feasibility study of H- sources which could potentially be developed to approach the goal of producing a single high-duty-factor H- beam of up to ~100 mA suitable for acceleration by an RFQ. Currently, no existing H- sources have been shown to operate anywhere near this goal so the study will focus on identifying the most promising approach and define the existing option space. Attention will be given to source concepts which are most likely to be scalable to continuous beam operation while offering lower-risk, lower duty-factor or beam current operation in the shorter term. The study will discuss the technology within this option space, tradeoffs and identify the best path forward. Once the best option is identified, an estimate of the expected output emittance, beam current, achievable beam duty-factor, hydrogen and cesium consumption rates, physical size and weight estimates of the source and supporting subsystems will be provided. This work will lay the foundation for developing a conceptual source design in Development 2 which will inform an actual mechanical design that will be used to either construct a prototype source or modify an existing source as part of a future technology development program for the project. The relative technology development risk and mitigations would also be defined for the candidate ion source technologies for a TRL 5 demonstration. Risk analysis for later TRL 6 phases and beyond would be part of a later phase of the project.

The SNS (Spallation Neutron Source) (radiofrequency) RF-driven, Hion source injects ~50 mA of Hbeam into the SNS accelerator at 60 Hz with a 6% duty factor. It injects up to 7 A·hrs of Hions during its ~14week service cycles, which is an unprecedented lifetime for small-emittance, high-current pulsed Hion sources. The SNS source also features unprecedented low cesium consumption and can be installed and started up in <10 h. Presently, the LANSCE (Los Alamos Neutron Science CEnter) accelerator complex in Los Alamos is fed by a filament-driven, biased converter-type Hsource that operates with a high plasma duty factor of 10%. It needs to be replaced every 4 weeks with a ~4 day startup phase. The measured negative beam current of 16-18 mA falls below the desired 21 mA acceptance of LANSCE’s accelerator especially since the beam contains several mA of electrons. LANSCE and SNS are exploring the possibility of using the SNS RF Hsource at LANSCE to increase the Hbeam current and the ion source lifetime while decreasing the startup time. For this purpose, the SNS Hsource has been tested at a 10% duty factor by operating it at 120 Hz with 840 μs plasma pulses generated with ~30 kW of 2 MHz RF power, and extracting ~25 mA around-theclock for 28 days. This, and additional tests and other considerations are discussed in this paper.

RE-driven (internal and external antenna), Cs-enhanced H- ion sources at the Spallation Neutron Source (SNS) facility are capable of producing high current (>60 mA), high duty-factor (1 ms, 60 Hz) H- beams lasting multiple months. A solid reaction Cs dispenser system is used to cesiate the ion source. This work reports on our continuous effort to optimize the ion source cesiation process guided by the Cs signal observed with the plasma emission spectroscopy to mitigate the Cs induced high voltage sparking while assuring to enable high If beam currents.

The U.S. Spallation Neutron Source (SNS) now operates with 1.2 MW of beam power on target with the near term 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 (WIT) 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.

A positive ion source with RF discharge in solenoidal magnetic field is described. In this paper we present an overview of positive ion production in saddle antenna (helicon discharge) radio frequency (SA RF) ion sources. The efficiency of H+ ion production in recently developed RF sources with solenoidal antennas was improved to 2.9 mA/kW. About 24 kW of RF power is typically needed for 70 mA beam current production from a 7 mm emission aperture. This efficiency is relatively low because in the RF discharge with a solenoidal antenna, the plasma is generated near the coil and diffuses to the axis creating a nearly uniform plasma density distribution in all cross sections of the discharge chamber, when the plasma flow is necessary only near an emission aperture. The efficiency of the extracted ion generation was improved significantly by using a saddle antenna with solenoidal magnetic field. In the RF discharge with the saddle antenna the plasma is generated near the axis and the magnetic field suppress the plasma diffusion from the axis, creating a peaked plasma density distribution on the emission aperture. With the SA the efficiency of positive ion generation in the plasma has been improved up to ~100 mA/cm2 per kW of RF power at 13.56 MHz. Continuous wave (CW) operation of the RF source has been tested on the small ORNL SNS test stand. The general design of the CW RF source is based on the pulsed version. A compact design of ion source is presented. Some modifications were made to improve the cooling and to simplify the design. Features of SA RF discharges and ion generation are discussed.

The Spallation Neutron Source operates reliably at 1.2 MW and will gradually ramp to 1.4 MW. This paper briefly recalls some of the struggles when the unprecedented project was started and ramped to 1 MW over a 3½ year period. This was challenging, especially for the H− ion source and the low-energy beam transport system, which make up the H− injector. It took several more years to push the H− injector to the 1.4 MW requirements, and even longer to reach close to 100% injector availability. An additional breakthrough was the carefully staged, successful extension of the H− source service cycle so that disruptive source changes became rare events. More than 7 A·h of extracted H− ions have been demonstrated with a single source without maintenance, more than twice the single-source quantity of ions produced by any other high-current H− accelerator facility. Achieving the 1.4 MW requirements with close to 100% availability and record-breaking source service cycles were the basis for the 2017 Brightness Award.

The Spallation Neutron Source (SNS) Linear Accelerator (Linac) delivers a high power proton beam (>1 MW) for neutron production with high neutron availability (>90%). For beam acceleration, the linac has both normal and superconducting RF sections, with the Superconducting Linac (SCL) portion providing the majority of beam acceleration (81 of 96 RF cavities are superconducting). Operationally, the goal is to achieve the highest possible beam energy by maximizing SCL cavity RF gradients, but not at the expense of cavity reliability [1, 2]. One mechanism that has negatively impacted both SCL cavity peak RF gradients and reliability is beam lost into the SCL due to malfunctions of upstream components. Understanding the sources and impact of errant beam on SCL cavity performance will be discussed.

It is assumed that persistent cesiation in the SNS RF SPS is related to deposition of carbon film on the collar converter. The work function dependence for graphite with alkali deposition has no minimum typical for metals and semiconductors and the final work function is higher. For this reason, the probability of H- secondary emission from cesiated metal and semiconductors can be higher than from cesiated carbon films but the carbon film maintains cesiation longer and can operate with low cesium consumption.

The Spallation Neutron Source (SNS) beam chopping system uses a segmented electrostatic lens in the Low Energy Beam Transport (LEBT) line to deflect the beam out of the Radio Frequency Quadrupole (RFQ) input aperture to create gaps in the 1 ms beam macro-pulse for extraction from the Ring, or fully displace the beam. The lens is split azimuthally into four quadrants which are pulsed independently by four bipolar high voltage pulse generators. The chopper timing control system creates trigger pulses to the pulse generators which deflect the beam sequentially to four positions on the chopper target. In the present chopper configuration, all four segments are powered simultaneously with a 1 MHz burst repetition rate within the macro-pulse. To improve chopping performance, faster switches and higher voltages are required. An alternative chopping system configuration which can meet this request has been proposed, where only two opposite segments are used at a time. This will facilitate pulse generator performance by reducing switching frequency and power dissipation in the high voltage switches while operating at increased voltages, and make beam deflection more effective, stable and reliable. The new chopping configuration requires changes in the LEBT timing control patterns, upgrading the pulse generator, and changing the azimuthal position of the lens segments in the LEBT structure. This paper will review the timing control patterns for present and suggested configurations, compare the pulse generator performance for both cases, and show the advantages of the new chopping modes. The results of the simulated beam distribution at the RFQ input for different deflecting voltages will also be presented.