Chip Piller

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.

Surface plasma source with saddle antenna radio frequency plasma generator

A prototype RF H(-) surface plasma source (SPS) with saddle (SA) RF antenna is developed which will provide better power efficiency for high pulsed and average current, higher brightness with longer lifetime and higher reliability. Several versions of new plasma generators with small AlN discharge chambers and different antennas and magnetic field configurations were tested in the plasma source test stand. A prototype SA SPS was installed in the Spallation Neutron Source (SNS) ion source test stand with a larger, normal-sized SNS AlN chamber that achieved unanalyzed peak currents of up to 67 mA with an apparent efficiency up to 1.6 mA∕kW. Control experiments with H(-) beam produced by SNS SPS with internal and external antennas were conducted. A new version of the RF triggering plasma gun has been designed. A saddle antenna SPS with water cooling is fabricated for high duty factor testing.

Performance of the H−Ion Source Supporting 1‐MW Beam Operations at SNS

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory reached 1‐MW of beam power in September 2009, and now routinely operates near 1‐MW for the production of neutrons. This paper reviews the performance, operational issues, implemented and planned mitigations of the SNS H− ion source to support such high power‐level beams with high availability. Some results from R&D activities are also briefly described.

Recent performance of and plasma outage studies with the SNS H⁻ source.

Spallation Neutron Source ramps to higher power levels that can be sustained with high availability. The goal is 1.4 MW despite a compromised radio frequency quadrupole (RFQ), which requires higher radio frequency power than design levels to approach the nominal beam transmission. Unfortunately at higher power the RFQ often loses its thermal stability, a problem apparently enhanced by beam losses and high influxes of hydrogen. Delivering as much H(-) beam as possible with the least amount of hydrogen led to plasma outages. The root cause is the dense 1-ms long ∼55-kW 2-MHz plasma pulses reflecting ∼90% of the continuous ∼300 W, 13-MHz power, which was mitigated with a 4-ms filter for the reflected power signal and an outage resistant, slightly detuned 13-MHz match. Lowering the H2 gas also increased the H(-) beam current to ∼55 mA and increased the RFQ transmission by ∼7% (relative).

Recent performance and ignition tests of the pulsed SNS H− source for 1-MW neutron production

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, wh...

Characterization of the CW starter plasma RF matching network for operating the SNS H− ion source with lower H2 flows

The Spallation Neutron Source H(-) ion source is operated with a pulsed 2-MHz RF (50-60 kW) to produce the 1-ms long, ∼50 mA H(-) beams at 60 Hz. A continuous low power (∼300 W) 13.56-MHz RF plasma, which is initially ignited with a H2 pressure bump, serves as starter plasma for the pulsed high power 2-MHz RF discharges. To reduce the risk of plasma outages at lower H2 flow rates which is desired for improved performance of the following radio frequency quadrupole, the 13.56-MHz RF matching network was characterized over a broad range of its two tuning capacitors. The H-α line intensity of the 13.56-MHz RF plasma and the reflected power of the 13.56-MHz RF were mapped against the capacitor settings. Optimal tunes for the maximum H-α intensity are consistent with the optimal tunes for minimum reflected power. Low limits of the H2 flow rate not causing plasma outages were explored within the range of the map. A tune region that allows lower H2 flow rate has been identified, which differs from the optimal tune for global minimum reflected power that was mostly used in the past.

The status of the SNS external antenna ion source and spare RFQ test facility

The Oak Ridge National Laboratory operates the Spallation Neutron Source, consisting of a H(-) ion source, a 1 GeV linac and an accumulator ring. The accumulated <1 μs-long, ∼35 A beam pulses are extracted from the ring at 60 Hz and directed onto a liquid Hg target. Spalled neutrons are directed to ∼20 world class instruments. Currently, the facility operates routinely with ∼1.2 MW of average beam power, which soon will be raised to 1.4 MW. A future upgrade with a second target station calls for raising the power to 2.8 MW. This paper describes the status of two accelerator components expected to play important roles in achieving these goals: a recently acquired RFQ accelerator and the external antenna ion source. Currently, the RFQ is being conditioned in a newly constructed 2.5 MeV Integrated Test Facility (ITF) and the external antenna source is also being tested on a separate test stand. This paper presents the results of experiments and the testing of these systems.

Saddle antenna radio frequency ion sources

Existing RF ion sources for accelerators have specific efficiencies for H(+) and H(-) ion generation ∼3-5 mA/cm(2) kW, where about 50 kW of RF power is typically needed for 50 mA beam current production. The Saddle Antenna (SA) surface plasma source (SPS) described here was developed to improve H(-) ion production efficiency, reliability, and availability. In SA RF ion source, the efficiency of positive ion generation in the plasma has been improved to 200 mA/cm(2) kW. After cesiation, the current of negative ions to the collector was increased from 1 mA to 10 mA with RF power ∼1.5 kW in the plasma (6 mm diameter emission aperture) and up to 30 mA with ∼4 kW RF. Continuous wave (CW) operation of the SA SPS has been tested on the test stand. The general design of the CW SA SPS is based on the pulsed version. Some modifications were made to improve the cooling and cesiation stability. CW operation with negative ion extraction was tested with RF power up to ∼1.2 kW in the plasma with production up to Ic = 7 mA. A stable long time generation of H(-) beam without degradation was demonstrated in RF discharge with AlN discharge chamber.

Status and Performance of ORNL Spallation Neutron Source Accelerator Systems

The Spallation Neutron Source (SNS) accelerator systems have been performing continuously and progressively since commissioning in 2006 to deliver the neutrons to beamlines. The 1.4 MW design beam power has been demonstrated during 24/7 operation while developments and investigations for system improvements are still ongoing to achieve the full design beam power and availability. Numerous difficulties that impeded reaching the full performance of the SNS accelerator systems have been identified and are being eliminated through repairs, upgrades, and developments. In this report, operational performance and developments of the accelerator systems are presented along with the efforts for future upgrades of the SNS.