K. Ikegami

Surface production dominating Cs-free H− ion source for high intensity and high energy proton accelerators

A Cs-free negative hydrogen (H−) ion source driven by pulsed arc plasma with a LaB6 filament is being operated for the beam tests of the Japan Proton Accelerator Research Complex (J–PARC) linac. A peak H− current of 38 mA, which exceeds the requirement of the J–PARC first stage, is stably extracted from the ion source with a beam duty factor of 0.9% (360 μs×25 Hz) by principally optimizing the surface condition and shape of the plasma electrode. The sufficiently small emittance of the beam was confirmed by high transmission efficiency (around 90%) through the following 324 MHz 3 MeV J–PARC radio frequency quadrupole linac (M. Ikegami et al., Proc. 2003 Part. Accel. Conf. 2003, p. 1509). The process of the optimization, which confirms the validity of hypothesis that H− ions are produced by surface reaction on a Mo plasma electrode dominantly in the ion source, is presented.

Interesting experimental results in Japan Proton Accelerator Research Complex H− ion-source development (invited)a)

The following interesting experimental results observed in Japan Proton Accelerator Research Complex (J-PARC) H(-) ion-source developments are reviewed. It was proven that almost all of H(-) ions were produced with surface reactions in cesium (Cs)-free J-PARC H(-) ion-sources. The worlds most intense class H(-) ion current of 38 mA in Cs-free ion sources for a high-energy linac was attained by an optimal shape and high temperature of the plasma electrode (PE), usage of a lanthanum hexaboride (LaB(6)) filament, and a newly devised high-power constant-current pulsed-arc power supply indispensable for it. It was also proven that the H(-) ion current could be increased to more than 40 mA by optimizing LaB(6)-filament shape. The surface elemental analysis of the PE after operation with a LaB(6)-filament showed that it was coated by boron (B) 95.5%, lanthanum (La) 2.5%, and oxygen (O) 1.9%. The H(-) ion current decreased by about 20% when a tungsten (W) filament was used instead of a LaB(6)-filament. The H(-) ion current could not be increased by seeding cesium (Cs) if the LaB(6)-filament was used. On the other hand, it was increased to more than 70 mA with much lower arc current of 150 A if Cs was seeded when a W-filament was used.

Acceleration of the polarized proton beam at the KEK 12 GeV ps

Abstract Acceleration of the polarized proton beam at the KEK PS up to 3.5 GeV is described. In the booster synchrotron, 75% of the linac beam polarization was preserved without any correction for the depolarizing resonances. In the main ring, almost 100% of the booster beam polarization was preserved at 3.5 GeV by the fast passage method for the intrinsic resonances and by the closed orbit correction for the imperfection resonances. The KEK PS is the first cascaded synchrotron which has demonstrated acceleration of a polarized beam.

Plasma sputter negative ion source with ECR discharge (invited)

A plasma sputter type of negative ion source with an electron cyclotron resonance (ECR) discharge has been developed at KEK. The ECR discharge was produced by a 2.45‐GHz microwave. In this ion source, negative heavy ions are produced at the surface of the metal which is placed in a Xe plasma confined by a cusp magnetic field. In preliminary experiments, the beam current of 7 mA for Cu− was obtained in pulsed mode operation.

Development of an H- ion source for Japan Proton Accelerator Research Complex upgrade.

A cesium (Cs) free H(-) ion source driven with a lanthanum hexaboride (LaB(6)) filament was adopted as an ion source for the first stage of the Japan Proton Accelerator Research Complex (J-PARC). At present, the maximum H(-) ion current produced by the ion source is 38 mA, using which J-PARC can produce a proton beam power of 0.6 MW by accelerating it with the 181 MeV linac and the 3 GeV rapid cycling synchrotron. In order to satisfy the beam power of 1 MW required for the second stage of the J-PARC in the near future, we have to increase the ion current to more than 60 mA. Therefore, we have started to develop a Cs-seeded ion source by adding an external Cs-seeding system to a J-PARC test ion source that has a structure similar to that of the J-PARC ion source except for the fact that the plasma chamber is slightly larger. As a result, a H(-) ion current of more than 70 mA was obtained from the ion source using a tungsten filament instead of a LaB(6) filament with a low arc discharge power of 15 kW (100 V, 150 A).

Development and operation of a Cs-free J-PARC H− ion sourcea)

A cesium-free H(-) ion source driven with a LaB(6) filament was developed for the J-PARC. It was operated for the J-PARC linac beam commissioning, which was started on 20 November 2006. Eight runs of 2 or 3 week beam commissioning were done until the end of June 2007. The source was mainly operated with a duty factor of 0.8% (320 micros and 25 Hz) while providing a 5 mA beam typically. Each interval of the runs, precise optimizations, such as the filament position, and so on, are examined. At present, a H(-) beam with a current of 38 mA and a rms normalized emittance of 0.22 pi mm mrad is extracted with a duty factor of 0.8% (320 micros and 25 Hz).

The cusp H− ion source at KEK

A cusp H− ion source has been operated at the 12 GeV proton synchrotron at KEK. The ion source is pulsed (200 μsec×20 Hz) and provides 15–20 mA H− beams to the linac. The long lifetime of the LaB6 cathodes resulted in stable operation.

Fine-tuning to minimize emittances of J-PARC RF-driven H⁻ ion source.

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H(-) ion source has been successfully operated for about one year. By the worlds brightest level beam, the J-PARC design beam power of 1 MW was successfully demonstrated. In order to minimize the transverse emittances, the rod-filter-field (RFF) was optimized by changing the triple-gap-lengths of each of pairing five piece rod-filter-magnets. The larger emittance degradation seems to be caused by impurity-gases than the RFF. The smaller beam-hole-diameter of the extraction electrode caused the more than expected improvements on not only the emittances but also the peak beam intensity.

Pre-conditioning procedure suitable for internal-RF-antenna of J-PARC RF-driven H− ion source

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H(-) ion source has been successfully operated for about 1 yr. By the world brightest level beam, the J-PARC design beam power of 1 MW was successfully demonstrated. Although no internal-RF-antenna failure, except for the once caused by an excess cesium due to a misoperation, occurred in the operation, many antennas failed in pre-conditionings for the first hundred days. The antenna failure rate was drastically decreased by using an antenna with coating thicker than a standard value and the pre-conditioning procedure repeating 15 min 25 kW RF-power operation and impurity-gas evacuation a few times, before the full power (50 kW) operation.

Status of the RF-driven H− ion source for J-PARC linac

For the upgrade of the Japan Proton Accelerator Research Complex linac beam current, a cesiated RF-driven negative hydrogen ion source was installed during the 2014 summer shutdown period, with subsequent operations commencing on September 29, 2014. The ion source has been successfully operating with a beam current and duty factor of 33 mA and 1.25% (500 μs and 25 Hz), respectively. The result of recent beam operation has demonstrated that the ion source is capable of continuous operation for approximately 1100 h. The spark rate at the beam extractor was observed to be at a frequency of less than once a day, which is an acceptable level for user operation. Although an antenna failure occurred during operation on October 26, 2014, no subsequent serious issues have occurred since then.