Hidetomo Oguri

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.

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).

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.

Dependence of beam emittance on plasma electrode temperature and rf-power, and filter-field tuning with center-gapped rod-filter magnets in J-PARC rf-driven H− ion sourcea)

The prototype rf-driven H(-) ion-source with a nickel plated oxygen-free-copper (OFC) plasma chamber, which satisfies the Japan Proton Accelerator Research Complex (J-PARC) 2nd stage requirements of a H(-) ion beam current of 60 mA within normalized emittances of 1.5 π mm mrad both horizontally and vertically, a flat top beam duty factor of 1.25% (500 μs × 25 Hz) and a life-time of more than 50 days, was reported at the 3rd international symposium on negative ions, beams, and sources (NIBS2012). The experimental results of the J-PARC ion source with a plasma chamber made of stainless-steel, instead of nickel plated OFC used in the prototype source, are presented in this paper. By comparing these two sources, the following two important results were acquired. One was that the about 20% lower emittance was produced by the rather low plasma electrode (PE) temperature (TPE) of about 120 °C compared with the typically used TPE of about 200 °C to maximize the beam current for the plasma with the abundant cesium (Cs). The other was that by using the rod-filter magnets with a gap at each center and tuning the gap-lengths, the filter-field was optimized and the rf-power necessary to produce the J-PARC required H(-) ion beam current was reduced typically 18%. The lower rf-power also decreases the emittances.

Recent operation status of Japan Proton Accelerator Research Complex H- ion source.

A cesium-free H(-) ion source driven with a LaB(6) filament is being operated at the Japan Proton Accelerator Research Complex for approximately three years without any serious trouble. In the beam commissioning or supply runs, the ion source has been operated in two different modes such as low current mode of 5 mA and high current mode of 30 mA. The total interruption time during the runs due to the ion source failure is approximately 50 h, which correspond to the ion source availability of 99%. After a long-term operation, the surface of the filament and the plasma electrode become discolored with dark partially. The result of surface analysis with field emission scanning electron microscope showed that most of the dark material is formed with boron. At the beam test performed in the interval of the run, we demonstrated that the H(-) current increased by miniaturizing the LaB(6) filament.

A survey of optimal filament shape in a Japan Proton Accelerator Research Complex H- ion source.

Dependence of various shapes of lanthanum hexaboride (LaB(6)) filaments on H(-) ion currents (I_H(-)s) was examined by using the first Japan Proton Accelerator Research Complex (J-PARC) test ion source. It is almost the same with the J-PARC H(-) ion source (J-PARC-IS) except for the maximum arc current (290 A instead of 400 A). An I_H(-) of 35.2 mA was extracted by using a cylindrical double-spiral LaB(6) filament with a diameter of 29.5 mm and a length of 35.5 mm, which is the same one used in J-PARC-IS. It increased to 43.4 mA with a flat triple-hairpin LaB(6) filament. The I_H(-) is considered to be increased by the enlargement of the high density plasma region near the plasma electrode aperture and the reduction of the LaB(6) filament unemission area located in the high density plasma region.

Numerical study of plasma generation process and internal antenna heat loadings in J-PARC RF negative ion source

A numerical model of plasma transport and electromagnetic field in the J-PARC (Japan Proton Accelerator Research Complex) radio frequency ion source has been developed to understand the relation between antenna coil heat loadings and plasma production/transport processes. From the calculation, the local plasma density increase is observed in the region close to the antenna coil. Electrons are magnetized by the magnetic field line with absolute magnetic flux density 30-120 Gauss which leads to high local ionization rate. The results suggest that modification of magnetic configuration can be made to reduce plasma heat flux onto the antenna.