The J-PARC linac has successfully accelerated the beam to the design value of 181 MeV
 
The J-PARC linac accelerated negative hydrogen beam up to 181 MeV on January 24th. The beam energy was measured by means of time-of-flight. The peak current, the beam pulse length, and the repetition rate were 5 mA, 20 μsec and 2.5 Hz, respectively. These values were selected for the purpose of the initial beam study, to avoid a possible damage to accelerator components, if something would go wrong at high beam power. During the beam commissioning, both RF power source and cavity system were very stable. This stability together with a very accurate alignment of all the magnets, especially in the drift-tube linac, were the two major elements to allow the rapid tuning of the linac. The acceleration to the full energy was achieved three months earlier than scheduled.

"J-PARC" is an acronym of the Japan Proton Accelerator Research Complex under construction at Tokai, Japan, which is approximately 110 km north of Tokyo. This is a joint project between the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Agency (JAEA). The accelerator system comprises a 400-MeV proton linac (181 MeV at the first stage of Phase I), a 3-GeV, 25-Hz Rapid-Cycling Synchrotron (RCS), and a 50-GeV Main Ring Synchrotron. The RCS provides the Materials and Life Science Experimental Facility (MLF) with the 1-MW beam to generate the pulsed muons and the pulsed spallation neutrons. Every three seconds, the RCS beam is injected to the MR four times. Then, the beam is ramped up to 50 GeV (40 GeV at Phase I) to be fast-extracted for the neutrino production or to be slowly extracted to the Hadron Experimental Facility (HDF). The neutrinos thus produced are sent to the SUPER KAMIOKANDE detector located 295-km west, while the slowly extracted beam will be used to produce secondary beams for the hyper-nuclei experiments, the Kaon rare decay experiments, hadron spectroscopy experiments and so forth, or to perform primary beam experiments in the HDF. The J-PARC construction started in April, 2001. The beam was accelerated by the Radio-Frequency Quadrupole (RFQ) linac up to 3 MeV on November 20th, 2006 which was just the same day as the beam commissioning started. Then, it was accelerated by the first tank of the Drift-Tube Linac (DTL) up to 20 MeV on December 19th, 2006 and by the second and third tanks of the DTL up to 50 MeV next day. All the thirty separated-type DTL (SDTL) cavities driven by fifteen klystrons were ready for the acceleration up to 181 MeV at midnight on January 19th. Afterwards, a phase scan for each pair of the SDTL cavities driven by one klystron was tried and, finally, the scan through fifteen pairs was completed on January 24th. In each scan the beam energy was measured by the time-of-flight method.

The J-PARC DTL has quadruple electromagnets, which are variable focusing elements, while driven by klystrons. In order to meet these conflicting requirements, the RF acceleration frequency was chosen as 324 MHz rather than widely used 350 MHz, the compact quadrupole electromagnets were developed by fully utilizing the electroforming and wire-cutting techniques, and the 324-MHz klystrons with a pulsed power of 3 MW (500 μsec and 50 Hz) were first developed by a manufacturer in close collaboration with the J-PARC linac team. The combination of 3-MeV RFQ linac and the 324-MHz RF source began to be considered as the best choice of the front end of proton linac by many future projects of FNAL, ISIS, GSI FAIR and CSNS, partly because the 324 (or 325) MHz, 3 MW pulsed klystrons are available now, and partly because the frequency of 324 (or 325) MHz is one quarter of L-band frequency, which will be used for the superconducting International Linear Collider.

The RCS beam commissioning will start this fall, while the beam commissioning of the MR and the muon and neutron production in the MLF will start in May, 2008. By the end of 2008, the beams will be provided for the J-PARC users. The earlier-than-scheduled success of the linac beam commissioning should be very encouraging for the users-to-be.
 

The author Professor Yoshishige Yamazaki is the Leader of the Accelerator Division at the J-PARC Center. He also serves as a Deputy Director of the J-PARC Center..