In Japan the KEK high-energy physics laboratory and the Japan Atomic Energy Research Institute are working together to finalize preparations for a major new high-intensity proton source, with applications in a number of sectors.
In Japan, plans for a major new proton complex also reflect a major administrative reorganization. Originally, the KEK high energy laboratory had a hadron accelerator project called the Japan Hadron Facility (JHF), which consisted of a 50 GeV proton synchrotron and a 3 GeV booster ring where the projected power of the latter was 0.6 MW.
The Japan Atomic Energy Research Institute (JAERI), on the other hand, had a high-power spallation neutron source project with a proton linac, in which 3 MW pulsed beams were planned for neutron scattering and 5 MW continuous beams were planned for nuclear transmutation.
Since both projects share a common goal to attain high-power proton beams, in the summer of 1998 the Government suggested a joint effort between KEK and JAERI for a single high-intensity proton facility in Japan.
Monbu-sho (the Ministry that supports KEK) and STA (the Science and Technology Agency, which supports JAERI) will merge in January 2001. Therefore, the 1998 suggestion also implied that the government wanted to initiate a project supported by both agencies.
Endorsement
After lengthy discussions, KEK and JAERI agreed in March 1999 to collaborate to create a single high-intensity proton accelerator proposal and a formal memorandum of understanding was signed by the directors of the two institutions. A joint proposal was published and reviewed in April 1999 by an international committee chaired by Yanglai Cho of Argonne. The committee strongly endorsed the proposal.
The project, which is to be constructed at the JAERI Tokai site, will consist of:
- A 400 MeV proton linac (normal conducting) to inject beams into the 3 GeV proton synchrotron;
- A superconducting linac to accelerate protons from 400 to 600 MeV, and used primarily for experiments toward nuclear transmutation;
- A 25 MHz 3 GeV proton synchrotron with 1 MW power, primarily for life and material sciences with neutrons and muons;
- A 50 GeV proton synchrotron delivering 15 mA and two extraction modes – slow extraction for kaon, pion and primary beams, and fast extraction for neutrino beams to the Superkamiokande detector.
The budget of the project is about ¥189 billion (approximately $1.89 billion when $1 = ¥100). According to a new law in Japan, any major scientific project must satisfy a government-organized third-party review committee. In this case the third party committee must include a range of people, such as scientists (physicists, chemists, biologists, etc), institutional administrators, journalists, economists and company presidents.
The Joint Project was assigned as the first case for such a third-party review, and the committee members were nominated in the late autumn of 1999. Their draft report strongly supports the project, despite its cost. This report will influence the policy decision, and it is hoped that official approval will be given for construction to start in the financial year beginning in April 2001.
Accelerator network
Phase I of the project comprises a 600 MeV linac, a 3 GeV 1 MW rapid-cycling synchrotron (RCS) and a 50 GeV main synchrotron. The Phase I facility could be upgraded to a 5 MW neutron source, which would be Phase II of the project.
One half of the 400 MeV beam from the linac will be injected to the RCS, while the other half will be further accelerated to 600 MeV by a superconducting (SC) linac. The 3 GeV beam from the RCS will be injected to the 50 GeV synchrotron. The 600 MeV beam from the SC linac will be transported to the experimental area for an accelerator-driven nuclear waste transmutation system (ADS). The 3 GeV beam from the RCS will be mainly used to produce pulsed spallation neutrons and muons. The muon-production and neutron-production targets will be located in series in the Life and Materials Science Experimental Area. Ten percent of the beam will be used for muon production.
The 50 GeV beam will be slow extracted to the Particle and Nuclear Physics Experimental Area and fast extracted for the neutrino experiment at the Superkamiokande detector 300 km away.
Producing the protons
A volume-production type negative hydrogen ion (H–) source is designed to produce a peak current of 53 mA with a pulse length of 500 ms and a repetition rate of 50 Hz. About 53% of the beam will be accelerated after the beam is chopped at both the 50 keV low-energy beam transport and the 3 MeV medium-energy beam transport.
If the present scheme is successful, one of the most important key technologies will be in place.
The radiofrequency quadrupole (RFQ) linac will accelerate the beam up to 3 MeV, the conventional drift-tube linac (DTL) up to 50 MeV and the separated DTL (SDTL) up to 200 MeV. Here, an acceleration frequency of 324 MHz was chosen. The frequency will be increased by a factor of three at 200 MeV.
Among the possible candidates for the coupled-cavity linac to be used from 200 to 400 MeV, the annular-ring-coupled structure (ACS) is most preferable because of its axial symmetry. Several prototypes of the L-band ACS have been developed and powered beyond the design value. The 400 MeV H– beam from the linac will be injected into the RCS during 500 ms, limited by the sinusoidally varying magnetic field of the 25 Hz RCS.
The beam will be chopped at twice the ring radiofrequency of 1.36 MHz (two bunches per ring) to avoid beam loss during injection. The RCS will thus accelerate two bunches (4 x 1013 protons per bunch) every 40 ms. Eight of the 10 buckets of the 50 GeV ring will be filled by four cycles of the RCS. Then the 50 GeV synchrotron will ramped for 1.9 s. The beam will be slowly extracted during 0.7 s. Afterwards it will take 0.7 s for the synchrotron to be ready for the next injection. In total, the period of one beam cycle will be 3.42 s, corresponding to an average current of 15.4 mA.
The purpose of the SC linac is to develop the necessary accelerator technology for the ADS nuclear waste transmutation experiment. If the present scheme is successful, one of the most important key technologies will be in place.
Construction of the 60 MeV proton linac began on the KEK site in 1998. The beam commissioning of the ion source and the RFQ linac will begin soon. Since these two components were designed for a peak current of 30 mA, they will be replaced for the JHF project. However, the beam could be used for testing the DTL and SDTL.
After construction and beam commissioning of the 60 MeV linac have been completed in the JAERI-KEK collaboration, the linac will be shipped to Tokai for the Joint Project.
