The 3 GeV-RCS (Rapid Synchrotron) in J-PARC (Japan Accelerator Research Complex) project accelerates a proton beam of 0.333 mA up to 3GeV. At 25 Hz repetition rate the RCS generates a high-power beam of 1 MW. In such an accelerated proton beam, neutrons as well as gamma rays will be generated. The cumulative energy dose will be of the order of 100 MGy for 30 years of operation. So as to minimize the radiation exposure during maintenance, it is necessary to compose the vacuum system with reliable components which have long life time in such a high level of radiation. In addition, it is necessary to keep the operating pressure with beam in ultra high vacuum for suppressing the pressure instability. Thus we should think of not only the outgassing mainly due to ion desorption but also the pumping efficiency.
From the above, the vacuum system was designed as follows. The ring is divided by the isolation valves into 6 sections (3 straight and 3 arc sections), which can be pumped down independently. For avoiding any eddy current loss ceramic ducts are used in the bending and focusing magnets. These ducts are connected to the Ti ducts, putting the Ti bellows between. Here, we adopt pure Ti as a material for the ducts and bellows because of its small residual radioactivity. The ring is evacuated with 20 ion pumps (0.7 m3/s) and 20 turbomolecular pumps (TMPs) (1.3 m3/s), which are attached to the Ti ducts. The TMPs are used for not only rough pumping but also evacuation during the beam operation. Especially a collimator system for localizing beam losses in a restricted area is evacuated with the TMPs, because the outgassing from this region will be probably the largest. In addition, the straight sections for beam injection and extraction are mainly pumped down by the TMPs. On the other hand, each arc section is pumped by 4 ion pumps and 2 TMPs.
To realize the above system, we have already succeeded in developing some components such as large aperture ceramic ducts and TMPs with high radioactive-resistance, as well as several kinds of heat treatment to reduce the outgassing. Now we have almost installed the vacuum system. The performance tests will be started before long.
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