Design issues for high-intensity, high-energy proton accelerators

Abstract

High-intensity, high-energy proton beams are required in various fields of science and industry, including pulsedspallation neutron experiments, nuclear-physics experiments, and nuclear-waste transmutation. We have various possible accelerator schemes for these purposes. The advantages and disadvantages of the parameter choices are summarized while emphasizing the importance of understanding the haloformation mechanisms in order to settle various controversial issues. The beam current to be accelerated is actually limited by the amount of beam loss, which is critically dependent upon the amount of beam halo, both longitudinal and transverse. The optimum design is also dependent upon the future performances of the key components, such as highintensity, low-emittance ion sources. Thus, we should concentrate our efforts on the development of these components in order to realize these machines. Some examples of the efforts being made in this direction are presented. Introduction and the Time Structure of a Beam The scope of this paper is to list various topical controversial issues concerning the design of high-intensity (typically more than 0.1 mA), high-energy (more than 1 GeV, but less than several 10 GeV) proton accelerators, and to hopefully present possible solutions, or to propose directions for further research and development. Examples of these machines are listed in Table 1 [1-8]. The optimum design of an accelerator is dependent upon its detailed specifications. The specifications for intensity and energy are still insufficient for optimizing the design. Other important factors are the time structure and emittance of the beam. Typical examples of useful time structures are shown in Fig. 1 ( a few 100 ns, a few 10 ns, CW or nearly CW). The beam as shown in Fig. 1 b) is required for spallation neutron experiments [10] with a high energy resolution, based upon the time-of-flight method. That shown in Fig. 1 c) is useful for muon spin rotation/resonance/relaxation experiments [11] in order to study mainly material science. An average current as high as possible is required for nuclear-waste transmutation/incineration [12], while a long-pulse or nearly CW beam is usually requested for nuclear-physics experiments(Fig. 1 d) [13]). A relatively low emittance (typically an unnormalized 90% emittance of around 2 π mm·mrad) is necessary for the latter. The beam represented by Fig. 1 b) and c) ( a peak current of a few 10 A) cannot be obtained directly from an ion source, the maximum peak beam current of which is on the order of 100 mA. This is the reason why we need a synchrotron ring with a revolution time of a few 100 ns. A typical schematic accelerator complex thus comprises an injector linac and a synchrotron ring. The highest possible beam current will be filled up in the ring, and will then be fast-extracted. The ring is used as a compressor with a pulse length equivalent to its revolution time in this case. Additional bunch compression with a bunch rotation is possible down to a few 10 ns (Fig. 1 c)) in a ring by applying a high voltage [9,14]. On the other hand, if what one needs is only a high average current, for example a few 100 mA, a unique solution would be a CW proton linac. However, if the necessary average current is much lower than the possible peak beam current in a linac, the CW proton linac scheme is extremely expensive. The best choice is again the accelerator complex comprising a linac and a ring, where the ring is used as a stretcher [9,14]. The beam is slowly extracted from the ring in this case. If the necessary energy exceeds around 3 GeV, one more ring should be built as in the case of JHP [7].