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Fig. 3-1 Composition of linac

Fig. 3-2 Focusing and acceleration concept Forming a series of proton bunches, a beam is accelerated by an electric field produced in the rf cavities, and focused with a magnetic field of the Q-magnets. Too weak a magnetic field gives too large a bunch size in the vertical direction, and too strong a magnetic field causes deterioration in the longitudinal focusing property. An optimum strength is selected to each Q-magnet in the design.

Fig. 3-3 Beam emittance Beam emittance is the product of beam size and beam divergence angle, and smaller emittance means better beam quality.

It is a high energy and a high current proton linac that constitutes the central part of the Neutron Science Project. The proton beam should obviously be continuous in order to produce sufficient neutron intensity, however, it should also be able to be pulsed, depending upon the type of experiments to be performed with the neutrons. The beam power aimed at ranges over several megawatts, which well exceeds any of those of presently existing similar accelerators by more than a factor of ten, bringing with it some severe technical challenges. These are; to suppress the loss of beams to prevent the accelerator from becoming radio-activated, to remove the tremendous amount of heat produced by the electric power, and etc. The composition of the linac is shown in Fig. 3-1. The main accelerating part is a superconducting linac, which has the advantage of not only making the rf power loss extremely small, but also of enabling the cavities to have a large bore radius allowing the beam to pass through the cavities with minimum beam loss. One of the remarkable characteristics in the accelerator design is the adaption of the recently proposed "equipartitioning" concept. This concept is that the beam becomes most stable when the longitudinal and the vertical motions are equally balanced. The beam focusing system is designed to control particle motions so that they obey the above concept in the beam; a typical focusing element is shown in Fig. 3-2. The resulting beam motions in the vertical and the longitudinal directions are shown in Figs. 3-3(a) and (b), respectively; the vertical axis is the beam emittance, i.e. the beam size multiplied by the beam divergence, and the horizontal is axis the energy. As is seen from the figures, an appreciable improvement in longitudinal motion is achieved. It is hopefully expected to realize the largest accelerator in the world with a well suppressed beam loss.

Reference K. Hasegawa et al., System Design of a Proton Linac for the Neutron Science Project at JAERI, J. Nucl. Sci. Technol., 36 (5), 621 (1999).

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