The Accelerators for the Joint Project

	The MA-loaded cavity

Toward the frontier of the world-highest beam power, the present accelerator technology does not surely guarantee the proton beam power beyond 1 MW. Among the three spallation neutron source projects in the world, the present design of the Joint Project is going to use the concept (page 1) of the accelerator complex comprising a linac, a rapid-cycling booster synchrotron (RCS), and a main proton synchrotron, while other two facilities will use a concept of full-energy linac (1- GeV class) and storage ring.

Some of the difficulties to realize the several-GeV RCS are summarized in the following. Since the number of protons accelerated at one time in a synchrotron is limited by the Coulomb force, the acceleration cycle of the synchrotron should be increased (up to 25 cycles per second in the present design) in order to obtain the high beam current required. The rapid cycling of the synchrotron implies rapid acceleration of the beam. The beam is accelerated by radio-frequency (RF) electric field. The rapid acceleration requires that the higher voltage per unit length (electric field) should be applied to the beam. For a realistic design of the present RCS, one needs a field of ~50 kV/m, which is not feasible with a conventional ferriteloaded RF cavity. Since the increase in velocity of the particles is faster, the circulating period of the beam in the ring is shorter. So, the other difficulty is that the frequency of the RF cavity should be adjusted rapidly to follow this rapid change.

Recently, KEK has solved both of these problems by developing an RF cavity loaded with a new type of magnetic alloy (MA) such as FINEMET. The MA-loaded cavity should withstand over 100 kV/m, and no tuning is necessary. The cavity has already been power-tested up to 50 kV/m, and has been beam-tested in various ways.