The Radiatron: a High Average Current Betatron for Industrial and Security Applications*

The fixed-field alternating-gradient (FFAG) betatron has emerged as a viable alternative to RF linacs as a source of high-energy radiation for industrial and security applications. For industrial applications, high average currents at modest relativistic electron beam energies, typically in the 5 to 10 MeV range, are desired for medical product sterilization, food irradiation and materials processing. For security applications, high power x-rays in the 3 to 20 MeV range are needed for rapid screening of cargo containers and vehicles. In a FFAG betatron, high-power output is possible due to high duty factor and fast acceleration cycle: electrons are injected and accelerated in a quasi-CW mode while being confined and focused in the fixed-field alternatinggradient lattice. The beam is accelerated via magnetic induction from a betatron core made with modern lowloss magnetic materials. Here we present the design and status of a prototype FFAG betatron, called the Radiatron, as well as future prospects for these machines. CONVENTIONAL BETATRON Betatron accelerators were introduced in the early 1940’s, and soon found common application in research, medicine and industry. From the viewpoint of industrial applications, betatron acceleration is still attractive because it avoids the expense and complication of using high-power RF. The conventional betatron [1] is a fixed-orbit circular induction accelerator. A changing magnetic field confines and focuses the particles, while also generating an azimuthal electric field which accelerates the particles. The condition relating the rate of change of the magnetic flux through the circular orbit, ̇ , and the local (at the design trajectory radius R) bending field Bz R ( ) to maintain this equilibrium orbit is ̇ = 2 R2 ̇ B z , or B z = 2Bz R ( ) +C, (1) where z B is the average is over the interior of the orbit, and C is a constant. Equation 1 is termed the betatron condition. In early betatrons the accelerating and bending fields were created by a single dipole, with a shaped gap to create the proportionality at the desired radius. Later, the functions of the accelerating core and the bending field were separated to reduce the energy needed to drive the core [2]; core biasing — setting C in opposition to the direction of increase in Bz — also allowed improvement in the total available acceleration. The correct proportionality between the core and bending fields in such devices was maintained by connecting the coils of the bending and accelerating magnets in parallel. Despite these and other improvements, the average current capability of the conventional betatron was limited by several factors: • The transverse focusing is weak. • The duty cycle is limited to a few percent – current may only be injected during a small portion of the betatron cycle because the bending fields change in time. • The momentum acceptance of the device is very low — there is a narrow range of equilibrium orbits of differing momenta allowed in the machine at a given time. • The cycle rate is limited by the rapid increase of eddy current losses at higher frequency.