No abstract

Stability of charged beam propagation through a relativistic hollow electron beam

A four-stage linear electron accelerator is described which uses pulsed radial transmission lines as the basic accelerating units. An annular electron beam produced by a foilless diode is guided through the accelerator by a strong axial magnetic field. Synchronous firing of the injector and the acccelerating modules is accomplished with self-breaking oil switches. The device has accelerated beam currents of 25 kA to kinetic energies of 9 MV, with 90% current transport efficiency. The average accelerating gradient is 3 MV/m.

Stable beam transport may be the limiting factor in the development of a new generation of high current linear induction accelerators. In this paper we analyze several important beam stability topics, including radial oscillations induced by an accelerating gap, the diocotron, resistive wall, and cyclotron maser instabilities, and the transverse beam breakup and imge displacement instabilities. At present image displacement appears to represent the most serious limitation to high current beam transport in linear accelerator structures.

Radial oscillations of hollow, relativistic electron beams have been observed on Sandia’s RADLAC I, a multistage high-current linear induction accelerator. These oscillations, which arise because the presence of the accelerating gaps prevents radial force balance, can be responsible for poor beam quality and low current-transport efficiency. The method of magnetic field shaping to suppress oscillations is investigated. Two analytical models are formulated to derive nonuniform fields necessary to maintain force balance in a single-gap geometry. Particle code simulations have been made to test beam propagation in contoured fields. Field shaping does suppress oscillations and is not sensitive to fluctuations in beam and gap parameters.