Concepts for high gradient, dielectric cavity, induction accelerators

Induction accelerator cavities comprising various arrangements of constant impedance transmission lines have been devised to provide a constant acceleration to a short pulsed beam of electrons. In most of the cavity concepts the lines are charged externally from a voltage source such as a Marx generator and switched internally; in others energy is fed from an external bipolar source. Several cavities based on the former concepts have now been fabricated and can be used to accelerate efficiently electron beams with pulse durations of 10-20 ns and currents of 10-100 kA. Acceleration gradients are, however, limited to a few MeV/m for efficient transfer of the energy from the cavity to the beam. Gradients an order of magnitude higher could be obtained by recirculating an undermatched beam several times through the cavities so that only a fraction of the cavity energy is transferred on each pass. Various ways of magnetically bending and confining the beam to achieve this recirculation have been proposed. An attractive alternative in terms of overall weight, however, is to exploit beam propagation in the ion focussed regime. In this scheme the beam is captured by and follows an ion trail formed by ionizing a very low pressure gas with a laser or, for a curved path, with a low energy electron beam. Efforts to demonstrate this recirculating linear induction technology are underway, notably at Sandia National Laboratory. (orig.)