Stability of compact recirculating accelerators

Beam stability in high-current recirculating accelerators using magnetic beam transport is investigated. The focus is on three instabilities which have the potential for causing beam disruption: the three-wave, negative-mass, and beam breakup instabilities. The three-wave instability is a parametric instability caused by strong-focusing fields like the helical quadrupole in the SLIA (Spiral Line Induction Accelerator). Approximate analytic expressions have been obtained for the growth rate and the convection velocity of this instability. A simple numerical simulation code E3WAVE has been written to model the linear behavior of the instability. E3WAVE was used to make predictions for the SLIA experiments presently under way. The short-wavelength negative-mass instability modes which interact resonantly with cavity modes of the drift tube were also investigated. Particle simulations show that the main nonlinear effect of the instability is to create an energy spread on the beam, rather than leading directly to current loss. Progress has been made in minimizing the beam breakup instability without resorting to focusing. By adding ferrite in the induction gaps to increase damping, and by using gaps that are considerably wider than usual, with correspondingly higher voltage, growth can be reduced to several e-foldings during a typical acceleration cycle