Long-term, correlated emittance decrease in intense, high-brightness induction linacs

Simulations of high-brightness induction linacs often show a slow, long-ter m emittance decrease as the beam is matched from the electron gun into the linac. Superimposed on this long-term decrease are rapid emittance oscillat ions. These effects can be described in terms of correlations in the beam's radial phase space. The rapid emittance oscillations are due to transverse plasma oscillations, which stay nearly in phase for different radial posit ions within the beam. The initial emittance, just after the electron gun, i s dominated by nonlinear focusing within the gun introduced by the anode ex it hole. Due to the large space-charge force of an intense electron beam, t he focusing of the beam through the matching section introduces an effectiv e nonlinear force (from the change in the particles' potential energies) wh ich counteracts the nonlinearities from the electron gun, leading to an ave rage, long-term emittance decrease. Not all of the initial nonlinearity is removed by the matching procedure, and there are important consequences bot h for emittance measurements using solenoid focal length scans and for focu sing the electron beam to a target. (C) 1999 American Institute of Physics. [S1070-664X(99)02909-2].