Energy transfer in nonequilibrium space-charge-dominated beams

Space-charge modes similar to those observed in recent experiments appear in simulations of nonequilibrium charged particle beams with anisotropy. The modes couple degrees of freedom, causing energy transfer and equipartitioning without halo formation in just a few betatron wavelengths. The rate depends on a single free parameter quantifying the space-charge intensity of the final state. Traditional stability analyses are shown not to apply to high-intensity laboratory beams originating with a large perturbation from equilibrium.     

Ping-pong modes: a new form of multipactor

The multipactor is a vacuum discharge based on a secondary electron emission. A novel resonant form is proposed that combines one- and two-surface impacts within a single period, provided the total transit time is an odd number of rf half-periods and the product of secondary yields exceeds unity. For low fD products, the simplest such mode is shown to significantly increase the upper electric field boundary of the multipacting region and lead to overlap of higher-order bands. The results agree nicely with 3D particle-in-cell code simulations. Practical implications of the findings are discussed.     

Power deposited on a dielectric by multipactor

We use a simple transmission line model to evaluate the RF power deposited on a dielectric window by a multipactor discharge. The calculation employs Monte Carlo simulation, using realistic secondary electron yield curves as input, and taking into account the distributions in the emission velocities and emission angles of the secondary electrons. Beam loading on the external RF, as well as the evolution of the DC electric field due to dielectric charging, are also accounted for. It is found that the buildup of the multipactor space charge, rather than beam loading, causes saturation. Over a wide range of operating conditions and materials, it is found quite generally that the multipactor delivers on the order of 1 percent, or less, of the RF power to the dielectric. A simple estimate is given in support of this ratio, using the susceptibility diagram that was constructed from kinematic considerations. Comparison with experimental results is given