Periodic Permanent-Magnet Quadrupole Focusing Lattices for Linear Electron-Beam Amplifier Applications

Periodic lattices of permanent-magnet quadrupoles (PMQs) have the potential to provide the strong focusing necessary to transport high-current space-charge-dominated electron beams commonly used in vacuum electronic power amplifiers. We describe a new design methodology and computational tools to produce optimized PMQ lattices, verifying the results with fully 3-D particle simulations of design examples. These examples include a PMQ system with a greater than fivefold reduction in magnet weight and volume compared with a periodic permanent-magnet (PPM) stack designed for a comparable beam and a PMQ system that can stably transport a factor of three times more current than a comparable PPM system. We conclude with a simple analysis of the interaction between the elliptical beam in the PMQ lattice with the waves on a helical traveling-wave circuit. In addition to enabling higher RF power operation, we show that the higher beam currents afforded by PMQ focusing enable a higher power gain per unit length and result in more compact form factors and higher amplifier power densities compared with comparable PPM-focused circuits.