Formation and transport of high-perveance electron beams for high-power, high-frequency microwave devices

One of the problems in the scaling of high-power vacuum microwave sources to higher frequencies is the need to transport beams with high space-charge density, since the rf circuit transverse dimensions tend to decrease with wavelength. Sheet electron beams offer a solution since the total beam current of a single device may be increased without high beam space-charge. A theoretical investigation of sheet beam transport in planar offset-pole periodically-cusped magnet arrays shows that stable, edge-focused transport of linear beams can be realized for very high current density beams ({dollar}>{dollar}100 A/cm{dollar}sp2{dollar}) at low beam voltages ({dollar}<{dollar}40 kV). A method of elliptical sheet beam formation using a normal round-beam electron gun and a lattice of quadrupole magnets was investigated. This method is particularly suited for laboratory experiments since the components are readily available or fabricated and the beam size may be varied through quadrupole magnet adjustments made externally to the vacuum envelope. Three-dimensional particle-in-cell simulations indicate that a high-quality 5.4 cm x 0.2 cm, 2 A, 10 kV elliptical beam can be produced with modest quadrupole gradients. An experimental investigation verified the basic potential of this method, however the beam size was limited to 4 {dollar}pm{dollar} 0.5 cm x 0.4 {dollar}pm{dollar} 0.1 cm due to beam rotation induced by leakage flux from a solenoid matching section between the gun and the quadrupole lattice.; Electron beam focusing by periodic permanent quadrupole magnet (PPQM) arrays was also studied. The focusing force from PPQM arrays can be an order of magnitude greater than that of conventional periodic permanent magnet (PPM) arrays for beam voltages from 10-40 kV. Analytic and numerical studies demonstrate the potential of this configuration but also show that beam ripple is more severe in PPQM systems. An experimental investigation using a rectangular-block PPQM array with a period of 2 cm and an adjustable gradient from 750-2000 g/cm and a 0.27 A, 10 kV electron beam was conducted. Measurements of the beam radius at different positions in the array are in qualitative agreement with the predictions of envelope and particle-in-cell simulations.