Numerical simulations of electron beam transport through a permanent-magnet quadrupole lattice for MM-wave TWT applications

Abstract

At millimeter-wavelengths, a key limitation to the peak and average power of linear-beam vacuum electronic amplifiers stems from the difficulty in controlling and confining high current density beams. Practical limits have been reached using axisymmetric solenoidal or periodic permanent magnet focusing. However, it has long been known in the accelerator community that alternating-gradient focusing using quadrupole magnets is capable of transporting higher beam currents for comparable field strengths. We have developed numerical design tools and a design methodology to synthesize a strong-focusing lattice employing permanent magnet quadrupoles (PMQs). The codes determine the optimum lattice parameters given a specified beam current, energy, emittance, desired average radius, and phase advance. Using these tools, a PMQ lattice was optimized for a 16 keV, 380 mA, 0.5 mm radius beam compatible with the requirements for a TWT operating in Ka-band (26.5 to 40 GHz). The resulting 3D magnetic field profiles were imported into the MICHELLE gun code. We will present the results of the 3D particle simulations, beginning with the formation of a round electron beam from a Pierce-type gun and following its transformation into an elliptical cross-section beam and subsequent transport through the PMQ lattice.