Design Study of a Permanent-Magnet Quadrupole Focusing Lattice for a mm-wave Traveling Wave Tube

Linear beam tubes have traditionally relied on axisymmetric solenoidal or PPM focusing to transversely confine the electron beam. In the particle accelerator community, however, it is well-known that alternating-gradient focusing using sequences of quadrupole magnets, known as "strong focusing", is capable of transporting more current for comparable field strengths (Reiser, 1994). At mm-wavelengths, a key limitation to the peak and average power of vacuum electronics devices stems from the difficulty of control and confinement of high-current density beams. In this paper we summarize an ongoing design study of a strong-focusing lattice employing permanent magnet quadrupoles (PMQs) capable of transporting a sub-mm radius electron beam for vacuum electronics applications. The study has the aim of producing a practical design for an experimental study at NRL for comparing quadrupole lattices with standard ones based on solenoidal periodic permanent magnets (PPMs). In addition, a design code and methodology for designing these PMQ channels is under development