250 MW, 11.4 GHz PPM coaxial ubitron

Abstract

Summary form only given. The design is presented for a 250 MW X-band coaxial ubitron-FEL amplifier. The device operates in the TE/sub 01/ mode of a coaxial circuit using a hybrid wiggler with inner and outer annular permanent magnets. The use of the TE/sub 01/ mode minimizes RF breakdown, since the electric field is zero at the walls. The coaxial geometry allows for a non-scalloping PPM-type stability condition with zero guide field. By choosing /spl omega//sub p//sup 2/=(/spl gamma//2) /spl Omega//sub W//sup 2/, where /spl omega//sub p/ is the plasma frequency and /spl Omega//sub W/=eB/sub /spl Gamma///mc, the beam's space-charge force balances the magnetic v/sub /spl perp///spl times/B/sub z/ radial force. The space-charge field and B/sub z/ both change sign at the radius of the magnetic minimum if the beam is properly centered, resulting in stable focused flow. Although this coaxial PPM confinement geometry is ideal for ubitrons since it produces a strong azimuthal quiver motion, it can also be utilized to propagate high current annular electron beams for higher power, coaxial analogs of conventional linear-beam devices such as klystrons. Kiloampere-level electron beams can be transported for voltages in the range of 100-500 kV. An 11.4 GHz ubitron using a 500 kV, 1 kA electron beam has been designed to produce 250 MW using a self-consistent slow-timescale simulation code. An efficiency of 37% with a saturated gain of 53 dB is predicted for an ideal beam in an untapered device with a length of 117 cm.