High power X-band coaxial amplifier experiments

The authors consider the continuing studies on the development of X-band coaxial microwave amplifiers as a source for next generation linear colliders. Coaxial amplifiers employ an annular electron beam propagating between inner and outer drift tube conductors, a configuration which allows large increases in beam current over standard pencil beam amplifiers. Large average diameter systems may still be used without mode competition since transverse-magnetic mode cutoff frequencies are controlled by the separation between conductors. A number of amplifier configurations are being studied, all primed by a driven initial cavity which resonates around 9 GHz. Simple theories of coaxial systems and particle-in-cell simulations are presented, as well as initial experimental results using a 420-keV, 7-8 kA, 9-cm-diameter annular beam.<>