Enhancement of traveling wave gain and efficiency with a gyroresonant phase filter

The growth rate and efficiency of a general traveling wave interaction (TWI) can be significantly enhanced by selectively suppressing those electrons which are phased to gain energy from the wave. By superimposing a periodic, weak transverse magnetic field onto the guiding field, a second resonance condition is introduced, that of gyroresonance with the "wiggler" field. The wiggler period is chosen such that the electrons which gain energy in the TWI become more resonant with the wiggler and thereby fall out of resonance with the TWI. Meanwhile, the favorably phased electrons which lose energy in the TWI are relatively unaffected by the wiggler. In this way only the electrons which are phased to gain energy are suppressed. We have observed in simulations of a particular TWI, a cyclotron autoresonant maser (CARM), the increase of efficiency by a factor of three and growth rate by two.