Megavolt gyro-traveling-wave amplifier experiment at 35 GHz

Summary form only given, as follows. An intense-beam gyro-traveling-wave amplifier (gyro-TWA) experiment has been carried out at 35 GHz, using a 1-MeV, 200-A beam from the VEBA pulseline accelerator. The experiment was carried out in a 10.8-mm-diameter drift tube and used a 6-mm-diameter solid electron beam with low initial traverse momentum and very low initial parallel velocity spread. The electron transverse momentum required by the gyro-traveling-wave interaction (β_⊥ approximately 0.3-0.4) was induced by transit through a one-period untapered bifilar helical wiggler magnet with a 4-cm period. A 35-GHz signal was launched in the TE₁₁ mode using a directional sidewall coupler. A massive iron collar (6-1/4-cm diameter, 6-cm long) was placed around the 1.27-cm outer diameter of the stainless-steel drift tube as a means to exclude the applied axial magnetic field and thus to control and vary the axial extent of the interaction. In experiments operating at 8 kG, near grazing incidence between the dispersion relations of the beam and waveguide modes, the the gyro-TWA demonstrated linear growth rates of approximately 2 dB/cm, total gain exceeding 30 dB, and an output power of ≳20 MW.