Wideband fast wave amplifiers

In a number of applications, wideband amplifiers1–5 provide significant advantages over oscillators of similar output power. For instance, in spherical tokamak fusion plasma heating, frequency agility allows targeted heating of specific plasma layers, possibly providing an additional plasma stabilization technique. Experimental and theoretical investigations of wideband amplifiers at MW power levels in the centimeter to millimeter wavelength range have shown that devices with greater than 20% bandwidth, suitable for long pulse to CW operation are a practical proposition. In cases where the ultimate in amplifier bandwidth is required, reversed guide magnetic field Free Electron Maser (FEM) amplifiers5 have demonstrated tunability over 70% of center frequency with 40% instantaneous bandwidth. Gyro-amplifiers1–4 using helically corrugated waveguides6 have shown exceptional gain, power, bandwidth and efficiency performance at cm and mm wavelengths. In all cases, the performance of a long pulse (and therefore high vacuum) system is strongly influenced by factors other than the intrinsic bandwidth of the interaction. For example, the stability of all of these devices can be greatly enhanced by appropriate control of the radiation polarization. However, polarization converters work optimally at only one frequency with declining efficiency at frequencies far from the center frequency. Also low reflection windows with 40% bandwidth for circularly polarized radiation require innovative design solutions. When the design must also be compatible with 500°C bakeout it becomes even more difficult.