Gyro-BWO experiment using a helical interaction waveguide

By using a helical corrugation on the inner surface of a cylindrical waveguide, it has been demonstrated that the wave dispersion has a near constant group velocity in the region of small axial wave number (G.G. Denisov et al., IEEE Trans. Plasma Sci. vol. 26, p. 508, 1998). This allows broadband microwave amplification to be achieved in a gyrotron travelling wave amplifier (G.G. Denisov et al., Phys. Rev. Lett. vol. 81, p. 5680, 1998; G.G. Denisov et al., ibid. vol. 81, p. 2746, 2000). Such a system is also favourable when used in a gyrotron backward wave oscillator (S.V. Samsonov et al., PS Spec. Issue on High Power Microwave Gen.; G.G. Denisov et al., Proc. IEEE IVEC 2003, pp. 338-339, 2003.) (gyro-BWO) in two ways. Firstly due to its larger group velocity as compared to normal gyro-BWOs, it has a wider frequency tuning range. Secondly due to the nonsymmetrical geometry of the helical interaction region, the electron beam interacts only with the wave in one direction (either backward or forward wave), and hence has a pure frequency component. In our experiment, a thermionic cathode electron gun in a Pierce-type geometry was used to generate a rectilinear electron beam of 1 μs pulse duration, 90-185 keV energy and ∼2.5 A in current.