No abstract

We present the formulation and experimental results of a new approach to designing internal mode converter reflectors for high-power gyrotrons. The method employs a numerical phase retrieval algorithm that reconstructs the field in the mode converter from intensity measurements, thus accounting for the true field structure in shaping the beam-forming reflectors. An outline for designing a four-reflector mode converter is presented and generalized to the case of an offset-fed shaped reflector antenna. The requisite phase retrieval and reflector shaping algorithms are also developed without reference to specific mode converter geometry. The design approach is applied to a 110 GHz internal mode converter that transforms the TE 22 6 gyrotron cavity mode into a Gaussian beam at the gyrotron window. Cold test experiment results of the mode converter show that a Gaussian beam with the desired amplitude and phase is formed at the window aperture. Subsequent high-power tests in a 1 MW gyrotron confirm the Gaussian beam observed in cold tests. The general development of the approach and its validation in a quasi-optical mode converter indicate that it is also applicable to other quasi-optical, microwave applications such as radio astronomy, free-space transmission lines, and mitre bends for overmoded waveguides.

Detailed studies of the spectral characteristics and spatial mode structure Three other modes are also present in the theoretical spectrum of the resonator; modes at 27.1 GHz, 28.1 GHz, and 28.3 GHz. For the three lower frequency modes observed in the experiment, all three had starting currents lower than the beam current at the nominal operating beam energy of 310-320 keV. For this reason, mode competition is a significant issue for the present experiment.The overall experimental setup is illustrated by Fig. 1. Apart from the Bragg resonator, the other main components of the device are the high-voltage modulator, the thermionic electron gun, the solenoids which produce the axial magnetic field, and the helical permanent magnet wiggler. The system operates with a flattop pulse length 1-= 1 As. Beam compression to a radius of 4 mm was achieved with minimal scalloping, in good agreement with adiabatic theory. The beam axial energy spread, before injection into the wiggler interaction region, is inferred from experimental data to be Ayli/yli < 0.5 %. The beam is transported through the interaction region by an axial magnetic field of magnitude 2.35 kG. A permanent magnet helical wiggler with 3 cm period and 500 G amplitude is used to provide the 4 perpendicular momentum of the interacting electrons. To ensure stable high-quality group I helical orbits in the interaction region, the wiggler has a 10-period long linearly tapered introduction. The resulting equilibrium orbits at 320 keV beam energy have /.3 = 0.18,