Nonlinear theory of gyroharmonic radiation from spatiotemporally modulated electron beams

Abstract

A multimode nonlinear particle simulation code is used to find the saturated efficiency for power transfer into modes of a cylindrical waveguide carrying a spatiotemporally modulated gyrating electron beam. For a TEsj-mode fifth harmonic 94-GHz harmonic converter using a 150-kV, 6.7-A cold beam, this code predicts a conversion efficiency of 57% when a linearly tapered guide magnetic field is used, and 70% when a nonlinear taper is used. Efficiency in the linear taper case is shown to be insensitive to beam axial velocity spread, while both cases show negligible power flow into competing modes.PACS numbers: 41.60.Ap, 52.75.Ms Efficient, high-power rf sources are in demand for many applications, such as drivers for next generation electron-positron colliders, as sources for fusion plasma heating and control, and as amplifiers for advanced mmwave radar systems. Extensions of proven technologies are being undertaken to meet these demands. Thus advanced klystrons [l] and gyroklystrons [2] show promise as 50-MW sources above 10 GHz for driving next generation colliders, cavity gyrotrons [3] have generated 940 kW at 140 GHz for plasma heating, and 35-GHz gyrotron traveling-wave amplifiers have shown 32% bandwidth and 30% efficiency at output levels of 25 kW [4]. This Letter reports first results on the nonlinear theoretical properties of a recently proposed [5] alternative mechanism to satisfy these demands, namely, gyroharmonic radiation from a spatiotemporally modulated gyrating electron beam. Prior to the work reported here, it was speculated that harmonic conversion based on this process could be highly efficient, that radiation into competing modes could be small, and that a moderate axial velocity spread on the electron beam could be tolerated [5,6]. Re-

A TE21 second-harmonic gyrotron backward-wave oscillator with slotted structure

“…Harmonic operation of gyrotron, effectively reducing the strength of the magnetic field, has been of growing interest. [19][20][21][22][23] However, it is subject to severe mode competition as well as high oscillation threshold. Figure 1͑a͒ shows the frequency-k z diagram of a Ka-band TE 21 second-harmonic gyro-BWO.…”