Design of MW-Class Coaxial Gyrotron Cavities With Mode-Converting Corrugation Operating at the Second Cyclotron Harmonic

This article presents investigations on the design of coaxial gyrotron cavities with mode-converting corrugations, operating at the second harmonic of the electron cyclotron frequency with output power of the order of megawatts. The suppression of the competing modes interacting at the fundamental cyclotron frequency is achieved by the combination of a corrugated coaxial insert and mode-converting corrugation on the outer wall. The outer corrugation couples the key competing modes to lower order modes with reduced quality factor. The design steps, which form a generally applicable design procedure, are described in detail. As an illustrative example, the proposed procedure is used for the design of a cavity for a fusion-relevant, second-harmonic MW-class gyrotron, operating at 170 GHz with the TE 37,1837,18 mode. From the simulations, it is found that for the proposed design, this mode is excited with an output power of around/ ∼ 1.5 MW. Two additional paths for cavity optimization toward even higher output power are also presented

High-Frequency MW-class Coaxial Gyrotron Cavities Operating at the Second Cyclotron Harmonic

Second-harmonic operation of gyrotron oscillators offers the possibility to generate millimeter and submillimeter radiation at half the value of the magnetic field required for operation at the fundamental cyclotron frequency (first harmonic). However, being inherently weaker than the interaction at the first harmonic, high-power second-harmonic continuous-wave (CW) operation employing high-order modes faces strong mode competition from the first-harmonic competing modes. It is shown that coaxial cavities with a corrugated insert allow to drastically enhance the mode selectivity at the second harmonic and suppress the first-harmonic competitors in MW-class gyrotrons. Detailed design considerations for coaxial cavities are presented and specific cavity designs for various candidate operating modes are given. We demonstrate numerically, with multimode interaction simulations, stable second-harmonic CW generation of 2 MW output power at 170 GHz and 0.7 MW at 280 GHz, using high-order modes with eigenvalues > 100. The presented results show the possibility to design second-harmonic gyrotrons with cost-effective magnet systems and achieve MW-class CW operation at frequencies above 250 GHz

Design Strategies for Second Harmonic Gyrotrons in Nuclear Fusion Applications

Gyrotrons are high-power microwave sources that play an important role in the heating of plasmas for magnetically confined thermonuclear fusion applications. This paper presents a comprehensive study of two potential strategies for operating high-power megawatt-class fusion gyrotrons at the second harmonic of the electron cyclotron frequency which requires only half of the gyrotron cavity magnetic field. The first approach focuses on a coaxial cavity design that effectively suppresses fundamental competing modes, making it a robust solution for second harmonic operation. The second strategy discusses the injection of an external locking signal. Therefore, a quasi-optical mode converter was designed and tested capable of handling both, co- and counter-rotating modes