Experimental Classification and Enhanced Suppression of Parasitic Oscillations in Gyrotron Beam Tunnels

Abstract

High-power gyrotrons may suffer from parasitic oscillations that are excited in the electron-beam compression zone. Different damping structures are proposed in the literature that reduce the possibility of parasitic excitation by increasing the starting currents of the modes. In this work we focus on a dielectric-loaded (stacked) beam tunnel. Based on our previous theoretical studies, we make targeted modifications to the beam tunnel in order to classify the parasitic signals and localize the areas where they are excited. After two successive modifications, the beam tunnel exhibits improved behavior with higher starting currents of the parasitic modes. The experiments are performed by using a modular 170 GHz, 1 MW short-pulse gyrotron, which due to its flanged construction gives the possibility to modify the beam tunnel without affecting the rest of the tube. Index Terms-Beam tunnel, dielectric-loaded, gyrotron, parasitic oscillations I. INTRODUCTIONYROTRONS are the only high-frequency, high-power millimeter-wave source that is able to cover the Electron Cyclotron Resonance Heating (ECRH) and Electron Cyclotron Current drive (ECCD) needs of modern fusion experiments. There are already numerous examples of successful Short-Pulse (SP) and Continuous-Wave (CW) gyrotrons that cover the frequency range from 100 GHz to 200 GHz with demonstrated RF power up to 2 MW and a total efficiency that in depressed collector operation can exceed 50% [1].In several recent experiments with MW-class gyrotrons, the tubes suffered from undesired parasitic oscillations that were excited in the region of the beam tunnel [2]- [4]. Such

Generation of 1.5 MW–140 GHz Pulses With the Modular Pre-Prototype Gyrotron for W7-X

“…With accelerating voltage higher than 85.5 kV and before reaching the nominal beam energy, parasitic oscillations tend to get excited, the power is saturated and the efficiency is reduced. Similar behavior has been observed in [17]. Despite the non-optimal operation it is still possible to reach 1.4 MW with 31 % efficiency.…”

Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

“…An alternative, further improved beam tunnel has also been designed [24]. It uses BeO/SiC ceramic material of different concentration, following the theoretical guidelines of [25], which have been recently validated experimentally [26]. The performance of this alternative design, in terms of number and quality factor of possible parasitics in the frequency range of 115-135 GHz, is improved by approximately 50%, compared to the beam tunnel of the 1 MW gyrotron.…”

Multifaceted Simulations Reproducing Experimental Results From the 1.5-MW 140-GHz Preprototype Gyrotron for W7-X

“…In gyrotrons, the RF parasitic oscillations that limit the power and appear at frequencies 5-15% lower than the operating frequency are attributed, as a rule, to parasitic beamwave interaction in the beam tunnel before the cavity (e.g. [25][26][27][28][29]). It can be argued that the onset of parasitic excitation in case 3 is related to the higher values of the electron velocity ratio (α > 1.25), as compared to cases 1 and 2.…”