The KIT two-dimensional beam optics code ESRAY is an efficient tool for the design and optimization of magnetron injection guns and collectors of gyrotrons or comparable vacuum tubes. The applied numerical methods and latest modifications and enhancements will be presented. To achieve more realistic results, a new emitter surface roughness model and the treatment of secondary electrons has been implemented recently. An extension to three dimensions (without taking space-charge effects into account) permits the simulation of various collector sweeping concepts, including the very efficient transverse sweeping.

Gyrotron electron gun and collector simulation with the ESRAY beam optics code

High power cw gyrotrons operate with an RF-power of up to 1 MW and efficiencies of 35–60 %. The waste power remains in the spent electron-beam and must be dissipated in the gyrotron-collector. Such collectors have strong sophisticated water-cooling systems for continuous operation. The electron-beam strike area on the collector surface is moved between an upper and a lower turning point by sweeping magnet fields in order to achieve a smooth power deposition.

Transverse field collector sweeping for the W7-X gyrotrons — Modulation techniques

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7-X Stellarator (W7-X), which is under construction at IPP- Greifswald. A 10 MW ECRH plant with CW-capability at 140 GHz is under construction to meet the scientific objectives. The microwave power is generated by 10 gyrotrons with 1 MW each. Three gyrotrons are already operational at IPP in Greifswald. The W7-X gyrotrons are designed for single frequency operation at 140 GHz. The operation regime of the W7-X stellarator experiments is restricted to magnetic fields matched to the ECRH-frequency and it would be interesting to extend the operation regime to a second resonant magnetic field. Therefore gyrotron operation at a second frequency was investigated. The standard vertical collector sweep systems of gyrotrons inherently display a pronounced peaking of the collector heat loading. An improved collector sweep system was successfully tested on the TED gyrotrons, which creates an almost homogenous heat loading. This system is of particular interest for next generation gyrotrons with an output power up to 2 MW.

Extended operation of the 1 MW, CW gyrotrons for W7-X

The high frequency power modulation of cw gyrotrons with an rf-output power up to 1 MW and efficiencies of 35–55 % is limited by collector loading Two alternative power modulation schemes are commonly applied which show a significant difference in the resulting collector loading In any case waste power remains in the spent electron-beam and must be dissipated in the gyrotron-collector Such collectors have strong sophisticated water-cooling systems and the electron-beam strike area is moved on the collector surface by sweeping the magnetic field The technically less demanding power modulation method unfortunately generates the highest level of collector loading Different sweep technologies have been developed in order to avoid overheating

Collector loading during high frequency power modulation

The EU CW gyrotrons for ITER [1] will operate at a frequency of 170 GHz with an output power of 1 MW per tube, at an efficiency of approximately 50 %. With this efficiency, 1 MW of power from the electron beam has to be absorbed in the gyrotron collector. To avoid local overheating of the collector structure, the electron beam is swept across the inner surface at low frequencies (5 Hz – 50 Hz). This leads to local cyclic thermal loads, which induce thermo-mechanical fatigue of the collector material. The mechanical effect on the collector structure is investigated by calculating time-dependent load profiles with the in-house beam optics code ESRAY which are then used in a transient thermo-mechanical FEM-analysis. The obtained strain-stress cycles are used to predict the lifetime of the collector with a Manson-Coffin Basquin equation. The work investigates the influence of different parameters, such as the collector wall thickness and the sweeping frequency on the estimated lifetime of the gyrotron collector.

Collector lifetime estimations for the EU-1 MW, 170 GHz CW gyrotron for ITER

Advanced gyrotron collector sweeping with smooth power distribution

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Advanced gyrotron collector sweeping with smooth power distribution