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Effective Cavity Length of Gyrotrons

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Abstract

Megawatt-class gyrotron oscillators for electron cyclotron heating and non-inductive current drive (ECH&CD) in magnetically confined thermonuclear fusion plasmas have relatively low cavity quality factors in the range of 1000 to 2000. The effective length of their cavities cannot be simply deduced from the cavity electric field profile, since this has by far not a Gaussian shape. The present paper presents a novel method to estimate the effective length of a gyrotron cavity just from the eigenvalue of the operating TEm,n mode, the cavity radius and the exact oscillation frequency which may be numerically computed or precisely measured. This effective cavity length then can be taken to calculate the Fresnel parameter in order to confirm that the cavity is not too short so that the transverse structure of any mode in the cavity is the same as that of the corresponding mode in a long circular waveguide with the same diameter.

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Acknowledgments

The author would like to thank his colleagues of the Institute Pulsed Power and Microwave Technology (IHM) at Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany, Mikhael Glyavin of the Institute of Applied Physics, RAS, Nizhny Novgorod, Russia, and Gregory Nusinovich of the Institute for Research in Electronics and Applied Physics, University of Maryland, USA for providing useful information and for very fruitful discussions.

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  1. Institute for Pulsed Power and Microwave Technology (IHM) and Institute of High Frequency Techniques and Electronics (IHE), Karlsruhe Institute of Technology (KIT), 76131, Karlsruhe, Germany

    Manfred Thumm

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  1. Manfred Thumm
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Correspondence to Manfred Thumm.

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Thumm, M. Effective Cavity Length of Gyrotrons. J Infrared Milli Terahz Waves 35, 1011–1017 (2014). https://doi.org/10.1007/s10762-014-0102-z

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  • DOI: https://doi.org/10.1007/s10762-014-0102-z

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