Parasitic Oscillations in Smooth-Wall Circular Symmetric Gyrotron Beam Ducts

  • J. GenoudEmail author
  • S. Alberti
  • T. M. Tran
  • G. Le Bars
  • P. Kaminski
  • J.-Ph. Hogge
  • K. A. Avramidis
  • M. Q. Tran


In order to study parasitic oscillation that may occur in a realistic beam duct upstream to the gyrotron cavity, the self-consistent linear and spectral code TWANGlinspec has been modified. The large inhomogeneities in the smooth-wall beam duct geometry or in the magnetic field profile required the implementation of a numerical approach using a hybrid finite element method. The new model permits to characterize a large number of potentially spurious TE modes. Compared to previous studies on gyrotron beam duct instabilities, an extended interaction space including also the gyrotron cavity has been considered. The role of the connecting part between the beam duct and the cavity, called spacer, is highlighted and it is shown that the gyro backward-wave TE modes excited in this region generally have their minimum starting current. The sensitivity of the minimum starting current on electron beam velocity spread is also evaluated.


Gyrotron Beam-duct Parasitic oscillations Backward-wave 



We dedicate this article to our late friend T.M. Tran. This work was supported in part by the Swiss National Science Foundation, by the EUROfusion WP-HCD programme and by Fusion for Energy under Grants F4E-GRT-432 and F4E-GRT-553 to the European Gyrotron Consortium (EGYC). EGYC is a collaboration among SPC, Switzerland; KIT, Germany; HELLAS, Greece; IFP-CNR, Italy. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein are the sole responsibility of the authors and do not necessarily reflect the views of the European Commission and F4E. The author would like to thank D. Wagner and S. Brunner for precious scientific discussions.


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Authors and Affiliations

  1. 1.Ecole Polytechnique Fédérale de Lausanne (EPFL)Swiss Plasma Center (SPC)LausanneSwitzerland
  2. 2.Institute for Pulsed Power and Microwave TechnologyKarlsruhe Institue of TechnologyKarlsruheGermany

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