Prevention of Parasitic Oscillations in Electron Beam Tubes

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The  innovation  relates  to  a  device  for  preventing  parasitic  oscillations  in  electron beam  tubes.  It  comprises  a  beam  tunnel subject  to  an  axial  static  magnetic  field.  The  tunnel  is  equipped  with  ceramic  and  metal rings arranged alternately in the axial direction. These rings yield a structure on the inner surface preventing the harmonic rise of spurious oscillations that could otherwise  damage  the  tube.  The  technology  is  ready  for  use  in  the  non-­‐fusion  domain and was patented by the inventors Manfred Thumm and Gerd Gantenbein.

Description of the Technology

Electron beam tubes are primarily used for the contactless transfer of high energy by means of high frequency electromagnetic oscillations. Typical application examples are  gyrotrons  (>1  MW &  100­200 GHz), magnetrons,  klystrons or  traveling-­wave tubes.  The physical principle  is  based  on the  interaction between  an  electron  beam  and  a  RF frequency  in  a  hollow waveguide,  where  the high frequency power is  taken from the cycling motion  of  the electrons. The electrons are generated by means of an electron emitter, accelerated by  means  of  a  static  electric  field  and  pass  a  beam  tunnel  prior  to  entering  a  resonator. The electrons are guided by strong external axial magnetic field. The electrons  are  travelling  on  spiral  shaped  trajectories.  The  rotation  frequency  is  defined by the strength of the magnetic field and the relativistic mass of the electrons. In the case of vacuum tubes, undesirable spurious oscillations can occur. Such oscillations reduce the efficiency significantly and may even cause damage and destruction  as  a  consequence  of  thermal  overload.  The  technology  innovation  suppresses such undesirable oscillations and reduces their impact on the electron beam itself.

Innovation and advantages of the offer

To  suppress  the  undesirable  oscillations,  alternately  arranged  metal  rings  with  corrugations on the inner surface and ceramic rings are used, that yield a structure on  the  inner  surface  preventing  the  harmonic  rise  of  spurious  oscillations.  The  advantage of this design over conventional methods is the easy retrofitting through simple  exchange  of  components.  The  metal  rings  are  typically  made  of  copper  yielding  a  high  electric  conductivity  combined  with  a  low  high  frequency  damping.  The ceramic rings are preferably made of oxide ceramic. They have to be suitable for high  vacuum,  provide  electric  isolation  and  mechanically  workable.  The  chosen  material provides a high damping of parasitic oscillations.

Non-fusion applications

The technology has successfully been applied on the Nuclear Fusion domain. Further application areas are space as well as (test) facilities outside the fusion domain applying contactless high-­‐energy transfers via electron beams.
Application to space domain for HF communication, i.e. Traveling Wave Tubes (TWT) for HF generation in communication satellites, as well as for TV and Radio.

Fusion heritage

The  innovative  means  of  suppressing  spurious  oscillations  in  electron  beam  tubes  was developed at the Karlsruhe Institute of Technology (KIT). Such tubes are used to for  the  contactless  transfer  of  energy  for  gyrotrons,  magnetrons,  klystrons  or  traveling-­‐wave tubes. It was successfully tested and patented and is ready for use in other (test) facilities outside the fusion domain.

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