Report on the progress in gyrotron research

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Report on the progress in gyrotron research

• UMd team G. S. Nusinovich, T. M. Antonsen, Jr.,
• V. L. Granatstein, A. Singh, M. Yeddulla, O. V.
  Sinitsyn, and M. Kollander
• SAIC A. N. Vlasov
• CPI S. Cauffman and K. Felch

2nd U.S./Japan/EU RF Technology
Workshop Amsterdam, The Netherlands, Oct. 4-5,
2004
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CONTENT

• Start-up scenarios in MW-class overmoded
  gyrotrons
• Advances in MAGY
• Depressed collector codes
• Interaction of wave envelopes
• Start currents in gyrotrons with azimuthally
  nonuniform emission
• International collaboration

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Start-Up Scenarios
The first results of this work were reported at
the 1st U.S./Japan/EU RF Technology Workshop
(Febr. 12-14, 2003). Then, the work was
completed, and the start-up scenario for the 140
GHz, CPI gyrotron was calculated with the use
of a multi-frequency, non-stationary code MAGY.

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Advances in MAGY

• MAGY MAryland GYrotron. Non-stationary,
  multi-frequency
• code analyzing the interaction processes in
  various gyrodevices.
• A. Coaxial version of MAGY. This work is in
  progress.
• The code development should be completed by the
  end of 2004.
• This version of MAGY is under development by T.
  Antonsen,
• Vlasov and some people from NRL.
• B. Version for gyrodevices with azimuthal
  non-uniformities of
• waveguide/resonator walls
• interaction of a spent beam with EM waves in
  Denisov couplers,
• Gyro-TWT with spirally corrugated walls,
• Gyrotrons with slot-and-vane resonant structures
  (LOGs).

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Depressed collectors

• The leader of this efforts, Dr. A. Singh, was
  retired in the end
• of September 2004. The work will be continued by
  T. Antonsen,
• M. Kollander and V. L. Granatstein.
• The latest progress was described in the last
  Special Issue of
• IEEE-PS on High-Power Microwave Generation
• tracking of multiple generations of backscattered
  electrons,
• modification of electrode geometry to minimize
  the effect of
• backscattered particles,
• -efficiency optimization (from 62 to 65 in the
  case of two-stage
• depressed collector).
• Also the studies of the effect of misalignment of
  electron beam on
• the performance of depressed collectors (peak
  heat dissipation
• density on the inner collector) were reported at
  IVEC-2004.

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Interaction of wave envelopes
In high-power gyrotrons, an electron beam can
excite many modes with both direction of
azimuthal rotation co- and counter rotating
modes. Superposition of co-rotating modes with
the same radial and different azimuthal indices
can be treated as a wave envelope. Typically, the
coupling of electron beams to the
co-rotating modes with the radial index p is
approximately as strong as the coupling to the
counter-rotating modes with the radial index
(p1). Therefore, the competition of such
envelopes can be a serious issue (as shown in the
start-up scenario above).
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Gyrotron model the axial distribution of the RF
field was approximated by the point-gap model.
The temporal evolution of the amplitudes of the
envelopes and their azimuthal structure was
analyzed.

• Two stages
• Competition between
• envelopes.
• 2) Transition of the winning
• envelope from one steady
• state to another.

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Evolution of the azimuthal structure
Phase I azimuthal structure flattens, i.e. the
envelope reduces to one rotating mode. Then, two
oppositely rotating modes compete, and the one
with a stronger coupling wins the competition.
Phase II in the winning envelope there is a
slow transition from the original mode, which
has the smallest starting current, to the mode
with a different azimuthal index.
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Start currents in gyrotrons with azimuthally
non-uniform emission
Effect of the azimuthal non-uniformity of
electron emission on the mode coupling and
gyrotron efficiency was analyzed earlier
(Nusinovich and Botton, PoP, March 2001
Nusinovich et al., PoP, July 2001).
For the analysis of the start-up scenario it is
important also to evaluate the effect of this
non-uniformity on the start currents.
Two approaches analytical theory and MAGY
simulations
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International collaboration with Helsinki
University of Technology (Finland) and FZK
(Germany)

• Review paper on coaxial gyrotrons
• O. Dumbrajs and G. S. Nusinovich,
• Coaxial Gyrotrons Past, Present and Future,
• IEEE-PS, 32, 934-946 (June 2004).
• 2. Effect of reflections in gyrotrons (ITER
  gyrotron)
• O. Dumbrajs, G. S. Nusinovich and B. Piosczyk,
• Reflections in gyrotrons with radial output
• consequences for the ITER gyrotron,
• Physics of Plasmas, 11, (Dec. 2004).
• 3. Instability of gyrotron radiation with respect
  to azimuthal perturbations of the RF field
• (collaboration with O. Dumbrajs, the work is in
  progress)