EXPERIMENTAL AND THEORETICAL STUDY OF GYROTRON MODE CONVERTERS

1
EXPERIMENTAL AND THEORETICAL STUDY OF GYROTRON
MODE CONVERTERS

• M.A. Shapiro, J.P. Anderson, E. Choi, C.
  Marchewka, I. Mastovsky, J.R. Sirigiri, R.J.
  Temkin
• MIT Plasma Science and Fusion Center
• T.S. Chu
• Communications and Power Industries

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OUTLINE

• Motivation
• 1.5 MW, 110 GHz pulsed gyrotron with internal
  mode converter
• New approach for mode converter design
• Collaboration with LHD
• Conclusions

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Motivation

• High power 110 GHz gyrotrons are needed for
  ECH/ECCD of DIII-D Tokamak at General Atomics.
• CPI gyrotrons have produced 1 MW at 110 GHz for 5
  s.
• Next step is to produce 1.5 MW at 110 GHz with
  gt50 efficiency using a depressed collector.
• MIT research in short pulses tests physics and
  engineering issues of the 1.5 MW, 110 GHz
  gyrotron design.
• MIT experimental system is demountable and
  changes / improvements can be made quickly.
• Successful experiments at MIT will contribute to
  industrial tube development at CPI.

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1.5 MW, 110 GHz Gyrotron

• Design Parameters

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1.5-MW Gyrotron Schematic
First Experimental Stage (Axial Configuration)
Superconducting Magnet
30 l/s Ion Pump
VAT Gate Valve
MIG Electron Gun
Collector
Thermionic Cathode
Microwave Resonator
Window
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Power and Efficiency
Power and Efficiency vs. Magnetic Field
2000
0.5
Vb 96 kV Ib 40 A
TE22,6 (MAGY)
1.4 MW (37 )
0.4
1500
1.1 MW (30 )
1.1 MW (30 )
0.3
Efficiency
1000
Power (kW)
TE21,6
0.2
TE22,6
TE23,6
500
0.1
0
0
4.2
4.3
4.5
4.6
4.4
Magnetic Field (T)
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Internal Mode Converter
IMC Gyrotron Experiment Schematic
VAT Gate Valve
Superconducting Magnet
220 l/s Ion Pump
Single-stage Depressed Collector
Microwave Resonator
Launcher
Window
4-Mirror Internal Mode Converter
8
Mirrors
Mirror 2
Launcher
Mirror 1
12 cm
Mirror 3
Mirror 4
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Mode Converter Design and Experiment

• Patterns measured in cold test at CPI
• Pattern after Mirror 2 measured on a series of
  planes near Mirror 3 location
• Mirrors 34 designed using cold test data.
  Iteration method used (Denison et al., IEEE PS,
  1999)
• Complex GB content 99.0
• 0.7 MW power observed so far
• Mirror misalignment
• Mirror holder rebuilt

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1.5 MW gyrotron accomplishments

• Experimental Results (axial configuration)
• 1.4 MW at 37 efficiency in TE22,6 mode
• Mode converter designed and installed in tube
• Initial testing to 0.7 MW at 96 kV, 40 A
• Second stage of testing with correct mirror
  location will begin late October
• Depressed collector will be tested this year

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New Approach Method of Irradiance Moments for
Mirror Synthesis
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Method of irradiance moments for synthesis of
phase correctors
?0(x,y)?01x?10y?20x2?11xy?02y2 ?30x3?21x2y
?12xy2?03y3
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Non-Gaussian amplitude
Amplitude distribution (dB contours) at Plane I
employed in phase corrector synthesis.
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PHASE CORRECTOR PROFILE Plane II
Left - Synthesized using irradiance moment
method. The phase contours are indicated in
radians. Right - Synthesized using iteration
method
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PHASE CORRECTOR PROFILE Plane I
Left - Synthesized using irradiance moment
method Right - Synthesized using iteration method
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Window pattern
99.0 Gaussian beam with flat phase front
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Conclusions on Moments Method

• New method used for phase corrector synthesis
• Gaussian amplitude at plane II
• No side lobes at plane I
• Advantages
• Can be used to identify measurement errors
• Smooth mirror surface, easy to fabricate
• Phase unwrapping not needed
• Method useful for mirror design for improved
  launcher
• UW-CCR design, presentation by Prof. R. Vernon

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Collaboration with LHD
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LHD collaboration 168 GHz gyrotron radiation

• 2003-2004 IR measurement of 168 GHz gyrotron
  radiation
• LHD research group (Dr. Shimozuma) measured 168
  GHz gyrotron radiation emanating from
• 8.9 cm diameter corrugated waveguide (using IR
  camera). Waveguide misaligned, multimode
  radiation.
• MIT method of Irradiance moments used to align
  waveguide.
• MIT retrieved phase distribution from IR
  amplitude data.
• Collaboration will continue
• Presentation by Dr. Idei at this workshop

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Conclusions

• Observed 1.4 MW pulsed operation of 110 GHz
  gyrotron in axial configuration
• Mode converter designed and installed in tube
• Initial measurement 0.7 MW
• New method of IMC mirror design developed based
  on irradiance moments. Results in smooth mirror
  profiles.
• LHD collaboration on 168 GHz gyrotron
  transmission line