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Design of a Novel 140 GHz, 1 kW Gyro-Amplifier Colin D. Joye , A. Cerfon, M. A. Shapiro,

Design of a Novel 140 GHz, 1 kW Gyro-Amplifier Colin D. Joye , A. Cerfon, M. A. Shapiro, J. R. Sirigiri, R. J. Temkin and A. C. Torrezan Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge, MA 02139 April 27 , 200 6 Session 21.4. Outline of Presentation.

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Design of a Novel 140 GHz, 1 kW Gyro-Amplifier Colin D. Joye , A. Cerfon, M. A. Shapiro,

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  1. Design of a Novel 140 GHz, 1 kW Gyro-Amplifier Colin D. Joye, A. Cerfon, M. A. Shapiro, J. R. Sirigiri, R. J. Temkin and A. C. Torrezan Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge, MA 02139 April 27, 2006 Session 21.4

  2. Outline of Presentation • Introduction, Specifications. • Confocal waveguide. • Simulations (linear, nonlinear, backward wave). • Novel quasi-optical severs. • Summary

  3. Introduction • Need for high gain, moderate power amplifiers at >100 GHz • Short pulse spectroscopy • Electron Paramagnetic Resonance: Nanosecond-scale pulse sequences. • Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance. • Emerging medical applications. • Explore a novel confocal structure • Distributed diffractive loss to stabilize against oscillations. • Avoids use of lossy ceramics or other custom absorbers. • Based on a successful experiment at MIT1. • Novel high order mode operation 1J. R. Sirigiri, M. A. Shapiro, R. J. Temkin, “High-power 140-GHz quasioptical gyrotron traveling-wave amplifier,” Phys Rev Lett, vol. 90, iss. 25, pp. 258302-4, 2003.

  4. Amplifier Diagram

  5. Miter bend 12.57 mm TE11 waveguide Beam tunnel Collector Uptapers Sever Input Transmission Line • 12.6 mm (0.5 inch) diameter overmoded TE11 transmission line • 3.6 ± 0.5 dB loss in 138-142 GHz band

  6. Design Parameters

  7. Superconducting Magnet • 6.2T superconducting magnet from Magnex Scientific. • Demonstrated to meet spec. • 28cm flat field (+/-0.5%). • 127mm warm bore diameter. • 1m bore length. • Shielded design reduces stray magnetic field outside the magnet.

  8. Varian VUW-8140 Triode • Varian VUW-8140 Triode MIG • V0 = 30 kV • Cathode Radius: 0.91 cm • Magnetic compression: 25 • Transverse spread < 4%

  9. Confocal Waveguide Dispersion Relation Loss rates HFSS

  10. Gaussian-like higher order mode Mode selectivity enhanced by diffraction Mode spectrum is sparser than a cylindrical waveguide circuit Suitable for harmonic operation Mechanical tuning capability in vacuum Surprisingly robust to misalignment Diffractive loss avoids custom lossy materials. Easily increase loss by cutting down the aperture. Simple sever implementations 2a=6.0mm Rc=6.9mm 3.7 mm Quasi-optical Confocal Structure Rc: Mirror curvature and separation. a: Aperture size.

  11. HFSS Model • Simple input coupler design: • 5.5 GHz BW • S11 < -15 dB

  12. Rc = 6.9 mm, a=3.0mm Dispersion Relation HEMN Modes are spaced equidistant in frequency and linearly in M and N, The loss rates can be calculated: HE06: -1 dB/cm HE15: -28 dB/cm HE05: -2 dB/cm Confocal waveguide can be highly selective. HE05 is the primary BWO mode.

  13. Theory vs. HFSS The simple Gaussian beam theory matches HFSS within a few tenths of a dB/cm.

  14. Simulations Linear Backward wave Nonlinear

  15. HE06 Linear Gain Rate Need ~3 dB/cm to get >50 dB total. V0 = 30kV I0 = 2A α = 0.75 v spread = 0% B0 = 4.94 T Mode: HE06 Rc = Lt = 6.9mm Rb = 1.85mm Linear theory predicts significant gain as required to meet spec.

  16. HE05 Backward Wave Oscillations BWO criterion for the HE05 mode using general start current equation1: f = 120.2 GHz V0 = 30 kV α= 0.75 B0 = 4.94 T Rc = 6.9 mm Rb = 1.85 mm (2nd max) For I0=2A, a=3.0mm, we have 7.2 cm section maximum lengths. (code does not include kzI yet for confocal) 1Nusinovich and Li, “Theory of gyro-travelling-wave tubes at cyclotron harmonics,” Int. J. Elec., v. 72 (5-6), pp. 895-907, 1992.

  17. Sev Confocal WG Confocal WG Confocal WG HE06 Nonlinear Simulation Sev v spread = 6% B0 = 4.97 T (99.7% grazing) Rc = Lt = 6.85mm a = 3.0 mm Pin = 10 mW Pout = 2 kW pk > 1 kW Sat. Gain = 53 dB > 50 dB BW(-3dB) = 2.4 GHz > 1GHz Gain profile at 140 GHz

  18. x y z z amin anom sever Backward wave Forward wave Sever Options Confocal waveguide allows severs to be constructed simply. • Diffractive sever: region of very small aperture. • HFSS: 30 dB loss over 3cm. • Quasi-optical sever: due to the high bounce angle of ~75o, only a small gap is needed to achieve high loss. • HFSS: Undergoing simulation. Waveguide top view: Waveguide side view:

  19. Summary • The design of a novel high-order mode gyro-TWT employing diffractive loss via confocal waveguide has been presented. • A simple theory for confocal waveguide was discussed. • Linear gain, backward wave thresholds were shown. • Nonlinear simulations: Requirements are within reach. • Concepts for quasi-optical severs look very promising. • Plan to run first experiment in August, 2006. This work is supported by NIH / NIBIB, grant R01-EB001965.

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