Doctoral Thesis: A 250 GHz Photonic Band Gap Gyrotron Amplifier

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Event Speaker: 

Emilio A. Nanni

Event Location: 

NW17-218

Event Date/Time: 

Friday, March 22, 2013 - 10:30am

Abstract: 
This thesis reports the theoretical and experimental investigation of a
novel gyrotron traveling-wave-tube (TWT) amplifier at 250 GHz. At present,
there are no other amplifiers in this frequency range that are capable of
producing either high gain or high output power. The gyrotron amplifier
designed and tested in this thesis has achieved a peak small signal gain of
38 dB at 247.7 GHz, with a 32 kV and 0.35 A electron beam and 8.9 T magnetic
field. The instantaneous -3 dB bandwidth of the amplifier at peak gain is
0.4 GHz. A peak output power of 45 W has been measured. The output power is
not saturated but is limited by the 7.5 mW of available input power. The
amplifier can be tuned for operation from 245-256 GHz. With a gain of 24 dB
and centered at 253.25 GHz the widest instantaneous -3 dB bandwidth of 4.5
GHz was observed for a 19 kV, 0.305 A electron beam. To achieve stable
operation at these high frequencies, the amplifier uses a novel photonic
band gap (PBG) interaction circuit. The PBG interaction circuit confines the
TE03-like mode which couples strongly to the electron beam. The PBG circuit
provides stability from oscillations by supporting the propagation of TE
modes in a narrow range of frequencies, allowing for the confinement of the
operation TE03-like mode while rejecting the excitation of oscillations at
lower frequencies. Experimental results taken over a wide range of
parameters, 15-30 kV and 0.25-0.5 A, show good agreement with a theoretical
model. The theoretical model incorporates cold test measurements for the
transmission line, input coupler, PBG waveguide and mode converter. This
experiment achieved the highest frequency of operation for a gyrotron
amplifier. With 38 dB of gain and 45 W this is also the highest gain
observed above 94 GHz and the highest output power achieved above 140 GHz by
any vacuum electron device based amplifier.
Thesis Supervisor: Dr. Richard J. Temkin, PSFC