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. 2010 Sep 24;105(13):135101.
doi: 10.1103/PhysRevLett.105.135101. Epub 2010 Sep 20.

Amplification of picosecond pulses in a 140-GHz gyrotron-traveling wave tube

H J Kim  1 , E A Nanni, M A Shapiro, J R Sirigiri, P P Woskov, R J Temkin
Affiliations

Amplification of picosecond pulses in a 140-GHz gyrotron-traveling wave tube

H J Kim et al. Phys Rev Lett. .

Abstract

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro-TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE₀₆ mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz. Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.

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Figures

FIG. 1
FIG. 1
(color online). Dispersion diagram of HE06 gyro-TWT in a confocal waveguide (left) and cross sectional view (right). The annular electron beam has a 4.2 mm diameter.
FIG. 2
FIG. 2
(color online). Schematic diagram of the picosecond pulse amplification in a quasioptical gyro-TWT.
FIG. 3
FIG. 3
(color online). Measured and calculated gain-bandwidth curves for beam voltages of 35.4 and 28 kV. The points labeled (a), (b), (c), and (d) correspond to the labels in Fig. 6 below.
FIG. 4
FIG. 4
(color online). Measured (dots) and calculated (solid line) transit time delay of 500 ps pulses in (a) 10 cm-long WR4 waveguide with a cutoff frequency of 139.25 GHz and (b) the 28.4 cm gyro-TWT confocal waveguide structure with a cutoff frequency of 135.0 ± 0.3 GHz.
FIG. 5
FIG. 5
(color online). Pulse shapes for an input pulse of 580 ps at the 35.4 kV. The output pulses at (a) 137.3 GHz and (b) 138.13 GHz have measured widths of 1045 ps and 660 ps, respectively.
FIG. 6
FIG. 6
(color online). Measured (dots) and calculated (solid lines) output pulse width after amplification as a function of input pulse width; (a) 28 kV and 137.4 GHz; (b), (c), and (d) are for 35.4 kV and (b) 138.13 GHz, (c) 137.3 GHz, and (d) 138.8 GHz.
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