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Experimental Demonstration of aW-Band Gyroklystron Amplifier

M. Blank, B. G. Danly, B. Levush, P. E. Latham, D. E. Pershing

1997
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Physical Review Letters
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The experimental demonstration of a four cavity W -band (93 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 67 kW peak output power and 28% efficiency in the TE 011 mode using a 55 kV, 4.3 A electron beam. The full width at half maximum instantaneous bandwidth is greater than 460 MHz, a significant increase over the bandwidth demonstrated in previous W -band gyroklystron amplifier experiments. The amplifier is unconditionally stable at this operating point. Experimental
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... sults are in good agreement with theoretical predictions. [S0031-9007 (97) 04699-1] PACS numbers: 84.40.Fe The continuing need for high power sources of microwave and millimeter wave radiation for such varied applications as high resolution radars, linear accelerators [1], magnetic resonance imaging [2] , and communications has led to extensive research on gyroklystron amplifiers [3] [4] [5] [6] [7] [8] [9] [10] . Much like a conventional klystron, the gyroklystron consists of several resonant cavities separated by drift sections cut off to the operating mode. As evidenced in numerous experiments, the interaction of the beam with the trapped mode in the cavity, based on the electron cyclotron maser instability, can reliably and efficiently generate high power, moderate bandwidth electromagnetic radiation at microwave and millimeter wave frequencies. For example, a three cavity C-band gyroklystron amplifier produced 54 kW peak output power and 30% efficiency in the TE 101 at 4.5 GHz [3]. The saturated gain was 30 dB and the FWHM bandwidth was 0.4%. A three cavity X-band gyroklystron achieved 16 kW peak output power and 45% efficiency with a FWHM bandwidth of 1% [4]. Fundamental and second harmonic two cavity gyroklystron amplifiers at 9.87 and 19.7 GHz, designed as drivers for linear colliders, achieved peak output powers of 20 and 30 MW, respectively, with efficiencies near 30% [5, 6] . A two cavity Ka-band gyroklystron, developed for radar applications, produced 750 kW at 35 GHz in the TE 021 mode at 24% efficiency [8] . In W -band, a pulsed four cavity gyroklystron amplifier achieved 65 kW peak output power at 26% efficiency with 300 MHz bandwidth [9] . A continuous wave version of the device demonstrated 2.5 kW average output power. The gyroklystron interaction, which takes place in standing wave cavities, inherently gives high efficiency, gain, and output power, but lower bandwidth than devices which rely on traveling wave interactions, such as the gyro-traveling-wave tube (gyro-TWT). Obtaining wider bandwidth without the concomitant problems of the absolute instability associated with the gyro-TWT interaction is an important area of study. It is the goal of the present work to demonstrate a high power, high gain, efficient, stable W -band gyroklystron amplifier with greater band-width than previously achieved, and to elucidate the basic physics of gyroamplifiers by comparing theoretical predictions with experimental results. This paper presents an experimental study of a four cavity W-band gyroklystron amplifier operating in the TE 011 mode near the fundamental cyclotron frequency, V c eB͞mg. The circuit consists of a drive cavity, two idler cavities, and an output cavity. The circuit was designed with a time-dependent version of the nonlinear code MAGYKL [11] . The wave equation solved in MAGYKL is given by where a is the complex amplitude of the fields, D v Q͕Re͕v c ͖ 2 v͖͞v is the normalized frequency shift, Q is the quality factor of the cavity, v is the drive frequency, v c is the cold resonant frequency, I b is the beam current, W EM is the stored energy, c is the speed of light, n perp and n z are the perpendicular and axial electron velocities, E c is the cold cavity electric field, and t is the time normalized to Q͞v, i.e., vt͞Q. In the formulation, the cavities are modeled by a series of straight uniform sections with abrupt discontinuities at the boundaries. This model is strictly accurate for the intermediate cavities, and the up taper of the output cavity is divided into 80 steps to approximate the smooth taper in radius. The fields in each section, expanded as a radial series of TE, TM, and TEM modes, are determined through a scattering matrix solution [12] . Theoretical studies have shown that it is necessary to include many terms in this radial mode series to correctly determine the resonant frequency of the cavity. In order to accurately predict the bandwidth of the amplifier, the formulation detailed in Ref. [11] was modified to include a frequency dependent drive power, as dictated by the resonant frequency and Q of the input cavity. The field amplitude in the drive cavity is given by 0031-9007͞97͞79(22)͞4485(4)$10.00

doi:10.1103/physrevlett.79.4485
fatcat:3ghdkza6ifh67mlfrkka4whudy