Abstract-The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE 0 i mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

No abstract

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 results are in good agreement with theoretical predictions. [S0031-9007 (97)04699-1] PACS numbers: 84.40.FeThe 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 bandwidth 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...

The experimental demonstration of a high average power W-band ͑75-110 GHz͒ gyroklystron amplifier is reported. The gyroklystron has produced 118 AW peak output power and 29.5% electronic efficiency in the TE 011 mode using a 66.7 kV, 6 A electron beam at 0.2% rf duty factor. At this operating point, the instantaneous full width at half-maximum ͑FWHM͒ bandwidth is 600 MHz. At 11% rf duty factor, the gyroklystron has produced up to 10.1 kW average power at 33% electronic efficiency with a 66 kV, 4.15 A electron beam. This represents world record performance for an amplifier at this frequency. At the 10.1 kW average power operating point, the FWHM bandwidth is 420 MHz. At higher magnetic fields and lower beam voltages, larger bandwidths can be achieved at the expense of peak and average output power.

A model of the self-fields associated with the charge density and current of the electron beam is incorporated into three-dimensional nonlinear formulations of the interaction in free-electron lasers for both planar and helical wiggler configurations. The model assumes the existence of a cylindrically symmetric electron beam with a flat-top density profile and a uniform axial velocity, and the self-electric and self-magnetic fields are determined from Poisson’s equation and Ampère’s law. Diamagnetic and paramagnetic effects due the electron beam interaction with the wiggler field are neglected; hence, the model breaks down when the wiggler-induced transverse displacement is comparable to the beam radius. The nonlinear formulations are based upon the arachne and wigglin codes, which represent slow-time-scale formulations for the evolution of the amplitudes and phases of a multimode superposition of vacuum waveguide modes. The electron dynamics in these codes are treated by means of the complete three-dimensional Lorentz force equations, and the representations for the self-fields are incorporated directly into this formulation. The results of the simulations are compared directly with an experiment at Lawrence Livermore National Laboratory based upon a planar wiggler and experiments at the Massachusetts Institute of Technology and the Naval Research Laboratory, which employed helical wigglers. These experiments employed intense electron beams with current densities of 200–1200 A/cm2 and comparable space-charge depressions of Δγself/γ0≊0.53%–0.78% across the beam. The simulations are in reasonable agreement with the experiments, and indicate that the self-fields tend to (1) reduce saturation efficiencies and (2) enhance beam spreading depending upon the magnitude of external beam focusing.