Stabilization of absolute instabilities in the gyrotron traveling wave amplifier

Design and verification of a backward wave oscillation suppression circuit for the Ka-band gyrotron travelling-wave tube

Backward wave oscillation seriously degrades the stability of gyrotron travelling-wave tubes (gyro-TWTs), especially during high average/continuous wave operation. To solve this problem, a selective mode suppression structure (SMSS) based on the mode coupling principle is proposed and applied in the nonlinear beam-wave interaction region to suppress the parasitic TE11 mode. It is capable of obtaining a high power and improving the tube stability. Simulation results demonstrate that the SMSS can raise the starting current from 10 to 18 A and the starting pitch factor from 1.2 to 1.6. Based on this proposed circuit, a Ka-band TE01 mode gyro-TWT was designed, and the particle-in-cell simulation shows that it can achieve a saturated output power of over 150 kW from 29.7 to 31.7 GHz with a velocity spread of 2.2%. For verification, a SMSS is manufactured and cold tested. The measurement of S-parameters reveals that it can effectively suppress the parasitic TE11 mode.

Broadband rectangular TEn0 mode exciter with H-plane power dividers for 100 GHz confocal gyro-devices

A generic approach to excite TE_n0 (n ≥ 1) modes in a rectangular waveguide for confocal gyro-devices is proposed. The exciter consists of a 3 dB H-plane power divider (n ≥ 3) and a mode-converting section. The injection power is split into two in-phase signals with equal amplitudes which simultaneously excite the secondary waveguide via two sets of multiple slots. Both the position and width of the slot are symmetrically distributed with respect to the center line for each set of slots. The slot width complies with a geometry sequence, with adjacent slots being spaced a quarter wavelength apart to cancel the backward wave out. A TE₄₀ mode exciter at 100 GHz is numerically simulated and optimized, achieving a 1 dB and a 3 dB transmission bandwidth of 18.2 and 21 GHz, respectively. The prototype is fabricated and measured. The cold test is carried out utilizing two identical back-to-back connected mode exciters, and the measured performances are in good agreement with the numerical simulation results when taking into account the wall loss and assembly tolerance.

Absolute Instability near the Band Edge of Traveling-Wave Amplifiers

Applying the Briggs-Bers "pole-pinch" criterion to the exact transcendental dispersion relation of a dielectric traveling wave tube (TWT), we find that there is no absolute instability regardless of the beam current. We extend this analysis to the circuit band edges of a linear beam TWT by approximating the circuit mode as a hyperbola in the frequency-wave-number (ω-k) plane and consider the weak coupling limit. For an operating mode whose group velocity is in the same direction as the beam mode, we find that the lower band edge is not subjected to absolute instability. At the upper band edge, we find a threshold beam current beyond which absolute instability is excited. The nonexistence of absolute instability in a linear beam TWT and the existence in a gyrotron TWT, both at the lower band edge, is contrasted. The general study given here is applicable to some contemporary TWTs such as metamaterial-based and advanced Smith-Purcell TWTs.

Demonstration of a 140-GHz 1-kW Confocal Gyro-Traveling-Wave Amplifier

The theory, design, and experimental results of a wideband 140-GHz 1-kW pulsed gyro-traveling-wave amplifier (gyro-TWA) are presented. The gyro-TWA operates in the HE(06) mode of an overmoded quasi-optical waveguide using a gyrating electron beam. The electromagnetic theory, interaction theory, design processes, and experimental procedures are described in detail. At 37.7 kV and a 2.7-A beam current, the experiment has produced over 820 W of peak power with a -3-dB bandwidth of 0.8 GHz and a linear gain of 34 dB at 34.7 kV. In addition, the amplifier produced a -3-dB bandwidth of over 1.5 GHz (1.1%) with a peak power of 570 W from a 38.5-kV 2.5-A electron beam. The electron beam is estimated to have a pitch factor of 0.55-0.6, a radius of 1.9 mm, and a calculated perpendicular momentum spread of approximately 9%. The gyro-amplifier was nominally operated at a pulselength of 2 μs but was tested to amplify pulses as short as 4 ns with no noticeable pulse broadening. Internal reflections in the amplifier were identified using these short pulses by time-domain reflectometry. The demonstrated performance of this amplifier shows that it can be applied to dynamic nuclear polarization and electron paramagnetic resonance spectroscopy.

Transition of absolute instability from global to local modes in a gyrotron traveling-wave amplifier

The gyrotron traveling-wave amplifier employing the distributed-loss scheme is capable of very high gain and effective in suppressing the global absolute instabilities. This study systematically characterizes the local absolute instabilities and their transitional behavior. The local absolute instabilities are analyzed using a model that incorporates the penetration of the field from the copper section into the lossy section. The axial modes were characterized from the perspective of beam-wave interaction and were found to share many characteristics with the global modes. The transition from global modes to local modes as the distributed loss increases was demonstrated. The electron transit angle in the copper section, which determines the feedback criterion, governs the survivability of an oscillation. In addition, the oscillation thresholds predicted using this model are more accurate than those obtained using a simplified model.

Absolute instabilities in a high-order-mode gyrotron traveling-wave amplifier

The absolute instability is a subject of considerable physics interest as well as a major source of self-oscillations in the gyrotron traveling-wave amplifier (gyro-TWT). We present a theoretical study of the absolute instabilities in a TE01 mode, fundamental cyclotron harmonic gyro-TWT with distributed wall losses. In this high-order-mode circuit, absolute instabilities arise in a variety of ways, including overdrive of the operating mode, fundamental cyclotron harmonic interactions with lower-order modes, and second cyclotron harmonic interaction with a higher-order mode. The distributed losses, on the other hand, provide an effective means for their stabilization. The combined configuration thus allows a rich display of absolute instability behavior together with the demonstration of its control. We begin with a study of the field profiles of absolute instabilities, which exhibit a range of characteristics depending in large measure upon the sign and magnitude of the synchronous value of the propagation constant. These profiles in turn explain the sensitivity of oscillation thresholds to the beam and circuit parameters. A general recipe for oscillation stabilization has resulted from these studies and its significance to the current TE01 -mode, 94-GHz gyro-TWT experiment at UC Davis is discussed.

Phase and gain measurements in a distributed-loss cyclotron-resonance maser amplifier

The control of gain and phase delay in a cyclotron-resonance maser (CRM) amplifier is essential for a variety of applications. In this experiment, the gain and phase-delay variations are measured with respect to controlling parameters; the electron-beam current and the axial magnetic field. Following Chu et al. [Phys. Rev. Lett. 74, 1103 (1995)], the CRM amplifier comprises of a distributed-loss waveguide to enable high gain without oscillations. Our experiment yields an amplification up to 26 dB, and a phase-delay control range of 360 degrees. In order to keep a fixed gain with the varying phase delay, the two controlling parameters (i.e., the solenoid field and the beam current) are operated together in a compensating mode. The experiment is conducted in a frequency of 7.3 GHz, with an electron beam of 18-kV voltage and 0.25-0.4-A current. The experimental results are compared with a theoretical model. Practical implementations of gain and phase control in CRM devices are discussed.