Gain broadening of two-stage tapered gyrotron traveling wave tube amplifier

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.

High efficiency and wideband gyro-traveling-wave-tube amplifier

A model is presented for a wideband gyro-traveling-wave-tube amplifier. The model's output bandwidth is comparable to that of the existing wideband model, but its efficiency is improved a lot. In this model, after choosing a proper initial magnitude, the axial magnetic field has a slightly linear decrease from a given position in the interaction section. The interaction mechanism of the beam wave is analyzed and verified by numerical simulation. When the beam current is 7 A, voltage is 90 KV, the velocity ratio is 1.0, and the axial velocity spread is 2%, numerical results show a constant bandwidth of 20% with an efficiency of 42%; the peak power and gain are, respectively, 260 KW and 47 dB.

High-gain wide-band gyrotron traveling wave amplifier with a helically corrugated waveguide

First bandwidth measurements of a novel gyrotron amplifier are presented. The coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 1.1 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 8.4 to 10.4 GHz ( 21% relative bandwidth), and an efficiency of 29%, in good agreement with theory.

Linear theory of multistage forward-wave amplifiers

A small-signal theory describing multistage gyro-traveling-wave tubes (gyro-TWTs) is developed. Multistage configurations of gyro-TWTs as well as conventional TWTs seem to be attractive because of their principal ability to operate stably with a high gain, since the stability can be provided by a shortening of each stage while the gain increases with the number of stages. Two regimes of operation, far from cutoff and near cutoff, are considered. In the former case, the equations can be reduced to those describing conventional TWTs and free electron laser amplifiers. Thus a part of the results obtained is also valid for these devices. For two-stage configurations (two waveguides separated by a drift region) the effect of the difference in waveguide cutoff frequencies on the gain and bandwidth is studied. This effect has a common nature with stagger-tuning of cavities, which is widely used in conventional klystrons and gyroklystrons for bandwidth enlargement. The trade-off in gain and bandwidth of such "stagger-tuned," two-stage gyro-TWTs and TWTs is analyzed. Also, the theory of two-stage gyro-TWTs with tapered waveguides and external magnetic field is discussed.