Helical gyro-TWT Research Articles

Time-Domain Simulation of Helical Gyro-TWTs With Coupled Modes Method and 3-D Particle Beam

A new self-consistent time-domain model for the simulation of gyrotron traveling-wave tubes with a helically corrugated interaction space (helical gyro-TWTs) is presented. The new model links classical methods using the approach of slowly varying variables together with an expansion of the electromagnetic field in eigenmodes and advanced full-wave particle-in-cell (PIC) solvers. The aim is to significantly reduce the required calculation time compared to full-wave PIC solvers, while less strict assumptions are introduced as in the classical approaches of slowly varying variables. For the first time, the classical theory of coupled circular waveguide modes for the description of the operating electromagnetic eigenmode in the helical interaction space is combined with a 3-D PIC representation of the electron beam. This allows the simulation of the beam–wave interaction over a broad bandwidth and at arbitrary harmonics of the cyclotron frequency. In addition, arbitrary electron beams (with spreads, offsets of the guiding center from the symmetry axis, and so on) can be investigated. The new approach is compared with the full-wave 3-D PIC code CST Microwave Studio. A good agreement of the simulation results is achieved, while the computing time is significantly reduced.

Design Procedure for a Broadband TE11/HE11 Mode Converter for High-Power Radar Applications

The HE11 hybrid mode, propagating in an overmoded corrugated circular waveguide, is widely used for low loss transmission of high-power microwaves. Due to the inherent broadband frequency behaviour, this will be also essential for future broadband high-power radar applications, like space debris observation in low earth orbit (LEO). A promising amplifier concept for such radar sensors is a helical gyro-TWT. However, since the HE11 hybrid mode is not suitable for electron-beam-wave interaction in this kind of vacuum electron device, an additional mode converter is required. The present paper addresses the design procedure of a broadband high-power mode converter, designed for a helical gyro-TWT intended for future broadband high-power radar applications in the W-band. The interaction mode of the helical gyro-TWT under consideration can be easily transferred to the circular waveguide TE11 mode. Therefore, a TE11 ↦ HE11 mode converter is addressed here. The design procedure is based on a scattering matrix formalism and leads to a high HE11 mode content of ≥98.6% within the considered frequency range from 92 GHz to 100 GHz. Inside this frequency band, the mode content is even better and reaches ≈ 99.7% at ≈95 GHz. This allows broadband frequency operation of a helical gyro-TWT and is suitable for broadband high-power radar applications.

Формирование сверхмощных микроволновых импульсов в системах стретчер-усилитель-компрессор.

We theoretically investigate the possibility of generating ultra-high-power ultrashort microwave pulses based on the Chirped-Pulse Amplification (CPA) method, which is widely used in laser physics. This method includes preliminary elongation of the initial pulse in a stretcher, sequential amplification of spectral components in a broadband amplifier, and compression in a line with negative dispersion (compressor). We consider the scheme in which waveguides with multi-fold helical corrugation are used as dispersing elements (stretcher and compressor), and a relativistic Cherenkov TWT or helical gyro-TWT is used as an amplifier. For the parameters of experimentally realized amplifiers in the 30 GHz range, we show that the peak pulse power in the stretcher-amplifier-compressor system significantly exceeds not only the saturation level of the amplifier, but also more than 4 times higher than the power of the used electron beam.

Startup scenarios for an ultrashort pulse generator based on two coupled helical gyro-TWTS operating in the amplification and nonlinear absorption modes

Startup scenarios are studied for a high-power microwave ultrashort pulse (USP) generator with passive mode locking and consisting of two coupled helical gyro-TWTs operating in the amplification and nonlinear absorption modes. A startup scenario is proposed for a USP generator with a delay of switching on the absorbing section that allows the stability of generated pulses to be improved with a simultaneous increase in the efficiency of the device.

Generation of trains of ultrashort microwave pulses by two coupled helical gyro-TWTs operating in regimes of amplification and nonlinear absorption

Based on a time-domain model, we demonstrate that a periodic train of powerful ultrashort microwave pulses can be generated in an electron oscillator consisting of two coupled helically corrugated gyrotron travelling wave tubes (gyro-TWTs) operating in regimes of amplification and saturable absorption, respectively. The mechanism of pulse formation in such an oscillator is based on the effect of passive mode-locking widely used in laser physics. Saturable absorption can be implemented in a gyro-TWT in the Kompfner dip regime by a proper matching of the guiding magnetic field. According to simulations with the parameters of an experimentally realized Ka-band gyro-TWT, the peak power of generated pulses with a duration of 200 ps can achieve 400 kW.

Ultrashort pulse generation based on two coupled helical gyro-TWTs

Ultrashort pulse generation based on two coupled helical gyro-TWTs

Mechanisms of amplification of short electromagnetic pulses in gyroresonance traveling-wave tubes

Traditional gyroresonance traveling-wave tubes (gyro-TWTs) with a smooth waveguide serving as the operating space and gyro-TWTs based on a waveguide with multiturn helical corrugation are considered. A nonstationary nonlinear theory providing for the description of amplification of short pulses in various circuits of such TWTs is proposed. The specific features of amplification of short pulses in helical gyro-TWTs are investigated for the regimes in which the dispersion curves of a normal wave and of an electron flow contact and intersect. In the first case, it is demonstrated that pulses having a spectrum width on the order of the amplification bandwidth can be amplified without distortion of their shapes. In the second case, it is shown that the slip of an extremely short input pulse relative to the electron beam ensures the infeed of one of the fronts of an electromagnetic pulse with an initially unmodulated electron flow. As a result, the pulse amplitude at the output of the interaction space can exceed the saturation amplitude in the regime of stationary amplification of monochromatic radiation.

Gyro-TWT with a helical operating waveguide: new possibilities to enhance efficiency and frequency bandwidth

A helical corrugation of the inner surface of an oversized cylindrical waveguide provides, for certain parameters, an almost constant value of group velocity and close to zero longitudinal wavenumber of an eigenwave for a very broad frequency band. The use of such a helical waveguide as an operating section of a gyrotron traveling wave tube (gyro-TWT) allows significant widening of its bandwidth and an increase in the efficiency at very large particle velocity spreads. In this paper, the new concept is confirmed by theoretical analysis and "cold" measurements of the helical waveguide dispersion. Results of a linear and nonlinear theory of the helical gyro-TWT as well as two designs for subrelativistic (80 keV, 20 A) and relativistic (300 keV, 80 A) electron beams are also presented. For both designs, parameters providing a very broad frequency band (about 20%) and high efficiency (above 30%) have been found. When the transverse velocity spread is increased from zero up to a very high value of 40%, simulations showed only a 20%-30% narrowing in the frequency band and a 20% decrease in electron efficiency. The theoretical analysis demonstrates important advantages of the helical gyro-TWT over the "smooth" one in frequency bandwidth, sensitivity to electron velocity spread, and stability to parasitic self-excitation.