Improved Mode Selection in Coaxial Cavities for Subterahertz Second-Harmonic Gyrotrons

A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for second-harmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length, thereby greatly increasing the starting currents. Such a method of mode selection is demonstrated to be more versatile, when compared to that provided by a tapered coaxial conductor. The advantage of a dielectric-coated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode

Theoretical Investigation on Injection Locking of the EU 170 GHz 2 MW TE34,19-Mode Coaxial-Cavity Gyrotron

Injection locking of gyrotron oscillators offers an improved mode stability and the precise phase and frequency control of the generated millimeter-wave signal. It might offer completely new possibilities for applications related to nuclear fusion plasma, spectroscopy, and radar. In this presentation it is shown that the theory of Kurokawa can be applied to understand the injection locking of gyrotrons and that it provides accurate prediction of the locking behavior. Based on that, the investigation on injection locking of the EU 170 GHz 2 MW TE 34,19 -mode coaxial-cavity gyrotron using self-consistent single and multimode simulations is presented. Detailed studies on injection signals containing competing modes to account either for signal impurities or for deliberate injection of competing modes are presented

Validation of a New Fast-Time Scale Code for Advanced Simulations of Gyrotron Cavities

Gyrotrons for fusion applications are microwave vacuum tubes that are capable to produce an output power in the megawatt range at long pulses up to continuous wave (CW) and at frequencies above 100 GHz. That is possible due to the working principle of gyrotrons which allows using cavities with a very large electrical size (in the order of several cm) compared to the operating wavelength (in the order of a few mm). This mandatory requirement for high output power is a challenge in simulating the interaction between the electromagnetic (EM) field and the electron beam in a gyrotron resonator. Due to this, the simulation of the electron interaction in gyrotrons are typically carried out by using computer codes which make use of the very specific properties of the EM problem to simplify the calculations. At KIT, a new code names “SimpleRick” is under development. A fast-time scale Particle-in-Cell (PIC) method is implemented to complement the classical models used for gyrotron simulation. The PIC code introduces significantly fewer assumptions than the classical model and may therefore represent more physical details. For example, in contrast to the classical models, the new model can represent non-symmetric electron beams. In this work, the numerical implementation and the performance of this PIC model are verified and a new method for the calculation of the eigenvalues of coaxial gyrotron resonators is shown in more detail

Extended Feedback System for Coupled Sub-THz Gyro-Devices to Provide New Regimes of Operation

A new type of high-power pulsed source in the millimeter and submillimeter frequency range, utilizing the method of passive mode locking, was proposed in 2015 by the Institute of Applied Physics (IAP-RAS) in Nizhny Novgorod. This principle, well known from laser physics, allows the generation of a periodic series of powerful, coherent, ultrashort pulses. In the millimeter and submillimeter wavelength range, this can be realized using an amplifier and a saturable absorber coupled in a feedback loop. For the coupling of the two devices, a sophisticated feedback system is required. Such a system, based on simple overmoded waveguide components, was previously proposed by the authors. The present article shows how the proposed feedback system can be extended, allowing for a wide range of possible operation regimes for two coupled gyro-devices. Particularly noteworthy is the application of the modified feedback system for the realization of a two-stage amplifier in the subterahertz (sub-THz) range. Furthermore, it seems to be possible to use two helical gyro-devices coupled in the proposed way as a source of coherent pulses, as a free-running or locked continuous wave (CW) source, and as a two-stage amplifier. In all cases, no design changes of the feedback system are required

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

Gyrotron multistage depressed collector based on E × B drift concept using azimuthal electric field. II: Upgraded designs

Multistage Depressed Collectors (MDCs) are nontrivial for high-frequency gyrotrons. A basic conceptual design of an E x B MDC using azimuthal electric fields was proposed in Part I of this series. In the present work, several upgraded design proposals based on the basic one will be elaborated. These proposals will significantly reduce the back-stream of electrons, which was the main drawback of the basic design proposal. Another upgraded design proposal will shrink the length and maximal radius of the MDC to be only a fraction of its full-length version. A conceptual design of the final MDC proposal will be given at the end

Multifaceted Simulations Reproducing Experimental Results from the 1.5-MW 140-GHz Preprototype Gyrotron for W7-X

A multifaceted simulation procedure, addressing the electron beam properties, the beam-wave interaction, and the internal losses, has been used for the simulation of the experimental operation of a 1.5-MW 140-GHz short-pulse preprototype gyrotron. The preprototype is related to the development of 1.5-MW gyrotrons for the upgrade of the electron cyclotron resonance heating system at the stellarator W7-X. A very good reproduction of experimental results has been achieved by simulation, without resorting to arbitrary speculations. This validated the numerical tools as well as the design and fabrication of the short-pulse preprototype, which fully reached the target of efficient 1.5-MW operation in millisecond pulses. Special attention has been given to simulating the possibility of parasitic after-cavity interaction in the gyrotron launcher. Also, parasitic backward-wave excitation in the gyrotron cavity has been demonstrated by simulation, at a frequency and voltage range in agreement with experimentally observed parasitic oscillations. This offers an additional possibility with respect to the origin of deleterious parasitic oscillations in high-power gyrotrons, which are usually attributed mainly to the gyrotron beam tunnel