Two-dimensional leapfrog scheme for trajectories of relativistic charged particles in static axisymmetric electric and magnetic field

A method for the calculation of two-dimensional particle trajectories is proposed in this work. It makes use of the cylindrical symmetry and the simplification of the static electric field, so that there should be no systematic error for the centered large-orbit rotations nor for the acceleration or deceleration in a uniform electric field. The method also shows a lower error level than the standard Boris method in many cases. Typical applications of this method are for example, electron microscopes, electron guns and collectors of gyro-devices as well as of other vacuum tubes, which can be described in axisymmetric cylindrical coordinates. Besides, the proposed method enforces the conservation of canonical angular momentum by construction, which is expected to show its advantages in the simulation of cusp electron guns and other components relying on non-adiabatic transitions in the externally applied static magnetic field

Progress of the Methods for Optimum of Quasi-Optical Mode Converters at KIT

The paper reports the progress of the methods for the synthesis of Quasi-Optical (QO) mode converters at KIT in the period from 2018 until today. Typically, the QO mode converter consists a waveguide launcher and a mirror system. The first progress done is the development of the spectrum reconstruction method for smoothing the launcher wall. The second is the improvement of the method for the design of quasi-parabolic mirrors. The third progress considers the synthesis of a Denisov-type launcher for the conversion of co- and counter-rotating modes

New Type of Pulsed High-Power sub-THz Source Based on Helical-Type Gyro-TWTs

A new type of microwave source for the generation of high-power ultra-short coherent pulses at 263 GHz is presented. The source is based on the idea of passive mode-locking of two microwave tubes as it was first proposed in [1]. While passive mode-locking is well-established in laser physics, this idea is new for microwave electron tubes and was first experimentally demonstrated in [2]. As electron tubes, two helical gyro-TWTs are used that are coupled by a quasi-optical feedback system. The configuration proposed in this publication extends the original setup to enable the operation of the passive mode-locked oscillator in the hard excitation regime and to allow in addition the operation of the coupled helical gyro-TWTs as two-stage amplifier and as frequency-tunable, phase-locked backward wave oscillators. The performed simulation show an expected output power above 500 W and pulse widths below 0.1 ns for an operation as passive mode-locked oscillator

New Type of sub-THz Frequency-Doubling Gyro-TWT with Helically Corrugated Circuit

A novel type of frequency doubling gyrotron traveling wave amplifier (FD-GTWT) for applications that require high-power microwave in the sub-THz frequency range is presented. The proposed FD-GTWT delivers high power and high gain over a broad bandwidth and simultaneously doubling the frequency of the input signal. Simulations of a first 263GHz FD-GTWT design are presented, which show for a 10mW driving signal at 131.5GHz an RF output power of 250Wat 263 GHz and a gain of >40 dB over a bandwidth of 17.5 GHz. The basis of the FD-GTWT are two interaction circuits separated by a long drift section. In the first circuit, the electron beam is pre-bunched at the fundamental cyclotron harmonic. In the second one, high-power RF is induced by the pre-bunched electron beam at the 2nd cyclotron harmonic. Both sections consist of helically corrugated waveguides that efficiently suppress parasitic interactions and allow broad bandwidth

Plasma Spraying of a Microwave Absorber Coating for an RF Dummy Load

The European fusion reactor research facility, called International Thermonuclear Experimental Reactor (ITER), is one of the most challenging projects that involves design and testing of hundreds of separately designed reactor elements and peripheric modules. One of the core elements involved in plasma heating are gyrotrons. They are used as a microwave source in electron–cyclotron resonance heating systems (ECRH) for variable injection of RF power into the plasma ring. In this work, the development and application of an alumina-titania 60/40 mixed oxide ceramic absorber coating on a copper cylinder is described. The cylinder is part of a dummy load used in gyrotron testing and its purpose is to absorb microwave radiation generated by gyrotrons during testing phase. The coating is applied by means of atmospheric plasma spraying (APS). The absorber coating is deposited on the inner diameter of a one-meter cylindrical tube. To ensure homogeneous radiation absorption when the incoming microwave beam is repeatedly scattered along the inner tube surface, the coating shows a varying thickness as a function of the tube length. By this it is ensured that the thermal power is distributed homogeneously on the entire inner tube surface. This paper describes a modeling approach of the coating thickness distribution, the manufacturing concept for the internal plasma spray coating and the coating characterization with regard to coating microstructure and microwave absorption characteristics

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

Development of a CUSP-Type Electron Gun for a W-Band Helical Gyro-TWT

To drive a broadband gyro-TWT with a helically corrugated interaction region, a high-quality axis-encircling electron beam is required. In this publication, a CUSP-type electron gun, capable of generating such a beam, is developed for a 94 GHz helical gyro-TWT. The design was optimized using the electron-beam-optics code ESRAY [1]. The final electron gun is optimized for the generation of an electron beam with a 50 kV beam voltage, 1.5 A current, and a pitch factor of α=1.0 with an RMS spread as low as 3.49 %. Additionally, tolerance studies, including the influence of deviations in the emitter position and the surface roughness of the emitter, are performed

Status and First Operation of Gyrotron Teststand FULGOR at KIT

FULGOR, the new KIT gyrotron teststand for megawatt-class gyrotrons, will be presented. Results of initial experiments using a 1.5 MW 140 GHz short pulse pre-prototype gyrotron will be discussed

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