Investigations on Improving Broadband Boundary Conditions in Gyrotron Interaction Modelling

Gyrotrons are microwave tubes capable of providing mega-watt power at millimetric wavelengths. The microwave power is produced by the conversion of the kinetic energy of an electron beam to electromagnetic wave energy. Simulations of the beam-wave interaction in the gyrotron cavity are essential for gyrotron design, as well as theoretical and experimental studies. In the usual gyrotron operation the spectrum of the generated radiation is concentrated around the nominal frequency. For this reason, the usual simulations consider only a narrow-band output spectrum (e.g. several GHz bandwidth comparing with the working frequency in the range of 100-200 GHz). As a result, the typical existing codes use a single-frequency radiation boundary condition for the generated electromagnetic field in the cavity. This condition is matched only at one frequency. However, there are two important aspects, which motivate an advanced formulation and implementation of the cavity boundary condition. Firstly, the occurrence of broadband effects (which may be several tens of GHz) in some cases, like dynamic after-cavity-interaction or modulation side-bands, requires a broadband boundary condition. Secondly, there are reflections from inside and outside of the gyrotron, which can only be considered in the simulation through a boundary condition with user-defined, frequency-dependent reflections. This master thesis proposes an improved formulation of the broadband boundary condition in the self-consistent, beam-wave interaction code Euridice. In this new formulation, two physical variables — the wave impedance and the axial wavenumber are expanded in polynomial series in the frequency domain. Because the beam-wave interaction process is simulated transiently in the time domain, the boundary condition should be also expressed in the time domain. This involves a non-trivial inverse Fourier transform, for which two solutions are proposed, tested and validated. It has been shown that, through the newly developed formulation, the existing matched boundary condition (that should yield zero-reflection in ideal case) can be improved by 15 dB even with a first-order polynomial series. Moreover, a user-defined, frequency-dependent complex reflection coefficient can be introduced. This was not possible with the previously existing boundary condition in Euridice

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

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

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