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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 250W at 263GHz and a gain of >40dB over a bandwidth of 17.5GHz. 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A continuous-wave (CW) high-harmonic gyrotron driven by a low-power electron beam is a compact radiation source demanded by terahertz applications. Its physical feasibility, however, is hampered by ohmic losses and mode competition in the gyrotron cavity. An ultralow-loss diamond loading of the cavity can give a clue to this problem. This article is concerned with theoretical aspects of mode selection and design for a gyrotron cavity loaded with coaxial rod made of chemical vapor deposition (CVD) diamond. As an example, the design of a high-Q diamond-loaded cavity for a third-harmonic 658-GHz gyrotron powered by a 0.1-A, 15-kV electron beam is presented. It is shown that the designed cavity enables the gyrotron to produce up to 116-W output power in a single oscillating mode. [ABSTRACT FROM AUTHOR]

One way to improve the mode selection in terahertz gyrotrons is the use of stepped cavity resonators that satisfy the resonance condition for two modes with the same azimuthal but different radial indices. Such sectioned resonators, or complex cavities, were analyzed in detail by several authors, recently and in the past. One common problem for such designs is the excitation of unwanted modes by both the stepped cavity structure and the cavity input and output tapers. Mode conversion becomes especially a severe problem for higher-order resonant modes in highly oversized resonators. In this article, we introduce ways to minimize the unwanted mode conversion in both the radial step of the cavity as well as in the tapered sections of the cavity and the up-taper to the collector and output window. Frequency tuning between 391.5 and 393.5 GHz has been achieved by operation at different axial mode numbers of the mode pair TE8,4/TE8,5. Due to their intrinsic higher output mode purity, the so-called antiphase modes of the complex cavity have been chosen for this output mode purity study. [ABSTRACT FROM AUTHOR]

An electron cyclotron system (ECS) is required in the European DEMO fusion reactor for plasma heating, magnetohydrodynamic control, especially the suppression of neoclassical tearing modes (NTMs), and thermal instability control. Microwave beams of 2 MW, generated by gyrotrons, shall propagate through corrugated HE11 waveguides (WGs) of 63.5-mm inner diameter and finally reach specific locations in the plasma by a dedicated set of mirrors. The deployment of the beam may be done either by angular steering, i.e., tilting moveable mirrors, or by frequency steering, i.e., tuning the gyrotron frequency. In the last option, for long pulse operations, broadband optical grade diamond windows are required as confinement and/or vacuum barriers both at the torus and the gyrotron side. The Brewster-angle window represents a promising solution due to its intrinsic simplicity. However, the WG aperture imposes a minimum Ø180 mm for the disk at the Brewster-angle of 67.2° for diamond. This is an ambitious target, considering 140 mm as the state of the art. First, this article shows the very promising results of diamond growth experiments carried out by Diamond Materials GmbH with the first worldwide free-standing optical grade Ø180-mm disk having an average unpolished thickness of 1.3 mm. For the first time, one side of this large disk was polished and loss tangent measurements were performed at the center and over a large area. Then, the performance of the window featuring these large disks was characterized by a consistent set of thermomechanical analyses in view of the forthcoming window prototyping activity. [ABSTRACT FROM AUTHOR]

In anticipation of an Electron Cyclotron Resonance Heating system upgrade for the stellarator Wendelstein 7-X, a 1.5 MW – 140 GHz continuous-wave gyrotron is under development. In order to provide a first experimental verification of the scientific RF and electron beam optics design of the gyrotron with ms pulses, the Karlsruhe Intitule of Technology has developed a short-pulse pre-prototype gyrotron. In this work, we present details regarding the construction of the pre-prototype as well as measurements from the first experimental campaign delivering up to 1.6 MW in short pulses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we present the current status and both the ongoing investigation and the continuous improvements to the operational performance of a unique gyrotron, which is built using a 20 T superconducting magnet and holds a world record of 10-W terahertz wave generation at the highest frequency (1.08 THz) in the continuous wave (CW) operation. In addition, it has demonstrated high-purity single-mode generation on a sequence of modes that cover a wide range from subterahertz to terahertz frequencies at both the fundamental and second-harmonic resonances of the electron cyclotron frequency. As an illustration, the measurements of the observed radiation patterns of eight output modes radiated from a currently used resonant cavity with a linear up-taper are presented and compared with the corresponding patterns simulated by scattering matrix calculations. A new design of an optimized cavity with a nonlinear up-taper, which further improves the mode purity of the generated output radiation has been proposed and is being currently implemented as a replacement of the existing resonator. The overall operational performance and the output characteristics of this gyrotron (called FU CW III in accordance with the nomenclature adopted at FIR UF Center) make it a versatile and appropriate source of coherent CW radiation for many novel applications in the fields of high-power terahertz science and technologies. [ABSTRACT FROM AUTHOR]

An iterative procedure is described, which models the influence of the thermal expansion of the gyrotron cavity on the expected gyrotron performance. It is a multiphysics simulation method, which involves electrodynamic, thermal-hydraulic, and thermo-mechanical simulations. The method is applied to the first European 170-GHz, 1-MW continuous wave prototype gyrotron for the ITER. According to the simulations, a performance reduction of ~15% is expected at nominal operating parameters, because of the thermal expansion of the cavity. Alternative operating points to mitigate this effect are proposed and numerically validated. The numerical results are discussed in light of experimental findings. [ABSTRACT FROM AUTHOR]

Single-stage depressed collector systems are widely used in high-power gyrotrons in order to increase the overall efficiency. The position where the deceleration of the beam takes place influences the collector efficiency. In this paper, several deceleration schemes are numerically investigated for the short-pulse prototype of the 170-GHz, 1-MW European gyrotron for International Thermonuclear Experimental Reactor. The results of this paper are in a qualitative agreement with the results of dedicated experiments. A simple idea is also proposed for the significant increase of the overall gyrotron efficiency without an important technological modification. [ABSTRACT FROM AUTHOR]

The possibility of dynamic after cavity interaction (ACI) in a down-scaled gyrotron model has been studied using a 3-D conformal finite-difference time-domain particle-in-cell simulation. The purpose has been to benchmark this type of simulation against previous numerical results obtained by other methods and tools and to obtain additional physical insight on the effect of dynamic ACI. The benchmarking has been successful, as a good agreement with the older results has been achieved. Thus, the algorithms developed and validated can be used for studying the issue in a larger scale or more realistic model with affordable computational resources in the near future. [ABSTRACT FROM AUTHOR]