J. Franck

A generic mode selection strategy for high-order mode gyrotrons operating at multiple frequencies

High-power, high-frequency gyrotrons for electron cyclotron resonance heating and current drive, such as proposed for the demonstration thermonuclear fusion reactor DEMO, require operating modes of very high order. As it is shown, the selection of the operating modes for such gyrotrons can be based on multi-frequency operability. A general selection strategy is derived, suitable for multi-purpose multi-frequency gyrotrons with quasi-optical mode converter and single-disc output window. Two examples, one of them relevant for future DEMO gyrotron designs, are discussed.

Systematic cavity design approach for a multi-frequency gyrotron for DEMO and study of its RF behavior

High frequency (>230 GHz) megawatt-class gyrotrons are planned as RF sources for electron cyclotron resonance heating and current drive in DEMOnstration fusion power plants (DEMOs). In this paper, for the first time, a feasibility study of a 236 GHz DEMO gyrotron is presented by considering all relevant design goals and the possible technical limitations. A mode-selection procedure is proposed in order to satisfy the multi-frequency and frequency-step tunability requirements. An effective systematic design approach for the optimal design of a gradually tapered cavity is presented. The RF-behavior of the proposed cavity is verified rigorously, supporting 920 kW of stable output power with an interaction efficiency of 36% including the considerations of realistic beam parameters.

Multi-frequency operation of DEMO gyrotron with realistic electron beam parameters

Along with high power and high efficiency operation around 240 GHz, the requirements of DEMO gyrotrons include the possibility of multi-frequency operation and fast frequency tunability in 2-3 GHz steps. In this paper, multi-frequency operation of a proposed design for a conventional cavity DEMO gyrotron is presented, including operating parameters at different frequencies. The performance of the gyrotron is simulated considering realistic electron beam parameters.

Direct Voltage Depression Calculation of Arbitrary Electron Beams in Misaligned Coaxial Gyrotron Cavities

Coaxial-cavity gyrotrons for electron cyclotron heating in plasma experiments for nuclear fusion can operate with very high-order modes, having reduced mode competition and decreased voltage depression compared with hollow-cavity tubes. However, since exact alignment of coaxial insert and cavity wall can only be ensured up to a certain precision, the effects of misalignment must be properly understood. In this paper, an efficient method is presented to determine the voltage depression on beam electrons for arbitrary misalignment between cavity wall and insert, and for a beam with arbitrary shape and density distribution. The method has been verified using a 3-D code, and it can be generalized to some other geometries.

Interaction circuit design and RF behavior of a 236 GHz gyrotron for DEMO

The Demonstration Fusion Power Reactor (DEMO) to follow ITER by 2050 demands high frequency (>230 GHz), high power (in the range from 1 MW to 2 MW) gyrotrons as RF sources for electron cyclotron resonance heating and current drive (ECRH&CD). In the frame of the EUROfusion programme at KIT, the designs of conventional-cavity type and coaxial-cavity type DEMO-compatible gyrotrons are under investigation. In this presentation, the physical design of the interaction circuit of a 236 GHz conventional cavity gyrotron and its RF behavior are presented. The simulation results show a stable single mode RF output power without serious mode competition.

On the dependence of the efficiency of a 240 GHz high-power gyrotron on the displacement of the electron beam and on the azimuthal index

Two issues in the cavity design for a Megawatt-class, 240 GHz gyrotron are addressed. Those are first, the effect of a misaligned electron beam on the gyrotron efficiency and second, a possible azimuthal instability of the gyrotron. The aforementioned effects are important for any gyrotron operation, but could be more critical in the operation of Megawatt-class gyrotrons at frequencies above 200 GHz, which will be the anticipated requirement of DEMO. The target is to provide some basic trends to be considered during the refinement and optimization of the design. Self-consistent calculations are the base for simulations wherever possible. However, in cases for which self-consistent models were not available, fixed-field results are presented. In those cases, the conservative nature of the results should be kept in mind.

RF Behavior and Launcher Design for a Fast Frequency Step-tunable 236 GHz Gyrotron for DEMO

Abstract As part of the EUROfusion project, the conceptual design of a 1 MW 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a DEMOnstration Power Plant (DEMO), along with a 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-steerable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been studied as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2–3 GHz is demonstrated in numerical simulations. A hybrid-type Quasi-Optical Launcher (QOL) has been designed for a step-frequency tunable gyrotron with sufficiently high Fundamental Gaussian Mode Content (FGMC).

Magnetron injection gun for a 238 GHz 2 MW coaxial-cavity gyrotron

Karlsruhe Institute of Technology (KIT) has started the development of gyrotrons for the first demonstration fusion power plant DEMO. A coaxial-cavity 238 GHz 2 MW gyrotron design is under investigation. After having obtained an initial cavity design, one focus of current studies is the associated triode-type magnetron injection gun (MIG). Constraints, design approaches and an initial gun design are presented. An outlook on further investigations is given.

Mode selection and resonator design for DEMO gyrotrons

Physical design studies towards DEMO-compatible gyrotrons (frequency above 230 GHz, output power above 1 MW, frequency-tunable) have been started at KIT. Usage of very high-order modes (eigenvalue above 120) is necessary and allows a novel mode-selection strategy. Two different resonator designs for 237.5 GHz gyrotrons are under investigation.

A fast frequency step-tunable 236 GHz gyrotron design for DEMO

As part of the EUROfusion WP HCD EC project, the conceptual design of a 1 MW, 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a Demonstration Power Plant (DEMO), along with the 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-movable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been confirmed as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2-3 GHz is demonstrated in numerical simulations.