Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.

The Gyrotron at 50: Historical Overview

The feasibility of manufacturing a 2-MW CW coaxial cavity gyrotron at 170 GHz has been demonstrated and data required for fabrication of an industrial tube have been obtained. An engineering design of a prototype started recently with the goal to provide gyrotrons with 2-MW microwave output power for International Thermonuclear Experimental Reactor (ITER). The design of critical components of the prototype tube as electron gun, cavity and RF output system will be verified under realistic conditions at short pulses using the experimental coaxial gyrotron at Forschungszentrum Karlsruhe.

A 2 MW, 170 GHz coaxial cavity gyrotron

Most of the present-day millimeter-wave gyrotrons developed for plasma experiments in controlled fusion reactors utilize cylindrical cavities operating in high-order modes. The choice of modes should obey certain restrictions dictated by the achievable mode selection and the maximum admissible level of the density of microwave ohmic losses in the cavity walls. Even with these restrictions, developers have successfully manufactured quasi-continuous-wave gyrotrons operating in the short millimeter wavelength bands that are capable of delivering microwave power on the order of 1 MW. To upgrade gyrotron power to the level of several megawatts, more complicated coaxial microwave circuits should be used. This statement is also valid for relativistic gyroklystrons, which are currently under development for driving future linear accelerators. This paper presents an overview of the history of the development of coaxial gyrodevices, a discussion of the physics-based issues which are the most important for their operation, a description of the state of the art in the development of coaxial gyrodevices for the above-mentioned applications, and a brief forecast for their future.

Coaxial gyrotrons: past, present, and future (review)

High power gyrotron oscillators are mainly used as millimetre wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for energy generation by controlled thermonuclear fusion. The maximum pulse length of commercially available 1.0 MW gyrotrons, employing open-ended cylindrical resonators and chemical vapour deposition diamond output windows is 12 s at 140 GHz and 9 s at 170 GHz, with efficiencies slightly above 30%. The energy world record of 160 MJ at a power level around 0.9 MW is held by the European FZK-CRPP-CEA-TED collaboration. Total efficiencies of up to 50% have been obtained using a single-stage depressed collector (SDC). To achieve output powers of around 2 MW in continous wave operation at the ITER reference frequency 170 GHz, it is necessary to use a coaxial cavity geometry. A maximum output power of 2.2 MW at 165 GHz was obtained at FZK with an efficiency of 28% (48% in SDC operation). The availability of gyrotrons with fast frequency tunability would permit the use of a simple, fixed, non-steerable mirror antenna for local current drive experiments on ITER. This work reports on the progress in gyrotron development and the status of advanced coaxial cavity gyrotrons and step-wise frequency tunable gyrotrons.

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MW gyrotron development for fusion plasma applications

We report, for the first time comprehensively, on the present status of the code-package EURIDICE for gyrotron interaction simulations and cavity design, developed at the National Technical University of Athens. The area of application, theoretical models, numerical implementation, and features of the codes are discussed.

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EURIDICE: A code-package for gyrotron interaction simulations and cavity design

This paper investigates coaxial gyrotron cavities with longitudinal slots on the inner conductor as a means to reduce the number of possible competing modes. In the analytic theory the corrugated surface is treated as a homogeneous impedance surface ("impedance corrugation") to obtain simple formulas for the characteristic equation of the eigenmodes, for the electromagnetic fields and the wall losses. The developed model applies if the number of slots is sufficiently high (cutoff wavelength much larger than the corruption period). The characteristic equation in terms of the ratio C of the outer wall radius to the inner conductor radius is solved numerically to determine a range of eigenvalues and C where the eigenvalue curves are monotonically decreasing. In such a region a cavity having its inner conductor downtapered (radius decreasing toward the cavity output) can be used to reduce the diffractive quality factors of several modes, leaving the working mode undisturbed and without favoring other modes. In addition the electromagnetic field profiles are investigated, and in particular it is shown that for certain cavity parameters a mode could have its energy concentrated close to the inner conductor. As a check on the validity of the theoretical approximations, simulations with the MAFIA code are carried out. These give good agreement with the results of the analytic equations.

C.T. Iatrou

Papers

Coaxial cavities with corrugated inner conductor for gyrotrons