This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

/pdf/state-of-the-art-of-high-power-gyro-devices-and-free-5avfirj4z9.pdf

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

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 Wendelstein 7X (W7-X) stellarator (R = 5.5 m, a = 0.55 m, B < 3.0 T), which at present is being built at Max-Planck-Institut fur Plasmaphysik, Greifswald, aims at demonstrating the inherent ste...

/pdf/electron-cyclotron-heating-for-w7-x-physics-and-technology-2pobx2z7e6.pdf

Electron Cyclotron Heating for W7-X: Physics and Technology

In many tokamak and stellarator experiments around the globe that are investigating energy production via controlled thermonuclear fusion, electron cyclotron heating and current drive (ECH&CD) are used for plasma start-up, heating, non-inductive current drive and MHD stability control. ECH will be the first auxiliary heating method used on ITER. Megawatt-class, continuous wave (CW) gyrotrons are employed as high-power millimeter (mm)-wave sources. The present review reports on the worldwide state-of-the-art of high-power gyrotrons. Their successful development during the past years changed ECH from a minor to a major heating method. After a general introduction of the various functions of ECH&CD in fusion physics, especially for ITER, Section 2 will explain the fast-wave gyrotron interaction principle. Section 3 discusses innovations on the components of modern long-pulse fusion gyrotrons (magnetron injection electron gun (MIG), beam tunnel, cavity, quasi-optical output coupler, synthetic diamond output window, single-stage depressed collector) and auxiliary components (superconducting magnets, gyrotron diagnostics, high-power calorimetric dummy loads). Section 4 deals with present megawatt-class gyrotrons for ITER, W7-X, LHD, EAST, KSTAR and JT-60SA, and also includes tubes for moderate pulse length machines as, ASDEX-U, DIII-D, HL-2A, TCV, QUEST and GAMMA-10. In Section 5 the development of future advanced fusion gyrotrons is discussed. These are tubes with higher frequencies for DEMO, multi-frequency (multi-purpose) gyrotrons, stepwise frequency tunable tubes for plasma stabilization, injection-locked and coaxial-cavity multi-megawatt gyrotrons, as well as sub-THz gyrotrons for collective Thomson scattering (CTS). Efficiency enhancement via multi-stage depressed collectors, fast oscillation recovery methods and reliability, availability, maintainability and inspectability (RAMI) will be discussed at the end of this section.

/pdf/high-power-gyrotrons-for-electron-cyclotron-heating-and-4i6ctmqvpg.pdf

High-power gyrotrons for electron cyclotron heating and current drive

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

The development of high-power gyrotrons (118 GHz, 140 GHz) in continuous-wave (CW) operation for heating nuclear fusion plasmas has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The 140-GHz gyrotron being under development for the installation at the W7-X stellarator now under construction at the IPP Greifswald, Germany, operates in the TE/sub 28,8/ mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high mode-purity low-Ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power. Long pulse operation (10 s at 1 MW) was possible without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30% without to about 50% with depression voltage. The best performance reached so far has produced an energy per pulse as high as 90 MJ (power 0.64 MW, pulse length 140 s) which is the highest value achieved in gyrotrons operating at this frequency and power level. The pulse-length limitations so far are mainly due to the external system.

Development of a 140-GHz 1-MW continuous wave gyrotron for the W7-X stellarator

Fast frequency-tunable gyrotrons are of interest for controlling instabilities in magnetically confined plasmas in large fusion reactors like e.g. ITER. The confining magnetic field depends on the radial position in tokamaks and stellarators. Hence, the electron cyclotron resonance interaction between the RF wave and the plasma electrons occurs only in a small plasma layer where the resonance condition is approximately fulfilled. Other plasma flux surfaces can be reached by a change of the gyrotron frequency or by mechanically steerable mirrors in the plasma vessel making use of the Doppler shift. Since it is difficult to find a material for such mirrors and since a reduction of the number of movable parts inside the plasma vessel is desired, it is obvious that the first solution is more advantageous. A 1 MW frequency-step-tunable gyrotron that operates between 114 and 166 GHz is under development at FZK. In a first step the frequency change has been performed by a slow variation of the magnetic field B by modification of the current in the superconducting (SC) coils. To prevent quenching of the SC solenoids, /spl part/B//spl part/t is limited to a maximum of /spl sim/0.1 Tesla/minute which results in a frequency change of /spl sim/42 MHz/s. To detect and suppress plasma instabilities in the ITER tokamak, the required frequency variation has been estimated to be of the order of seconds for several GHz. To fulfil these demands a hybrid magnet system including two normal conducting (NC) solenoids was build at FZK.

Fast-frequency step-tunable high-power gyrotron with hybrid magnet system

The TE/sub 22,6/ 1 MW gyrotron operated at Forschungszentrum Karlsruhe (FZK) at frequencies between 114 and 166 GHz has been investigated with respect to the fast-frequency tunability in the frequency range from 132 to 147 GHz. For that purpose, the gyrotron has been equipped with a special hybrid-magnet system consisting of superconducting (sc) magnets in the cryostat and additional normal conducting (nc) copper magnets with a fast time constant. Some special problems due to the magnetic coupling between the different magnets were investigated by calculation and experiment. Making use of these investigations some different current regulation systems for the nc magnets were implemented and tested experimentally. Finally a step-tuning operation between the modes from TE/sub 20,6/ to TE/sub 24,6/ in time steps of 1 s has been achieved.

Fast frequency-step-tunable high-power gyrotron with hybrid-magnet-system

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Development of a 140-GHz 1-MW continuous wave gyrotron for the W7-X stellarator

Fast-frequency step-tunable high-power gyrotron with hybrid magnet system

Fast frequency-step-tunable high-power gyrotron with hybrid-magnet-system