With intensities 108–1010 times greater than other laboratory sources, X-ray free-electron lasers are currently opening up new frontiers across many areas of science. In this Review we describe how these unconventional lasers work, discuss the range of new sources being developed worldwide, and consider how such X-ray sources may develop over the coming years.

X-ray free-electron lasers

This paper describes the genealogical tree of the gyrotron, stimulated emission of cyclotron radiation in microwave electronics from magnetron to gyrotron, and arrangement of the gyrotron. The structure of the alternating field in the gyrotron is also given, along with motion and bunching of electrons near cyclotron resonance, equations of the gyrotron, and varieties of the gyrotron. A review of experimental studies is discussed, with problems of utilization of stimulated emission cyclotron radiation.

http://ieeexplore.ieee.org/iel6/22/25041/01129149.pdf

The Gyrotron

As follows from theory, supported by experiment, if the electron energy grows to ∼ 1 McV the gyroton preserves its capability to produce single-mode radiation in the millimetere wave range with rather high efficiency. If the electron energy is ultra-relativistic (≳ 1 McV), preference must be given to the eyelotron auto-resonance maser, which is, in principle, the most effective type of free electron laser (‘ doppertron’) at milimetere and submillimetre wavelengths.

Relativistic gyrotrons and cyclotron autoresonance masers

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

FEL's based on the stimulated undulator radiation (ubitrons) are compared with those based on the stimulated cyclotron radiation [cyclotron autoresonance masers (CARM's)]. If the high-current accelerators are used as electron injectors, then from the viewpoint of simplicity of oscillatory electron energy pumping, criticality with respect to electron velocity dispersion, and efficiency, CARM's seem to be more effective than ubitrons at mm and sbmm waves. For such HF generators, resonators based on selective Bragg reflection of electromagnetic waves in corrugated metallic tubes are most atractive. CARM's of this type yield 6 MW at a 4 mm wavelength and 10 MW at a 2 mm wavelength in the single-mode regime.

FEL's with Bragg reflection resonators: Cyclotron autoresonance masers versus ubitrons

In a gyrotron (amplifier or oscillator) having an electrodynamic system in the form of a slightly irregular waveguide having a low effective Q, the frequency of the variable field is close to the critical frequency of one of the natural waves of the waveguide in operating modes having a high efficiency; as consequence of this:

1)

a fairly accurate determination of the optimal parameters of the system may be accomplished by extrapolation of the results of the theory based on the fixed-variable-field-structure approximation;




2)

a constraint on the output power exists which is caused on the one hand by mode competition and on the other hand by rebunching of electrons in an excessively powerful variable field, which according to the constraint imposed by the diffraction Q(1), may develop in the electrodynamic system of a gyrotron when a powerful electron beam is injected into it.

Theory of gyrotrons with a nonfixed structure of the high-frequency field

Common properties of free electron lasers

Nonstationary generation in free electron lasers

Introductory experiments with a projected 30 GHz/5 MW/30 dB three-cavity gyroklystron [1] are described. The electron gun was tested in a configuration with self-excited cavity: the 50% efficiency at the 12 MW output power of the oscillator confirmed the high quality of the electron beam. At the first experiment with the gyroklystron, 3.5 MW output power at 18 % efficiency and 27 dB gain were obtained at 0.5 μs pulse duration.

M. I. Petelin

Papers

FEL's with Bragg reflection resonators: Cyclotron autoresonance masers versus ubitrons

Theory of gyrotrons with a nonfixed structure of the high-frequency field

Common properties of free electron lasers

Nonstationary generation in free electron lasers

Ka-band gyroklystron operating at a combination of high-order modes