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).

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

Record-breaking high power coherent radiation at a subterahertz frequency region from a gyrotron utilizing second harmonic resonance modes was attained with a simple cavity. In order to aim at high power and high frequency simultaneously, the oscillation mode was selected carefully enough to realize stable radiation free from mode competition. The cavity radius was determined from the viewpoints of the oscillation frequency, the coupling coefficient between the electron beam, and the rf-electric field. The cavity length was also optimized for the highest perpendicular efficiency. In addition, a new electron gun which is capable of generating a thin laminar beam for a large current was introduced. Consequently, single mode second harmonic radiation with powers of 52 and 37 kW at frequencies of about 349 and 390 GHz, respectively, was achieved.

Development of a Novel High Power Sub-THz Second Harmonic Gyrotron

This report gives an update of the worldwide experimental achievements in the development of long-pulse gyrotron oscillators for fusion plasma heating, plasma diagnostics and materials processing. In addition, this work gives a short overview of the present development status of coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, quasi-optical gyrotrons, cyclotron autoresonance masers, gyro-amplifiers, gyro-BWO's, free electron masers and of vacuum windows for such high-power mm-wave sources.

/pdf/state-of-the-art-of-high-power-gyro-devices-and-free-1mmeiw5vph.pdf

State-of-the-Art of High Power Gyro-Devices and Free Electron Masers. Update 2011. (KIT Scientific Reports ; 7606)

A “Sandwich” type of neutron shielding composite filled with boron carbide reinforced by carbon fiber

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800-MHz dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic ( $\omega = {2}\omega _{c}$ ) of the electron cyclotron frequency. It produces up to 9.3-W continuous microwave (CW) power at 527.2-GHz frequency using a diode type electron gun operating at ${V} = {16.65}$ kV, ${I}_{b} = {110}$ mA in a TE11,2,1 mode, corresponding to an efficiency of ~0.5%. The gyrotron is tunable within ~0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE11 mode of an internal 12-mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12- to 16-mm i.d. waveguide followed by 3 m of the 16-mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ~84% HE11 mode content.

Second Harmonic 527-GHz Gyrotron for DNP-NMR: Design and Experimental Results

Boron carbide (B4C) is one of advanced materials and is being used in a wide rage of applications. The unique feature of this material is its large neutron-absorbing cross-section. Some of its most prominent applications are controlling rods in nuclear reactors and radiation protection. 24 GHz microwave processing for B4C ceramics were performed under flowing argon gas using the sintering system. Sintering at the high temperature (up to 2200°C) was achieved using thermal insulation system consists of fiber-board, boron nitride powder, and boron nitride case. The sintered samples were achieved 90 % of theoretical.

High Temperature Thermal Insulation System for Millimeter Wave Sintering of B4C

The effect characteristic of high frequency and strong electric field processing was observed. One of interest effects is so called non-thermal effect or microwave effect. In order to widely study the non-thermal effect of ceramics sintering. We developed a material processing system by using a 300 GHz gyrotron. This system consists of a 300 GHz, 3.5 kW, CW gyrotron (gyrotron FU CW I) with a cryogen-free 12 T superconducting magnet, a corrugated circular waveguide and applicator. Boron carbide is one of advanced materials and is being used in a wide rage of applications. 24 GHz microwave processing for boron carbide ceramics were performed. Enhanced sintering (non-thermal effect) was observed.

Millimeter and submillimeter wave sintering of ceramics

Sintering at high temperature (up to 2200/spl deg/C) was achieved. Boron carbide has been sintered to 83% of theoretical density without carbon or sintering aides by using a 24 GHz gyrotron as a radiation source. The densification of B/sub 4/C was occurred at 2000/spl deg/C. However the densification showed the plateau at the temperature 2100/spl deg/C. The SEM shows the sintering process for small pores occur at the early stage of sintering below the temperature 2100/spl deg/C. These results suggest the green specimens which doesn't have large pores, make more dense sintered pellets below 2100/spl deg/C.

Ceramics sintering using a 24 GHz gyrotron system

Boron carbide (B/sub 4/C) is one of advanced materials and is being used in a wide rage of applications. The unique feature of this material is its large neutron-absorbing cross-section. Some of its most prominent applications are controlling rods in nuclear reactors and radiation protection. 24 GHz microwave processing for B/sub 4/C ceramics were performed under flowing argon gas using the sintering system. Enhanced sintering was observed in B/sub 4/C ceramics for sintering with the microwave power of 2.5 kW as compare to 1.5 kW one, even at the same sintering temperature and holding time. The experiment results in different sintering conditions are presented.

Non-thermal effects on B/sub 4/C ceramics sintering

Boron carbide (B4C) is one of advanced materials and is being used in a wide rage of applications. The unique feature of this material is its large neutron-absorbing cross-section. Some of its most prominent applications are controlling rods in nuclear reactors and radiation protection. 24 GHz microwave processing for B4C ceramics were performed under flowing argon gas using the sintering system. The sintered samples were characterized by the density, the shrinkage and SEM micrographs of fracture surface. Above the temperature of 2000°C, the shrinkage and the grain grows were observed.

http://ieeexplore.ieee.org/iel5/6213246/6220476/06220598.pdf

T. Saji

Papers

High Temperature Thermal Insulation System for Millimeter Wave Sintering of B4C

Millimeter and submillimeter wave sintering of ceramics

Ceramics sintering using a 24 GHz gyrotron system

Non-thermal effects on B/sub 4/C ceramics sintering

Ultra high temperature sintering by using 24 GHz gyrotron