Development Of Gyrotrons Research Articles

Dependence of the gyrotron efficiency on the azimuthal index of non-symmetric modes

Development of MW-class gyrotrons for future controlled fusion reactors requires careful analysis of the stability of high efficiency operation in very high-order modes. In the present paper, this problem is analyzed in the framework of the non-stationary self-consistent theory of gyrotrons. Two approaches are used: the one based on the wave envelope representation of the resonator field and the second one based on representation of this field as a superposition of eigenmodes, whose fields are determined by a self-consistent set of equations. It is shown that at relatively low beam currents, when the maximum efficiency can be realized in the regime of soft self-excitation, the operation in the desired mode is stable even in the case of a very dense spectrum of competing modes. At higher currents, the maximum efficiency can be realized in the regimes with hard self-excitation; here the operation in the desired mode can be unstable because of the presence of some competing modes with low start currents. Two 170 GHz European gyrotrons for the international thermonuclear experimental reactor are considered as examples. In the first one, which is the 2 MW gyrotron with a coaxial resonator, the stability of operation in a chosen TE34,19-mode in the presence of two sideband modes with almost equidistant spectrum is analyzed and the region of magnetic fields in which the oscillations of the central mode are stable is determined. The operation of the second gyrotron, which is the 1 MW gyrotron with a cylindrical cavity currently under development in Europe, is studied by using the wave envelope approach. It is shown that high efficiency operation of this gyrotron in the TE32,9-mode should be stable.

Current status of the development of the problem-oriented software package GYREOSS

A great number of problem-oriented software packages (e.g., DAPHNE, ESRAY, GYROSIM, just to name a few) are used for computer-aided design (CAD) and optimization of high-power gyrotrons for fusion research (most notably for ECRH and ECCD). In them, adequate self-consistent physical models formulated in a two-dimensional space (2D) are implemented. Most of the problems encountered in recent years in the development of megawatt-class gyrotrons, however, can be attributed to physical factors and phenomena that are both non-stationary (for instance various electron beam instabilities, dynamics of the trapped particles) and inherently three-dimensional by nature (e.g., non-uniformity of the emission, misalignment of the electrodes and magnetic coils, etc.). In order to address these problems and to take into account most of the physical factors that have so far been neglected, a work on the development of a novel problem-oriented software package called GYREOSS (which stands for Gyrotron Electron-Optical System Simulation) was initiated recently. Since then, GYREOSS has evolved into a test bed for experimenting with different numerical methods, algorithms, and programming techniques for ray tracing (trajectory analysis) and particle-in-cell (PIC) simulations. In this paper, we outline the current status of GYREOSS and present both its physical model formulated in 2D and 3D and its program realization. We illustrate the capabilities of the latest version of the code, which includes a novel 3D field solver and an efficient relativistic particle pusher. Finally, we present an outlook for a further development of GYREOSS.

From Series Production of Gyrotrons for W7-X Toward EU-1 MW Gyrotrons for ITER

Europe is devoting significant joint efforts to develop and to manufacture MW-level gyrotrons for electron cyclotron heating and current drive of future plasma experiments. The two most important ones are the stellarator Wendelstein W7-X at Greifswald and the Tokamak ITER at Cadarache. While the series production of the 140 GHz, 1 MW, CW gyrotrons for the 10-MW electron cyclotron resonance heating system of stellarator W7-X is proceeding, the European GYrotron Consortium is presently developing the EU-1 MW, 170 GHz, CW gyrotron for ITER. The initial design had already been initiated in 2007, as a risk mitigation measure during the development of the advanced ITER EU-2-MW coaxial-cavity gyrotron. The target of the ITER EU-1-MW conventional-cavity design is to benefit as much as possible from the experiences made during the development and series production of the W7-X gyrotron and of the experiences gained from the earlier EU-2-MW coaxial-cavity gyrotron design. Hence, the similarity of the construction will be made visible in this paper. During 2012, the scientific design of the ITER EU-1-MW gyrotron components has been finalized. In collaboration with the industrial partner Thales electron devices, Vélizy, France, the industrial design of the technological parts of the gyrotron is being completed. A short-pulse prototype is under development to support the design of the CW prototype tube. The technological path toward the EU ITER-1 MW gyrotron and the final design will be presented.

Recent Advances in the Worldwide Fusion Gyrotron Development

Progress in the worldwide development of high-power gyrotrons for magnetic confinement fusion plasma applications is presented. Gyrotron oscillators are used for electron cyclotron heating, electron cyclotron current drive, stability control, and plasma diagnostics. After technology breakthroughs in the research on gyrotron components in the 1990s, significant progress has been achieved in the 2000s, in the field of long-pulse and continuous wave (CW) operation for a wide range of frequencies. Currently, the development of 1-MW-class CW gyrotrons for the tokamak ITER (170 GHz), the stellarator Wendelstein 7-X (140 GHz), and the tokamaks DIII-D and JT-60SA (110 GHz) has been very successful in EU, Japan, Russia, and USA. The Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW at 1-h pulse duration). For this progress in the field of high-power long-pulse gyrotrons, innovations such as the realization of high-efficiency stable oscillation in very high-order cavity modes with low ohmic losses, the use of single-stage depressed collectors for energy recovery (efficiency enhancement and simpler power supplies), highly efficient internal quasi-optical (q.o.) mode converters (low level of internal stray radiation), and synthetic diamond windows have essentially contributed. The total tube efficiencies are around 50% and the purity of the linearly polarized fundamental Gaussian output mode is 97% and higher. Power modulation technologies for stabilization of neoclassical tearing modes have proceeded. Future prospects of advanced high-power fusion gyrotrons are in the areas of two- and three-frequency gyrotrons, fast step-wise frequency tuneability, higher unit power (coaxial cavities), and higher frequencies for more efficient plasma stabilization and noninductive current drive as well as reliability, availability, maintainability and inspectability for next step fusion power stations. The GYCOM step-tuneable 1-MW gyrotron for ASDEX Upgrade employing a broadband travelling-wave-resonator window (with two diamond disks) operates at 105, 117, 127 and 140 GHz. The EU 170 GHz coaxial-cavity gyrotron prototype achieved in millisecond pulses the power of 2.1 MW at 46% efficiency and 96% Gaussian mode purity. A new power record of second harmonic oscillation has been achieved in the subterahertz band (83 kW/389 GHz/3 ms) in Japan for application to collective Thomson scattering diagnostics. A fundamental frequency 670-GHz gyrotron with pulsed magnet generated 210 kW in 20-30- μs pulses at 20% efficiency (collaboration of institutions in Russia and USA).

Dispersion characteristics of plasma-filled cylindrical waveguide

An analytical and numerical study is made on the dispersion properties of a cylindrical waveguide filled with plasma. An electron beam and static external mag- netic field are considered as the mechanisms for controlling the field attenuation and possible stability of the wave- guide. The effects of plasma warmness and inhomogeneity are also considered. Dispersion relations in x describing different physical situations that govern the mode propa- gation in the waveguide are obtained. The plasma dielectric tensors and the dispersion relations which describe E and H waves and, hence, the damping rate of these waves are calculated and studied. The necessary conditions for the field stability in the waveguide and amplification coeffi- cients for these waves are also obtained. H-wave modes are always attenuated by collisional effect. The growth of the excited E wave is calculated in the resonance case, and the stability condition for the E wave is obtained. E waves are found to be more stable in warm plasma compared to cold plasma. The results obtained here are of great interest and may be used to analyze how the plasma affects the electromagnetic properties of the cavity of the 1-2 MW 140-170 GHz continuous-wave gyrotron (for W7-X stell- arator and ITER), for MW gyrotron development for fusion plasma applications, and for second harmonic generation in a plasma-filled parallel plane waveguide.

Development of second harmonic gyrotrons, Gyrotron FU CW GII and Gyrotron FU CW GIII, equipped with internal mode converters

Second harmonic gyrotrons, Gyrotron FU CW GII and Gyrotron FU CW GIII, were developed at the Research Center for Development of Far-Infrared Region, University of Fukui, Japan to achieve two goals. Each gyrotron was equipped with an internal quasi-optical mode converter. Using Gyrotron FU CW GII allowed the design of the cavity and mode converter to be validated, which was the first goal. After that, Gyrotron FU CW GIII, which is an improved version of Gyrotron FU CW GII, allowed us to achieve a high power output of up to 420 W, which was the second goal, with a cathode voltage setting of −21 kV and a beam current of 0.57 A. This was achieved using a newly developed electron gun and with the careful sitting of the gyrotron on the magnet.

Development of Advanced Gyrotrons in Europe

The German PMW project and the European EGYC consortium are driving gyrotron developments for the two major plasma fusion experiments in Europe using ECRH, Wendelstein W7-X at Greifswald and the international ITER at Cadarache. 1 MW CW, 140 GHz conventional cavity gyrotrons have been developed and are being delivered to W7-X whereas advanced 2 MW CW, 170 GHz coaxial-cavity gyrotron technology has been tested for ITER. Additionally, an 1 MW, 170 GHz conventional cavity design is under development for ITER. Furthermore, research work on gyrotron concepts for future fusion experiments, focusing on frequency-step tunable gyrotrons and multi-MW coaxial-type gyrotrons is ongoing at Europe, in particular at KIT. This paper is reporting some of the important results and the ongoing research work.

Development of 154 GHz 1 MW Gyrotron for ECRH of LHD

At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. Development of a new 154 GHz 1 MW gyrotron for ECRH system of Large Helical Device (LHD) has been done. The design studies of electron gun (MIG), cavity oscillator, mode converter, transmission mirrors, collector and superconducting magnet (SCM) have indicated more than 1 MW output with efficiency ~33 % for cavity oscillation mode of TE28,8. In the short pulse test of the first test tube, the maximum output power of 1.06 MW and the maximum total efficiency of 35.2 % with CPD were obtained.

GAMMA 10/PDX Project Status and Future

GAMMA 10/PDX project is the development of the mirror devices to aim the fusion-reactor relevant research using the potential control and the end loss as the particle and heat fluxes for transport, divertor plasma physics and Plasma Wall Interaction (PWI) studies. It was obtained that more than 10 MW/m2 heat flux density for PWI studies. The closed divertor-simulator module has been newly installed. The first studies of this module have been done and obtained the expected plasma flow. We will strengthen the module functions and diagnostic capability and heating systems to enable to be more reactor-relevant simulator. In the core transport control by the ECH, correlation studies of the radial electric field profile and the fluctuations are made and the high coherency is confirmed by using the new two-point potential detector of the Gold Neutral Beam Probe(GNBP). The maximum electron temperature obtained so far is - 150 eV during the ECH by Thomson Scattering, but we need more studies for the high power ECH optimization. As one of the key tools for the new GAMMA 10/PDX project, 2 MW gyrotron development at 28 GHz has been started, based on the success of 77 GHz-1.9 MW output. The maximum outputs of 1 MW at 28 GHz in short pulse and 540kW for 2 s are obtained.

Development of multi-purpose MW gyrotrons for fusion devices

The latest development achievements in the University of Tsukuba of over-1 MW power level gyrotrons required in present-day fusion devices, GAMMA 10, Large Helical Device (LHD), QUEST, Heliotron J and NSTX, are presented. The obtained maximum outputs are 1.9 MW for 0.1 s on the 77 GHz LHD tube and 1.2 MW for 1 ms on the 28 GHz GAMMA 10 one, which are new records in these frequency ranges. In long-pulse operation, 0.3 MW for 40 min at 77 GHz and 0.54 MW for 2 s at 28 GHz are achieved. A new programme of 154 GHz 1 MW development has started for high-density plasma heating in LHD. On the first 154 GHz tube, 1.0 MW for 1 s is achieved. As a next activity of the 28 GHz gyrotron, an over-1.5 MW gyrotron is designed and fabricated to study the multi-MW oscillation. The possibility of 0.4 MW continuous wave and 2 MW level output in operations of a few seconds, after the improvements of output window and mode converter, is shown. Moreover, a new design study of dual-frequency gyrotron at 28 and 35 GHz has started, which indicates the practicability of the multi-purpose gyrotron.

Gyrotron and power supply development for upgrading the electron cyclotron heating system on DIII-D

An upgrade of the electron cyclotron heating system on DIII-D to almost 15MW is being planned which will expand it from a system with six 1MW 110GHz gyrotrons to one with ten gyrotrons. A depressed collector 1.2MW 110GHz gyrotron is being commissioned as the seventh gyrotron. A new 117.5GHz 1.5MW depressed collector gyrotron has been designed, and the first article will be the eighth gyrotron. Two more are planned, increasing the system to ten total gyrotrons, and the existing 1MW gyrotrons will subsequently be replaced with 1.5MW gyrotrons.Communications and Power Industries completed the design of the 117.5GHz gyrotron, and are now fabricating the first article. The design was optimized for a nominal 1.5MW at a beam voltage of 105kV, collector potential depression of 30kV, and beam current of 50A, but can achieve 1.8MW at 60A. The design of the collector permits modulation above 100Hz by either the body or the cathode power supply, or both, while modulation below 100Hz must use only the cathode power supply.General Atomics is developing solid-state power supplies for this upgrade: a solid-state modulator for the cathode power supply and a linear high voltage amplifier for the body power supply. The solid-state modulator has series-connected insulated-gate bipolar transistors that are switched at a fixed frequency by a pulse-width modulation regulator to control the output voltage. The design of the linear high voltage amplifier has series-connected transistors to control the output voltage, which was successfully demonstrated in a proof-of-principle test at 2kV. The designs of complete power supplies are progressing.The design features of the 117.5GHz 1.5MW gyrotron and the solid-state cathode and body power supplies will be described and the current status and plans are presented.

Development of High Power ECH Systems for High and ELM-like Heat Flux in GAMMA 10 Tandem Mirror

Development of high power gyrotrons and electron cyclotron heating (ECH) systems for the power modulation experiments in GAMMA 10 have been started in order to generate and control the high heat flux and to make the ELM (edge localized mode) like intermittent heat load pattern for divertor simulation studies. ECH for potential formation at the plug region (P-ECH) produces the electron flow with high energy along the magnetic filed line. From the power scaling of confining potential and electron flux in GAMMA 10, the higher ECH power generates the higher confinement potential and the higher electron flux. The peak heat-flux of more than 10 MW/m 2 has been obtained during the ECH injection within the available power of ECH. This value almost corresponds to the steady-state heat load of the divertor plate of ITER. However, we need substantial upgrade of the heating power to approach the ITER level ELM energy density. For this purpose, MW power gyrotrons at 28 GHz have been started and the development is undergoing efficiently in the ECH system upgrade program with the collaboration of Japan Atomic Energy Agency (JAEA). c

Development of a High Power Gyrotron Operating at 28 and 35 GHz

An experimental and design study has been commenced for a dual-frequency gyrotron (28 and 35 GHz). The target output power at 28 GHz is 2 MW. For a modified 28 GHz 1 MW gyrotron, an output power of 1.25 MW and operation of 0.6 MW for 2 s have been achieved at 28 GHz. For the 35.45 GHz oscillation test, a cavity oscillation power of 1.2 MW and an efficiency of 33.9% have been confirmed by considering the calculated transmission efficiency of 72%. These results support the development of dual-frequency gyrotrons for lower frequency ranges.

Development of high-power gyrotrons with gradually tapered cavity

In high power gyrotrons, the parasitic modes coupled with the operating mode cannot be avoided in the beam-wave interaction. These parasitic modes will decrease the efficiency of the gyrotrons. The purity of the operating mode affected by different tapers should be carefully studied. The steady-state self-consistent nonlinear theory for gyrotron with gradually tapered cavity is developed in this paper. A steady-state calculation code including “cold cavity” and “hot cavity” is designed. By comparison, a time-domain model analysis of gyrotron operation is also studied by particle-in-cell (PIC). It is found that the tapers of gyrotron have different influences on the modes coupling between the operating mode and the parasitic modes. During the study, an example of 94 GHz gyrotron with pure operating mode TE03 has been designed. The purity of the operating mode in the optimized cavity is up to −77 dB, and in output waveguide of the cavity is up to −76 dB. At the same time, the beam-wave interaction in the designed cavity has been simulated, too. An output power of 120 kW, corresponding to 41.6% efficiency and an oscillation frequency of 94.099 GHz have been achieved with a 50 kV, 6 A helical electron beam at a guiding magnetic field of 3.5485 T. The results show that the power in spurious modes of the optimized cavity may be kept far below than that of the traditional tapered cavity.

Dynamic nuclear polarization at 700 MHz/460 GHz

We describe the design and implementation of the instrumentation required to perform DNP-NMR at higher field strengths than previously demonstrated, and report the first magic-angle spinning (MAS) DNP-NMR experiments performed at 1H/e− frequencies of 700MHz/460GHz. The extension of DNP-NMR to 16.4T has required the development of probe technology, cryogenics, gyrotrons, and microwave transmission lines. The probe contains a 460GHz microwave channel, with corrugated waveguide, tapers, and miter-bends that couple microwave power to the sample. Experimental efficiency is increased by a cryogenic exchange system for 3.2mm rotors within the 89mm bore. Sample temperatures ⩽85K, resulting in improved DNP enhancements, are achieved by a novel heat exchanger design, stainless steel and brass vacuum jacketed transfer lines, and a bronze probe dewar. In addition, the heat exchanger is preceded with a nitrogen drying and generation system in series with a pre-cooling refrigerator. This reduces liquid nitrogen usage from >700l per day to <200l per day and allows for continuous (>7days) cryogenic spinning without detrimental frost or ice formation. Initial enhancements, ε=−40, and a strong microwave power dependence suggests the possibility for considerable improvement. Finally, two-dimensional spectra of a model system demonstrate that the higher field provides excellent resolution, even in a glassy, cryoprotecting matrix.

Generation of high power sub-terahertz radiation from a gyrotron with second harmonic oscillation

New power records of second harmonic gyrotron oscillation have been demonstrated in the sub-THz band. The first step gyrotron of demountable type had succeeded in oscillation with power more than 50 kW at 350 GHz and nearly 40 kW at 390 GHz [T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009)]. Then, the second step gyrotron of sealed-off type was manufactured. A cavity mode was carefully selected to avoid mode competition with a neighboring fundamental harmonic mode. Matching of the selected mode with the electron gun was also circumspectly considered. The second step gyrotron has attained higher power radiation than the first gyrotron. The maximum single mode power was 62 kW at 388 GHz. Then, the electron gun was modified for use of a different cavity mode with a higher coupling coefficient than that for the 62 kW mode. The new mode proved single mode oscillation power of 83 kW at about 389 GHz. These results are new second-harmonic-oscillation power records for sub-THz gyrotrons. The present study constitutes foundations of development of high power second harmonic sub-THz gyrotron for application to collective Thomson scattering measurement on fusion plasmas, especially on high-density plasmas such as those produced in LHD [N. Ohyabu et al., Phys. Rev. Lett. 97, 055002 (2006)]. This paper reports the design consideration to realize high power single mode gyrotron oscillation at second harmonic and the examination of oscillation characteristics of the gyrotron.

Gyrotron Development for High Power THz Technologies at IAP RAS

The review summarizes recent experimental results in the field of THz gyrotrons developed for various applications. A CW gyrotron with the operation frequency of 0.26 THz has been successfully used for DNP spectroscopy A pulsed high-harmonic Large Orbit Gyrotron (LOG) with the frequency of 0.55 THz and kW level of output power has been used for THz breakdown and obtaining dense plasma in gases. A powerful pulsed 0.67 THz/200 kW gyrotron is under development for remote detection of ionization sources.

Development of a kW Level-200 GHz Gyrotron FU CW GI with an Internal Quasi-optical Mode Convertor

Development of gyrotrons with an internal mode convertor has started in Research Center for Development of Far-Infrared Region, University of Fukui (FIR FU). As the first gyrotron of such a kind, we have designed and manufactured Gyrotron FU CW GI. It operates at 203 GHz at fundamental cyclotron resonance. We have designed a cavity and a mode convertor under some constraints such as reuse of an electron gun and small diameter of a magnet bore. Designed output power is about 1 kW. We have succeeded in observation of a circular radiation pattern. The maximum observed output power is 0.5 kW for the setting cathode voltage of 20 kV and the beam current of 0.5 A. This success makes gyrotron development in FIR FU to proceed to a new stage.

Development of a high-power pulsed subterahertz gyrotron for remote detection of sources of ionizing radiation

We describe the main stages of the development of a gyrotron in the sub-terahertz band with a power of several hundreds of kilowatts, a duration of the output radiation pulses of about 10 μs, and the generation frequency corresponding to an atmospheric transparency window. The gyrotron having a working frequency of 0.67 THz, a power of 200–300 kW, and the focal diameter of the output beam 1–2 mm can be used to detect sources of ionizing radiation from a distance of several tens of meters. The detection principle is based on the appearance of a microwave frequency discharge in the focal spot if the number of free electrons exceeds the natural background by 1–2 orders of magnitude. The electron-optical system of such a gyrotron has been calculated and optimized. The scenario of the gyrotron switch-on has been analyzed for the electron beam formed in such a system, and the possibility of stable single-mode generation on the TE31.8 mode is demonstrated. The results of analysis of thermal loads, which demonstrate their acceptable level in the cavity and the tube cavity for pulse durations of about 10 μs are presented.

Development of dual frequency gyrotron and high power test of EC components

In JAEA, development of high-power long-pulse gyrotrons is underway. The output power of the gyrotron was applied for high-power long-pulse tests of the transmission line (TL) and the equatorial launcher (EL) mock up for ITER. The feature of design in the dual frequency gyrotron is the simultaneously satisfying the matching of both frequencies at a window and the same radiation angle at an internal mode convertor for both frequencies. The dual frequency gyrotron was developed and high power operations at 170 GHz and 137 GHz were carried out. The 170 GHz high power experiment of 40 m length ITER relevant TL was carried out and transmission efficiency and mode purity change caused by long pulse operation were measured. The mock-up model of EL was also tested using 170 GHz gyrotron. The power transmission through the quasi-optical beam line in EL was demonstrated using the full scale mock up model. Furthermore, the high power test results of the transmission components will be summarized.