High Power Gyro-Devices for Plasma Heating and Other Applications

Gyrotron oscillators 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 generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 1 MW gyrotrons employing synthetic diamond output windows is 5 s at 110 GHz (CPI and JAERI-TOSHIBA), 12 s at 140 GHz (FZK-CRPP-CEA-TED) and 10 s at 170 GHz (GYCOM and JAERI-TOSHIBA), with efficiencies slightly above 30%. Total efficiencies of 45–50 % have been obtained using single-stage depressed collectors (SDC). The energy world record of 160 MJ (0.89 MW at 180 s pulse length and 140 GHz) at power levels higher than 0.8 MW has been achieved by the European FZK-CRPP-CEA-TED collaboration at FZK. Operation at the 1^st and the 2^nd harmonic of the EC frequency enables gyrotrons to act as medium power step-tunable mm- and sub-mm wave sources in the frequency range from 38 GHz (fundamental) to 889 GHz (2nd harmonic) for plasma diagnostics, EC plasma discharges for generation of multi-charged ions, high-frequency broadband electron paramagnetic resonance (EPR) spectroscopy and medical applications. Gyrotrons have also been successfully used in materials processing. Such technological applications require gyrotrons with the following parameters: f ≥ 24 GHz, P_out = 4–50 kW, CW, η ≥ 30%. Future applications which await the development of novel high-power gyro-amplifiers include high resolution radar ranging and imaging in atmospheric and planetary science as well as deep-space and specialized satellite communications and RF drivers for next-generation high-gradient linear accelerators (supercolliders). The present paper reviews the state-of-the-art and future prospects of gyro-devices and their applications.     

用于受控热核聚变的兆瓦级同轴腔回旋振荡器的注-波互作用分析

相对于高阶工作模式的单腔回旋管,同轴腔回旋管具有缓解模式竞争,提高单模工作的稳定性,以及增大功率容量的优点,宜用于受控热核聚变中的电子回旋共振加热和电子回旋电流驱动而受关注.详细地研究了工作频率为170 GHz,TE_(34,11),模同轴腔回旋管的结构参数、电子束参数及腔壁损耗对注-波互作用的影响.首先对170 GHz兆瓦级功率模式选择进行分析,给出了工作模式.再次,基于时域自洽非线性理论,编写了时域单模稳态注-波互作用程序,分析了电流、磁场强度和腔壁欧姆损耗对互作用的影响,并对工作参数进行了优化.模拟结果表明:当电子束电流为68 A,工作电压为65 kV,引导磁场强度为6.58 T时,可获得2.18 MW的输出功率,49.23%的效率,外腔壁上的欧姆损耗密度峰值为1.94 kW/cm~2,内导体表面的小于0.15 W/cm~2;互作用效率随速度零散增大而降低,输出频率向下偏移;电子注厚度对互作用也有相似的影响.     

140 GHz, 1 MW CW Gyrotron Development for Fusion Applications��Progress and Recent Results

A 10 MW, 140 GHz ECH system is currently under construction for the stellarator W7-X. The RF power will be provided by 10 gyrotrons. A European collaboration has been established to develop and build 9 (out of 10) tubes each with an output power of 1 MW for continuous wave (CW) operation. This contribution reports on recent results with the series gyrotrons.     

170 GHz兆瓦级回旋管横向磁场扫描线圈组设计

170 GHz兆瓦级回旋管是热核聚变等离子体加热用电子回旋谐振系统最重要的功率源。在获得兆瓦级连续波输出功率的同时,收集极功率密度过大成为限制整管平均功率和脉宽进一步提升的主要问题。因此,设计了具备周期性变化的横向磁场扫描线圈组。通过仿真分析收集极内的电子注分布可知,采用该线圈组可以有效降低功率密度,从而保证整管稳定工作。同时,对不同电流相位的线圈组的性能进行对比分析,优化后的横向磁场扫描线圈组可使收集极平均功率密度由4.7 kW/cm2下降到0.29 kW/cm2,将被应用到未来170 GHz兆瓦级回旋管的热测中。     

Development in Russia of Megawatt Power Gyrotrons for Fusion

Tests of the Russian ITER gyrotrons were continued after debugging of the test facility at Kurchatov Institute including the evacuated transmission line and power supplies capable to provide gyrotron CW operation. A new ITER gyrotron (V-10) with upgraded body insulation, liquid cooling system for the DC-break ceramic and improved design of the relief window has been manufactured and is currently under test. A maximal total efficiency near 55% at 1 MW power level was attained. The maximal pulse duration of 1000 s at 0.8 MW power was limited at that time by the test stand. At 1 MW power level the gyrotron V-10 up to now reached a pulse duration of 570 s. An advanced short-pulse (100 ms) gyrotron model operating in the TE_28.12 mode demonstrated a very robust operation at relatively high electron energies (up to 100 keV) necessary to achieve 1.5-2 MW output power. Three double frequency gyrotrons have been delivered to ASDEX Upgrade. The development of a multi-frequency gyrotron with a broadband or tuneable synthetic diamond window is in progress.     

Thermal Modelling of Edge-Cooled Single Disc Gyrotron Windows Using a One-Dimensional Finite Difference Computer Code

A one - dimensional finite - difference computer code has been developed and shown to be quite effective in thermal design and analysis of various plane edge-cooled single - disc windows for gyrotrons with Gaussian rf beam output. The program has a modular structure and is flexible enough to suit specific requirements of users; it can handle nonlinearity. Results obtained for various CVD diamond window cases compare well with the experiment and with the results obtained using ABACUS, a well-established 3-D Finite Element code. Results indicate that a CVD diamond disc with 94 mm diameter and edge cooled by water at 293 K will ensure the passage of 1 MW CW at 140 GHz. A fused quartz window, with 60 mm diamter and operating at 165 GHZ on a 1.5 MW pulsed coaxial gyrotron has also been analysed for its transient characteristics and results are presented.     

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.     

Very long pulse high power klystrons for FEL and their generating conditions

High power RF sources for major scientific machines (accelerators, plasma heating) are often either short pulse devices with very high peak power levels (e.g. 30-40 MW peak, 5μs pulses in S-band) or CW devices (e.g. 500 kW in S-band). Free electron lasers, on the other hand, require klystrons operating in a long-pulse mode (100 μs and 10 ms). In this case, the technology used in the two other cases (short pulse, CW) is not necessarily appropriate. When the peak power levels are very high in nearly-CW operation, new problems are encountered, for example thermal cycling and HV breakdown modes. The most critical parts are the output cavity, output window and the electron gun. We will discuss a family of 1·3 GHz klystrons as an example: 35-40 MW/10μs; 10-15 MW/200 μs and a few MW/1-10ms     

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.     

0.6 THz三次谐波回旋管的研究

 太赫兹回旋管是一类基于电子回旋受激辐射机理的快波器件,同时也是目前最具发展前景的高功率太赫兹辐射源.本文根据回旋管的线性理论和自洽非线性理论对三次谐波、工作频率0.6 THz的回旋管进行了研究,重点讨论了引导中心分别为0mm的实心回旋电子注和0.315mm的空心回旋电子注的模式竞争.通过分析比较,发现工作在0.6THz 、三次谐波的众多模式中TE37模是一比较理想的工作模式,它不仅有相对较高的功率输出,而且还有相对较少的模式竞争.本文中的设计采用55kV/1.0A,电子注的速度横纵比为1.5,在工作磁场7.86T下,数值计算结果表明输出功率达4.73kW.     

Linear and Non-Linear Inserts for Genuinely Wideband Continuous Frequency Tunable Coaxial Gyrotron Cavities

We consider two continuous frequency tunable CW coaxial gyrotron oscillators, one 330 GHz with 3 GHz bandwidth and output power 50 – 400 W for scientific applications and one 30 GHz with 0.4 GHz bandwidth and output power 40 – 140 kW for industrial applications. The continuous tuning of both gyrotrons is achieved by moving the linearly tapered inner conductor in the axial direction in combination with the proper adjustment of the operating magnetic field. We consider also a non-linear tapering, which makes it possible to reduce the length of the insert and to improve efficiency of the device.     

28GHz二次谐波回旋振荡管的自洽非线性计算

中等功率的连续波回旋管在工业领域有着重要的应用前景．本文对工作在28GHz频率的工业应用回旋振荡管进行了自洽非线性计算．结果表明,在二次谐波工作条件下,选取TE02模式,当电压为32kV、电流为6A时,可以获得连续波输出功率-50kW,效率-28％．     

Thermo-Mechanical Analysis of Single-Disc Edge-Cooled Silicon Nitride Millimeter Wave Window for 200 kW CW Gyrotrons

This paper investigates the possibility of using Silicon Nitride Composite (Kyocera SN-287) as single-disc, edge-cooled window for gyrotrons operating below 200 kW CW in the frequency range 28-42 GHz. Rotationally symmetric TE_0n, and TEM₀₀ Gaussian modes of rf transmission through the window have been considered. Analysis performed using a one dimensional (1D) finite difference (FD) code reveals that thermal stresses developed due to non-uniform temperature distribution on the disc surface are well within manageable limits for a 200 kW 42 GHz gyrotron proposed for some ECRH applications. For industrial gyrotrons, for microwave material processing and operating at a maximum power level of 100 kW CW, Si₃N₄ windows may be a cost effective replacement for sapphire windows. It is found that a TE₀₂ profile results in lower peak temperature at the window disc in comparison to a Gaussian beam profile and allows the use of smaller discs.     

Recent Developments on High-Power Gyrotrons—Introduction to This Special Issue

Gyrotrons with output powers of several 100 kW are mainly used as high-power millimeter (mm)-wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for generation of energy by controlled thermonuclear fusion. Other applications include ECR ion sources, materials processing and plasma chemistry, high-resolution Doppler radar, radar ranging and imaging in atmospheric and planetary science, active denial systems, as well as detection of concealed radioactive materials. The present review summarizes the status of recent developments on high-power gyrotrons and introduces this Special Issue on Gyrotrons.     

High peak power and high repetition rate characteristics in acurrent-pulsed Q-switched CO2 laser with a mechanical shutter

An electro-mechanical Q-switched (EMQ) CO₂ laser is Q-switched by a mechanical beam chopper in combination with a pulsed discharge current. Such a system can produce pulses with high peak powers (>10 kW) and high repetition rates (>1 kpps). In order to analyze the output characteristics, the peak power and the duration of the output pulses have been measured experimentally in detail over a wide range of Q-switching times up to 250 ns. For a low-pressure (<4 kPa) CO₂ gas system, the standard rate equations adequately explain the experimental results by introducing a new switching function for the form of the cavity loss for the mechanical chopper. In an EMQ-laser with a high initial inversion density (4.5·10¹⁵ /cm³ at 150 mA peak current), multiple peak pulses or pulse distortion have been observed. This is due to the plasma screening effect induced by the burning of the metal shutter blades placed inside the cavity. It is found that tungsten metal shutter blades can be used up to a power density of 259 MW/cm² for a focused beam without this effect occurring, The solutions of the rate equations show that optimum coupling can prevent the plasma screening effect even for a Q-switching time longer than the pulse buildup time. The EMQ-laser configured for optimum coupling has produced a peak output power of 30 kW for the 9P20 transition branch in the CO₂ spectrum without any pulse distortion. This value has been obtained even though the discharge length was only 1.3 meters     

Development of 28?GHz and 77?GHz, Mega-Watt Gyrotrons for Fusion Devices

A 28 GHz 1 MW with TE_8,3 cavity for GAMMA10 tandem mirror and a 77 GHz 1.5 MW gyrotron with TE_18,6 for Large Helical Device (LHD) have been developed to upgrade their Electron Cyclotron Heating (ECH) systems. In the 28 GHz gyrotron, the maximum power of 1.05 MW was obtained, which is in agreement with its design target value. And the high efficiency of 40% without collector potential depression (CPD) was obtained with 0.8 MW. In the first and second 77 GHz gyrotrons, 0.8 MW 3.6 sec., 0.3 MW 60 sec operations have been achieved, but several issues due to the stray RF and velocity dispersion have been found, too. In consideration of these, the design improvement was performed to aim at 1.5 MW in the third tube. In the short pulse test, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec with 1 MW, 1800 sec with 0.1 MW, 1.6 sec with 1.5 MW. Total injection power of 3.1 MW to LHD plasma has been achieved. It is shown that the electron beam pitch factor ?? decreases with increasing of beam current, comparing with the experimental and calculation results.     

High-Power CW Klystron for Fusion Experiments

A 3.7-GHz 700-kW klystron in continuous-wave (CW) operation has been developed to upgrade the lower hybrid RF plasma heating power in a tokamak up to 10 MW. The klystron is equipped with a diode gun, a five-cavity RF structure, two BeO-disk RF windows, and a large-size X-ray-shielded hypervapotron collector. The output power is recombined in a four-port junction which we also developed. The tube is designed to deliver 620-kW CW RF power with a mismatched load (VSWR = 1.4) and 700-kW CW with a matched load. Several prototypes have been built with successive design improvements. The major improvement was to change one single RF output into two RF outputs. The most recently built prototype meets all design specifications at 73.1 kV and 20.7 A, with an efficiency of 47% on a matched load and 40% with a 1.4 : 1 VSWR load, worst case phase. The power losses dissipated in the body have been measured as low as 17 kW, which corresponds to the RF heating and implies low beam interception. The measured temperatures of the output cavity noses and collector wall have been kept below 130degC and 200degC, respectively, which results in large thermal margin.     

CONCEPTUAL DESIGN STUDIES OF AN 84 GHz, 500 kW, CW GYROTRON

The feasibility of an 84 GHz, 500 kW, CW gyrotron for ECRH on an experimental tokamak will be presented in this paper. Mode competition and mode selection procedures are carefully investigated by considering various candidate modes and the TE_10,4 mode is chosen as the operating mode. A conventional cylindrical cavity resonator with weak input and output tapers and parabolic roundings is considered for interaction studies. Self-consistent, both single mode and time-dependent, calculations are carried out and power and efficiencies are computed for a typical set of beam parameters. The results show that an output power of well over 500 kW, CW and efficiency around 40% can be reached without a depressed collector.     

带状电子注驱动的准光谐振腔回旋管

回旋管作为一种重要的真空电子源,能够在毫米波和太赫兹频段生成高峰值以及高平均功率的电磁辐射,在波谱学、雷达、通信、生物医学等领域具有广泛的应用前景。传统的回旋管将不可避免地面对激烈的模式竞争问题,引入准光谐振腔将有望大大减小模式竞争的激烈程度。本文基于电子回旋脉塞理论,将准光谐振腔及带状电子注进行结合,以期实现更高的输出功率及效率。仿真结果表明,在电子电压为40 kV,背景磁场为8.4 T时,设计的准光回旋管能在220 GHz频点处产生6.1 kW的输出,电子效率达到6.1%且在一段时间内能稳定运行。本文结构将可能为高频段乃至高次谐波工作的回旋管设计提供全新方案,从而进一步用于通信、雷达等领域。     

Thermal effects and pulse mode operation of IMPATT diodes

The peak output power and efficiency of a silicon IMPATT diode operating in the CW and pulse modes are compared. It is shown that because of thermal effects, the peak pulse output power for a given bias current is less than the CW output power for the same bias level.