High-Power Low-Loss Micro- and Millimeter Wave Transmission
High-power micro- and millimeter wave sources such as gyro-devices operate in higher-order modes of circular waveguides. For interfaces of these vacuum electron tubes to high-power microwave transmission lines operating with waves with simple field structures, mode converters for higher-order modes are required. The appropriate mode conversion and transmission technology is mostly determined by the output mode and power of the source and by its frequency.
In high-power millimeter wave systems, long-distance transmission from the source to the load with very low ohmic attenuation and high mode purity can be accomplished by the following methods:
(1) Closed, highly overmoded smooth-wall circular waveguides propagating a TE0n mode (preferably TE01).
(2) Closed, highly oversized, circumferentially corrugated or dielectrically coated smooth-walled circular HE11-mode waveguides.
(3) Open, quasi-optical (QO) transmission of a Gaussian beam (TEM00 mode) using focusing metallic mirrors as phase-correcting elements.
Method (1) is adapted for high-power millimeter wave systems employing TE0n-mode gyrotrons with output power of up to 200 kW and external waveguide mode converters or external QO mode transducing antenna radiators to produce a pencil beam with well-defined direction and polarization. Such waveguides are employed in small ECRH installations and in technological gyrotron systems for materials processing. Transmission methods (2) and (3) are appropriate for ECRH complexes composed of 1 MW gyrotron oscillators, for example high-order TEmm-mode gyrotrons with built-in QO mode converter which directly converts the complicated cavity-interaction mode into a linearly polarized free-space Gaussian beam. The present lecture introduces into techniques and strategies for development of the various components of these different types of transmission lines.
State-of-the Art of High-Power Gyro-Devices
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 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 minutes (CPI and European KIT-CRPP-CEA-TED collaboration). The world record parameters of the European megawatt-class 140 GHz gyrotron are: 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 was generated with the JAEA-TOSHIBA 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 achieved 0.9 MW with a pulse duration of 1000 s and 55% efficiency. The short-pulse pre-prototype tube of the European 2 MW, 170 GHz coaxial-cavity gyrotron for ITER achieved at KIT the record power of 2.2 MW at 30% efficiency (without SDC) and 96% Gaussian mode purity. Russian gyrotrons for plasma diagnostics or spectroscopy applications deliver Pout = 40 kW with τ = 40 μs at frequencies up to 650 GHz (η > 4%), 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 gyrotrons with the following parameters: f > 24 GHz , Pout = 4-50 kW, CW, η > 30%. This lecture introduces into the principles of gyro-interaction and gives 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 plasma diagnostics. In addition a short overview of the present gyrotrons for technological and spectroscopy applications, gyro-klystrons, gyro-TWT amplifiers, gyro-twystron amplifiers, gyro-BWO’s, and of vacuum windows for such high-power mm-wave sources will be presented.
The Physics of Vacuum Electron Devices
This lecture covers basic principles of vacuum electron devices (microwave tubes), the electron beam formation, the various types of electron beam-electromagnetic wave interaction, types of vacuum electron devices (Klystron, Reflex-Klystron, IOT, TWT, BWO, EIK, EIO, Orotron, Clinotron, Magnetron, Cross-Field Amplifier, Gyrotron, Gyro-Amplifiers CARM, FEM and Vircator) and their components. Modern vacuum electron devices such as Vacuum Micro-Electronic and THz Frequency Sources also will be discussed. After a short review of high power microwave measurement techniques and different types of high-power microwave transmission lines the lecture ends with a brief discussion of different applications.
Manfred Thumm (SM’94-F’02) was born in Magdeburg, Germany, on August 5, 1943. He received the Dipl. Phys. and Dr. rer. nat. degrees in physics from University of Tübingen, Germany, in 1972 and 1976, respectively.
At the University of Tübingen he was involved in the investigation of spin-dependent nuclear forces in inelastic neutron scattering. From 1972 to 1975 he was Doctoral Fellow of the Studienstiftung des deutschen Volkes. In 1976 he joined the Institute for Plasma Research in the Electrical Engineering Department of the University of Stuttgart, Germany, where he worked on RF production and RF heating of toroidal pinch plasmas for thermonuclear fusion research. From 1982 to 1990 his research activities were mainly devoted to electromagnetic theory and experimental verification in the areas of component development transmission of very high power millimeter waves through overmoded waveguides and of antenna structures for RF plasma heating with microwaves. In June 1990 he became a Full Professor at the Institute for Microwaves and Electronics of the University of Karlsruhe, Germany, and Head of the Gyrotron Development and Microwave Technology Division, Institute for Technical Physics, Research Center Karlsruhe (Forschungszentrum Karlsruhe: FZK). From April 1999 to September 2011, he was the Director of the Institute for Pulsed Power and Microwave Technology, FZK, where his current research projects have been the development of high power gyrotrons, dielectric vacuum windows, transmission lines and antennas for nuclear fusion plasma heating, and industrial material processing. On October 1, 2009, the University of Karlsruhe and the FZK have merged to the Karlsruhe Institute of Technology (KIT). M. Thumm has authored/co-authored four books, 13 book chapters, 274 research papers in scientific journals, and more than 1220 conference proceedings articles. He holds 12 patents on active and passive microwave devices.
He is member of the IEEE EDS Vacuum Devices Technical Committee and the NPSS PSAC Executive Committee, a member of the Chapter 8.6 Committee Vacuum Electronics and Displays of the Information Technical Society in German VDE (Chairman from 1996 to 1999) and a member of the German Physical Society. From 2007 to 2008 he was an EU member of the ITER Working Group on Heating and Current Drive, the vice chairman of the Scientific-Technical Council of the FZK and the vice chairman of the Founding Senate of the KIT. From 2008 to 2010 he was the deputy head of the Topic Fusion Technology of the KIT Energy. He was the General Chair of the IRMMW-THz 2004 and IEEE ICOPS 2008 Conference in Karlsruhe, Germany. He has been a member of the International Organization and Advisory Committees of many International Conferences and a member of the Editorial Boards of several ISI refereed journals. From 2003 to 2010 he was the ombudsman for upholding good scientific practice at FZK/KIT and since 2012 he has been Associate Editor for Vacuum Electronics Fast Wave Devices for IEEE Trans. Electron Devices.
He was awarded with the Kenneth John Button Medal and Prize 2000, in recognition of outstanding contributions to research on the physics of gyrotrons and their applications. In 2002, he was awarded the title of Honorary Doctor, presented by the St. Petersburg State Technical University, for his outstanding contributions to the development and applications of vacuum electron devices. He received the IEEE-EDS 2008 IVEC Award for Excellence in Vacuum Electronics for outstanding achievements in the development of gyrotron oscillators, microwave mode converters and transmission line components, and their applications in thermonuclear fusion plasma heating and materials processing. Together with two of his colleagues he received the 2006 Best Paper Award of the Journal of Microwave Power and Electromagnetic Energy and the 2009 CST University Publication Award. In 2010 he was awarded with the IEEE-NPSS Plasma Science and Applications Award for outstanding contributions to the development of high power microwave sources (in particular gyrotrons) for application in magnetically confined fusion plasma devices as well as for stimulation and establishing of extensive international co-operations. He is a winner of the 2010 open grant competition of the Government of the Russian Federation to support scientific research projects implemented under supervision of Leading Scientists at Russian institutions of higher education. Together with A. Litvak and K. Sakamoto he has been the recipient of the EPS Plasma Physics Innovation Prize 2011 for outstanding contributions to the realization of high power gyrotrons for multi-megawatt long-pulse electron cyclotron heating and current drive in magnetic confinement nuclear fusion plasma devices. In 2012 he was awarded with the Heinrich Hertz Prize of the EnBW Foundation and the KIT for outstanding contributions to generation, transmission and mode conversion of high and very high microwave power for nuclear fusion and the HECTOR School Teaching Award in Embedded Systems Engineering.