Fast frequency-step-tunable high-power gyrotron withhybrid-magnet-system

The TE_22,6 1 MW gyrotron operated at Forschungszentrum Karlsruhe (FZK) at frequencies between 114 and 166 GHz has been investigated with respect to the fast-frequency tunability in the frequency range from 132 to 147 GHz. For that purpose, the gyrotron has been equipped with a special hybrid-magnet system consisting of superconducting (sc) magnets in the cryostat and additional normal conducting (nc) copper magnets with a fast time constant. Some special problems due to the magnetic coupling between the different magnets were investigated by calculation and experiment. Making use of these investigations some different current regulation systems for the nc magnets were implemented and tested experimentally. Finally a step-tuning operation between the modes from TE_20,6 to TE_24,6 in time steps of 1 s has been achieved     

Investigation of a Broadband Quasi-Optical Mode Converter for a Multi-Frequency 1 MW Gyrotron

A broadband quasi-optical (QO) mode converter for a multi-frequency gyrotron has been designed and tested at Forschungszentrum Karlsruhe (FZK). The launcher is optimized for the TE_22,8 mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE_17,6) and 143 GHz (TE_23,8). Combining with a beam-forming mirror system, which consists of a quasi-elliptical mirror and two phase-correcting mirrors with non-quadratic surface contour, further calculations show that efficiencies of more than 94% have been achieved for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode. Low power (cold) measurements show a good agreement with theoretical predictions. This QO mode converter can be used for the broadband operation of a multi-frequency 1 MW gyrotron.     

Development of multimegawatt gyrotrons for fusion plasma heating and current drive

High-frequency gyrotrons with high output power are mainly used for microwave heating and current drive in plasmas for thermonuclear fusion. The development of high-power gyrotrons in continuous wave (CW) operation has been in progress for several years in a joint collaboration between different European research centers and an industrial partner. The status of the development of the 140-GHz continuously operating gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. An output power of 890 kW has been achieved with a pulse length of 3 min. limited by the available high-voltage power supply at an electron beam current of 40 A. At a reduced beam current of 27 A , an output power of 540 kW was measured with a pulse length of 939 s. For the next fusion plasma device international thermonuclear experimental reactor, gyrotrons with a higher output power of about 2 MW are desirable. In short-pulse experiments, the feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, CW, has been demonstrated, and the information necessary for a technical design has been obtained. An output power of 2.2 MW has been reached in stable operation (without mode competition). At the nominal output power of 1.5 MW an efficiency of 48% could be obtained with single-stage depressed collector. The development of frequency tunable gyrotrons operating in the range from 105 to 140 GHz for stabilization of current driven plasma instabilities in fusion plasma devices (neoclassical tearing modes) is another task in the development of gyrotrons at the Forschungszentrum Karlsruhe.     

The Influence of Window Parameters on the Transmission Characteristics of Millimeter Waves

This work reports about the influence of some window parameters, such as the mechanical tolerance of disk thickness, the variation of distance between two disks, and the frequency drift during gyrotron operation on the transmission characteristics of millimeter waves. Detailed calculations of the transmission characteristics for a single-disk gyrotron window and frequency tunable double-disk plasma fusion torus windows have been performed. The geometry of the window units has been optimized in order to obtain a suitable transmission characteristic, i.e. power reflection less than ?20 dB within a frequency bandwidth of about 1 GHz around the chosen frequencies.     

Design of a Quasi-Optical Mode Converter for a Frequency Step-Tunable Gyrotron

The quasi-optical mode converter for a frequency step-tunable gyrotron which consists of a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system has been optimized for 9 modes from TE_17,6 at 105 GHz to TE_23,8 at 143 GHz. The first mirror is a large quasi-elliptical focusing one; the second and third are phase-correcting mirrors with a non-quadratic shape of the surface. The results of calculations show that for these modes the Denisov-type launcher has a well-focused beam with low diffraction losses, and the radiation pattern presents an almost identical field shape for all modes considered. A multi-mode optimization of the phase-correcting mirrors with two different methods has been tested. The simulations show that the phase-correcting mirrors can be used for broadband operation in the frequency range from 105 GHz up to 143 GHz in the various design modes. This quasi-optical mode converter can achieve efficiencies of 94%-98% for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode.     

A CVD-Diamond Disk Brewster Window for a Frequency Step-Tunable 1 MW Gyrotron

Output window design is one of the key issues for realizing broadband output of a multi-frequency gyrotron. Corresponding to the recent development of a frequency step-tunable 1 MW gyrotron at FZK, this paper reports about a newly designed ultra-broadband CVD-diamond disk Brewster window. The detailed calculations of mm-wave transmission characteristics are given, and measurement results of the disk loss tangent and its distribution across the disk are also presented. The geometry of the window unit has been optimized to keep the window reflections low over a wide angular range around the Brewster angle.