E. Borie

Development of a 140-GHz 1-MW continuous wave gyrotron for the W7-X stellarator

The development of high-power gyrotrons (118 GHz, 140 GHz) in continuous-wave (CW) operation for heating nuclear fusion plasmas has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The 140-GHz gyrotron being under development for the installation at the W7-X stellarator now under construction at the IPP Greifswald, Germany, operates in the TE/sub 28,8/ mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high mode-purity low-Ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power. Long pulse operation (10 s at 1 MW) was possible without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30% without to about 50% with depression voltage. The best performance reached so far has produced an energy per pulse as high as 90 MJ (power 0.64 MW, pulse length 140 s) which is the highest value achieved in gyrotrons operating at this frequency and power level. The pulse-length limitations so far are mainly due to the external system.

Mode competition using TE03 gyrotron cavities

Abstract The gyrotron is a powerful source of millimetre wave radiation. Fusion applications require more power per tube than is currently available This in turn means that the resonator must be highly overmoded, with a dense mode spectrum, which leads to mode competition. The influence of external parameters, such as the applied magnetic field, electron beam, and resonator geometry, on mode competition in tubes designed to operate at 150 and 140 GHz in the TE03 mode is investigated theoretically and experimentally. It can be shown that even when the mode spectrum is fairly dense, single mode operation of a gyrotron is possible. The influence of startup conditions on output power is investigated in the 150 GHz experiment. In some cases, modes with an axial index of 2 were excited.

Possibilities for multifrequency operation of a gyrotron at FZK

We investigate the possibility of multifrequency operation of a 140-GHz gyrotron, which is designed to operate in the TE/sub 22,8/ mode at 140 GHz and the TE/sub 19,6/ mode at 111 GHz or the TE/sub 17,6/ mode at 105 GHz, for which existing equipment can be used. The present calculations compute beam properties for a given set of coil currents, accelerating voltage and current, and then use these beam properties to compute the output power and efficiency. These calculations are performed separately for each mode.

Experimental results and numerical simulations of a high power 140 GHz gyrotron

The design of a 0.5 MW 140 GHz gyrotron with axial RF output operating in the TE/sub 10,4/ mode is presented. Experimental results and numerical simulations are compared. In particular the effects of RF reflections at the output window, velocity spread of the helical electron beam and startup of oscillations by simultaneous increase of pitch factor /spl alpha/ and relativistic factor /spl gamma/ are discussed. In short pulse operation (/spl les/5 ms) an output power of 690 kW has been obtained with an electronic efficiency of 31% and a mode purity of about 99%. The experiments have shown that it is possible to use for step frequency tuning the azimuthal neighbours of the TE/sub 10,4/ mode, TE/sub 9.4/ at 132.6 GHz (420 kW) and TE/sub 11,4/ at 147.3 GHz (300 kW) as working modes by decreasing and increasing the magnetic field, respectively. >

140-GHz gyrotron with multimegawatt output power

The operational features of a 140-GHz, transverse electric TE/sub 22,6/ mode gyrotron oscillator with an advanced quasi-optical mode converter, a Brewster window, and a single-stage depressed collector at 140 GHz with an output power of 2.1 MW and an efficiency of 34% without depressed collector and 53% with depressed collector are presented. The high output power level is possible due to an almost reflectionless termination of the radio frequency (RF) beam line outside the tube. The operation of the TE/sub 22,6/ mode gyrotron is described in detail, and the significant features for achieving the high-output power are pointed out.

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 experiments. Electron cyclotron resonance heating (ECRH) has proven to be an important tool for plasma devices, especially for stellarators, as it provides both net current free plasma start up from the neutral gas and efficient plasma heating. The development of high power gyrotrons (118 GHz, 140 GHz and 170 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. This paper describes the work of the Forschungszentrum Karlsruhe for the development of conventional-cavity 1-MW CW gyrotrons, coaxial cavity 2-MW short-pulse gyrotrons and a frequency step-tunable gyrotron in the frequency range between 105-140 GHz.

Design of a high order volume mode cavity for a l MW/140GHz gyrotron

Abstract A high-power cylindrical cavity gyrotron operating at 140 GHz is studied. The possible working modes TE22,4, TE22,5, and TE22,6,are compared. The output power envisaged is 1 MW. The influence of the space charge effect of the electron beam and the RF losses in the resonator are discussed. Results of numerical calculations of the interaction of the electron beam and the RF wave are presented. The problem of single-mode excitation is addressed in mode competition calculations. Stability regions of the working modes are shown for some interesting parameter ranges. From these plots an optimized variation of the parameters during start-up of the gyrotron can be deduced in order to ensure single-mode operation. On the basis of kinetic theory the influence of misalignment of the electron beam and resonator axis (eccentricity) on the starting currents and frequency shifts of the operating and parasitic modes is studies for several values of eccentricity.

Development of a 140 GHz, 1 MW, continuous wave gyrotron for the W7-X stellarator

The development of high power gyrotrons in continuous wave (CW) operation for heating of plasmas used in nuclear fusion research has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. A recent RD program aims at the development of 140 GHz gyrotrons with an output power of 1 MW in CW operation for the 10 MW ECRH system of the new stellarator plasma physics experiment Wendelstein 7-X at IPP Greifswald, Germany. The work is performed under responsibility of FZK Karlsruhe in collaboration with CRPP Lausanne, IPF Stuttgart, IPP Garching and Greifswald, CEA Cadarache and TED Velizy. The gyrotron operates in the TE28.8 mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high-mode purity low-ohmic loss cavity, an optimized non-linear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power, and an efficiency of 49 % has been achieved. Long pulse operation of the gyrotron was possible with an output power of 1 MW at a pulse length of 10 s without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30 % without depression voltage to about 50% with depression voltage. At an output power of 640 kW, a pulse length of 140 s could be achieved.

Possible operation of a 1.5-2-MW, CW conventional cavity gyrotron at 140 GHz

We present a study of the feasibility of continuous wave (CW) operation of a 140-GHz conventional cavity gyrotron at high power levels operating in the TE/sub 31,8/- and TE/sub 25,10/-modes. The question of mode selection is discussed, and a possible design of such a gyrotron with beam energy up to 90 keV and a current of 60-70 A is given. We find that it should be possible to operate a 140-GHz gyrotron at power up to 2 MW if a sufficiently high-order mode is used, although CW power may be somewhat lower.

Development of Advanced High Power Gyrotrons at Forschungszentrum Karlsruhe

The R&D activities at Forschungszentrum Karlsruhe (FZK) on advanced gyrotrons for use in magnetically confined fusion plasmas consist of: (1) the development of a coaxial cavity gyrotron capable of delivering 2 MW, continuous wave (CW) at 170 GHz, (2) the demonstration of fast frequency step tuning and (3) the work on a slowly tunable multi frequency gyrotron. In the case of the coaxial gyrotron a proof of principle has been demonstrated at pulses around 1 ms and specific problems related to the coaxial system have already been partly investigated. An RF-output power of 2.2 MW has been measured at 165 GHz in good agreement with theoretical results. The design of critical components for a CW tube is in progress. Fast frequency tuning has been performed by a stepwise frequency variation up to about 15 GHz around a central frequency of 140 GHz within 1 s at a power level of about 1 MW. As a first step towards application, a slowly tunable multi frequency 1 MW gyrotron operated between 105 and 140 GHz is under development for use at the ASDEX-Upgrade tokamak experiment in Garching. Übersicht Die F&E-Arbeiten am Forschungszentrum Karlsruhe (FZK) an fortschrittlichen Hochleistungsgyrotronen können in folgender Weise aufgeteilt werden: (1) Arbeiten zur Entwicklung eines koaxialen Gyrotrons mit einer Ausgangsleistung von 2 MW im Dauerstrich bei 170 GHz, (2) Demonstration einer schnellen stufenweisen Frequenzdurchstimmung mit Gyrotrons und (3) Entwicklungsarbeiten im Zusammenhang mit dem Bau eines langsam verstimmbaren Multi-Frequenz-Gyrotrons. Beim koaxialen Gyrotron wurde die prinzipielle Funktionsweise im Kurzpulsbetrieb bei 1 ms demonstriert. Spezifische Probleme des koaxialen Aufbaus wurden bereits größtenteils untersucht. Eine HF-Ausgangsleistung von 2.2 MW wurde bei 165 GHz in guter Übereinstimmung mit Rechnungen gemessen. Abschließende experimentelle Untersuchungen sind vorbereitet und der Entwurf kritischer Komponenten wird zur Zeit durchgeführt. Eine schnelle stufenweise Frequenzdurchstimmung wurde demonstriert. Im Frequenzbereich um 140 GHz wurde eine maximale Frequenzdurchstimmung von 15 GHz innerhalb einer Sekunde bei einer Ausgangsleistung von etwa 1 MW erreicht. Als ersten Schritt zur Anwendung wird ein zwischen 105 GHz und 140 GHz langsam durchstimmbares 1 MW-Gyrotron für einen Einsatz am ASDEXUpgrade Tokamak Experiment in Garching entwickelt. Für die Dokumentation koaxiales Gyrotron / Frequenzdurchstimmung