O. Drumm

A 2 MW, 170 GHz coaxial cavity gyrotron

The feasibility of manufacturing a 2-MW CW coaxial cavity gyrotron at 170 GHz has been demonstrated and data required for fabrication of an industrial tube have been obtained. An engineering design of a prototype started recently with the goal to provide gyrotrons with 2-MW microwave output power for International Thermonuclear Experimental Reactor (ITER). The design of critical components of the prototype tube as electron gun, cavity and RF output system will be verified under realistic conditions at short pulses using the experimental coaxial gyrotron at Forschungszentrum Karlsruhe.

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.

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

Design of a 42-GHz 200-kW gyrotron operating at the second harmonic

The design of a 42-GHz 200-kW continuous-wave gyrotron operating in the TE/sub 4,2/ mode at the second cyclotron harmonic is presented. This study includes mode selection, cavity design, and interaction calculations for power and efficiency as a function of various parameters, as well as preliminary design of the beam optical system, launcher, and window. This device is intended to serve as a heating source for a small experimental steady-state Tokamak, which requires microwave power, preferably in the form of a Gaussian beam.

Development of frequency step tunable 1 MW gyrotrons in D-band

Gyrotrons are widely used in electron-cyclotron-plasma heating of fusion installations. Modern and future installations require microwave sources with power of at least 1 MW. There are two main directions in the development of megawatt gyrotrons at GYCOM. The first line is an enhancement of parameters for conventional gyrotrons. The main efforts are applied now to implementing of CVD diamond windows into the gyrotrons and providing gyrotron operation in CW (or very long pulses of tens seconds) regime.

Design of a multifrequency high power gyrotron at FZK

The design of a multifrequency tunable 1 MW, 10 s gyrotron (105-140 GHz) for controlling plasma instabilities in fusion Tokamaks is presented. It operates in the TE/sub 22,8/ mode at 140 GHz and the TE/sub 17,6/ mode at 105 GHz and some other modes in between. It is equipped with a magnetron injection gun (diode type), a cylindrical cavity, an advanced quasi-optical launcher, three specific beam forming mirrors and a CVD-diamond Brewster window.

Progress toward optimization of phase-correcting mirrors for a multifrequency 1-MW gyrotron

Due to the requirement of a newly designed ultra-broadband chemical vapour deposition (CVD)-diamond window for a multifrequency 1-MW short-pulse gyrotron at Forschungszentrum Karlsruhe (FZK), the existing beam-forming mirror system inside the gyrotron tube has to be modified. This paper describes the design scheme for shaping the output beams from the launching antenna waveguide into the desired fundamental Gaussian profile by using adapted phase-correcting mirrors with nonquadratic surface contour function. Simulation results show these phase-correcting mirrors can be used for broadband operation of a frequency step-tunable gyrotron, which operates in nine modes from the TE17,6 mode at the frequency of 105 GHz to the TE23,8 mode at 143 GHz. Further calculations predict that efficiencies of more than 94% can be obtained for converting the high-order cylindrical cavity modes into the usable fundamental Gaussian mode

Experimental results and technical requirements for a 2 MW, CW, 170 GHz coaxial cavity gyrotron

In recent experiments on a 165 GHz, coaxial cavity gyrotron, the data basis necessary for a technical design and for industrial manufacturing of such a tube has been completed. In particular, the following investigations have been performed: measurement of the amount of microwave stray radiation captured in the mirror box and of the losses at the coaxial insert with improved accuracy; influence of misalignment of the coaxial insert on the gyrotron performance; extension of the microwave pulse length; measurement of the single-mode operating range and of the hysteresis effect due to variation of the beam voltage. The recently obtained results are presented and discussed. The results concerning the hysteresis effect are given in a separate paper at this conference (O. Dumbrajs et al., ibid., paper TH4.2, 2002). Based on these results, the design work for a 2 MW, CW, 170 GHz coaxial cavity gyrotron has started.

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 TE17,6 at 105 GHz to TE23,8 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.

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 TE22,8 mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE17,6) and 143 GHz (TE23,8). 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.