Development of Multimegawatt Gyrotrons

for Fusion Plasma Heating and Current Drive


G. Dammertz1a, S. Alberti2, A. Arnold1a,3, E. Borie1a, V. Erckmann4, G. Gantenbein5, E. Giguet7, R. Heidinger1b, J.P. Hogge2, S. Illy1a, W. Kasparek5, K. Koppenburg1a, M. Kuntze1a, H. Laqua4, G. Le Cloarec7, F. Legrand7, Y. Le Goff7, W. Leonhardt1a, C. Lievin7, R. Magne6, G. Michel4,

G. Müller5, G. Neffe1a, B. Piosczyk1a, T. Rzesnicki1a, M. Schmid1a, M. Thumm1a,3, M.Q. Tran2


1Forschungszentrum Karlsruhe, Association EURATOM-FZK, aIHM, bIMF I,

Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany,

2Centre de Recherche en Physique des Plasmas, Association Euratom-Confédération Suisse, EPFL Ecublens, CH-1015 Lausanne, Suisse

3Universität Karlsruhe, IHE, D-76128 Karlsruhe, Germany

4Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, Association EURATOM,

Wendelsteinstr. 1, D-17491 Greifswald, Germany

5Institut für Plasmaforschung, Universität Stuttgart, D-70569 Stuttgart, Germany

6CEA/Cadarache, 13108 Saint Paul-lez-Durance Cédex, France

7Thales Electron Devices, 2 Rue de Latécoère, F-78141 Vélizy-Villacoublay, France


e-mail:; phone: (+49) 7247-82 4160; fax: (+49) 7247-82 4874



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.


Conventional Cavity Gyrotron

The development of gyrotrons with 1-MW output power for CW operation is mainly linked with the construction of the new superconducting stellarator Wendelstein 7-X at IPP Greifswald, Germany. For this facility a total power of 10 MW of electron cyclotron waves (ECW) is planned and will be produced by ten gyrotrons each with a power of 1-MW. The gyrotrons are equipped with a diode-type magnetron injection gun,  a conventional TE28,8 mode cavity, an advanced quasi-optical (q.o.) mode converter system, an output RF-window with a single edge-cooled CVD-diamond disk and a depressed collector for energy recovery. After the construction and test of two prototypes, the series gyrotrons have been ordered. The second prototype yielded an output power of 890 kW for 180 s, and at reduced beam current a power of 540 kW for 937 s. After these tests, the tube had been disassembled for visual inspection. It was rebuilt with a new  cathode emitter ring in order to increase the uniformity of the electron beam. Further the cooling for the internal ion getter pumps was improved. The tube is now ready for tests.


Coaxial Cavity Gyrotron

In previous experiments, an output power as high as 2.2 MW was achieved during short-pulse operation at 165 GHz in the TE31,17 mode. For longer pulse operation, the built up of a Penning discharge in the rear part of the electron gun occurred. This could be suppressed by a modification of the gun geometry. At a beam current of 50 A, a pulse length of 22 ms could be achieved with an RF-output power of about 1 MW. The pulse-length was limited  due to the temperature rise of the collector. At reduced currents, a pulse length of more than 100 ms could be achieved without Penning discharge.

A leakage current to the insert has been measured which increased linearly with the depression voltage (retarding voltage of the collector). This leakage current reaches about 37 mA at a depression voltage of 27 kV. However, no influence of the accelerating voltage was found. It is assumed that the leakage current is created by trapped electrons generated by ionization of the background gas. In case of an existing depression voltage, the trapped electrons only can escape by diffusion across the magnetic field.

Based on the results of these measurements a conceptual design of an industrial prototype CW gyrotron with a coaxial cavity for an RF-output power of 2 MW at 170 GHz has been completed. The manufacturing process has been launched. The design of the cavity has been verified by using two different time-dependent, multimode and self-consistent codes. To reduce the internal stray radiation, an advanced RF-output system has been designed. A single disk CVD-diamond window with a thickness of 1.852 mm and a novel single-stage depressed collector will be used.

In order to verify the design of the critical components as electron gun, cavity, and output system, the 165 GHz coaxial gyrotron will be modified for operation at 170 GHz in the TE34,19 mode. The tests will allow to prove the RF generation, the mode competition and as well as the efficiency of the RF output system and to measure the amount of internal stray radiation. The tests are expected to start soon.


Frequency Step-Tunable Gyrotrons

The availability of MW gyrotrons with fast frequency step tunability permits the use of a non-movable antenna for local electron cyclotron current drive and plasma stabilization in thermonuclear fusion devices.

The existing TE22,6 mode gyrotron was modified to be operated in the TE22,8 mode. This mode is suitable for long-pulse operation at 1MW level. The resonator was optimized for a series of modes from 105-140 GHz. A q.o.-mode converter system has been designed, fabricated and tested in low-power measurements. The converter consists of a dimpled-wall antenna (in order to minimize the internal stray radiation) and a beam-forming mirror system with a large quasi-elliptical mirror and two additional phase-correcting mirrors. The field distribution measurement showed very good agreement with the calculated one. The power measurements with a quartz Brewster window gave an output power of 700 kW for different modes.

The use of a Brewster window from CVD-diamond even with a diameter of 140 mm needs a special construction of the window and an optimized RF beam to fit into this construction. A new q.o.-mode converter to be used with a diamond Brewster window has to be designed and will be constructed.