Advanced High Power Gyrotrons

M. Kuntze1, S. Alberti5, G. Dammertz1, V. Erckmann3, E. Giguet2, S. Illy1, W. Kasparek4, Y. Le Goff2, W. Leonhardt1, G. Michel3, G. Müller4, B. Piosczyk1, M. Schmid1, M. Thumm1,6, M.Q. Tran5

1 Forschungszentrum Karlsruhe, Association Euratom-FZK,

Institut für Hochleistungsimpuls- und Mikrowellentechnik,

Postfach 3640, D-76021 Karlsruhe, Germany

2Thales Electron Devices, 2 rue Latécoère, F-78141 Vélizy-Villacoublay, France

3 Max-Planck-Institut für Plasmaphysik (IPP), Wendelsteinstr. 1,
D-17491 Greifswald, Germany

4 Universität Stuttgart, Institut für Plasmaforschung, Pfaffenwaldring 31,
D-70569 Stuttgart, Germany

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

6Uni Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik,

Kaiserstr. 12, D-76128 Karlsruhe, Germany

 

A 1 MW, 140 GHz gyrotron with diamond window for continnous wave (CW) operation and with a single-stage depressed collector for energy recovery and improvement of efficiency has been designed and constructed in collaboration with CRPP Lausanne and TTE Vélizy. It operates in the TE28,8 cavity mode and provides a linearly polarized TEM0,0 Gaussian rf beam. The gyrotron consists of a magnetron injection gun (diode type), an improved beam tunnel, an improved cavity with rounded corners and low surface loss densities (< 2 kW / cm2), an optimized non-linear up-taper and an improved launcher.

First rf power measurements at pulse lengths of 1 ms gave 0.65 MW output power at 40 A and 82 kV. With pulse lengths up to 150 ms an output power of 0.6 MW was measured. Despite careful optimization it has not been possible, to improve the output power for this beam current. Measurements of the beam profile showed a strong shift of the beam with respect to the window axis by 30 mm downwards. This deviation leads to power reflections of 20% back into the gyrotron, and this might cause a further reduction of the generated power. The deviation is caused by a design error. The depressed collector could be operated up to 33 kV with less than 10 mA body current, which gives an improvement factor of 1.6 for the efficiency. The collector has been tested successfully with long pulse operation (30 s) without rf generation at 1 MW power level (50 kV, 20 A). The measurements are currently continued, new results will be given.

With the 165 GHz coaxial cavity gyrotron which has been developed at Forschungszentrum for higher output power due to reduced mode competition problems an output power of 2.2 MW with an efficiency of 28% has been measured recently in short pulse operation (1 ms). This value has been measured at a beam voltage of 94,6 kV and a beam current of 84 A. The maximum efficiency has been obtained at 1.5 MW output power and 56 A beam current. With a voltage depression of about 35 kV an efficiency of 48% is calculated (30% without SDC). Up to that depression voltage no reduction in output power has been observed, and almost no increase in body current has been measured. These results are in very good agreement with numerical calculations, if one includes internal losses of 10% and a velocity spread of 5% for the electron beam.