D. Wagner1, F. Leuterer1, A. Manini1, F. Monaco1, M. Münich1, F. Ryter1, H. Schütz1,
J. Stober1, H Zohm1, T. Franke1, R. Heidinger3, M. Thumm2, W. Kasparek4, G. Gantenbein2,
A.G. Litvak5,L.G. Popov6, V.O. Nichiporenko6, V.E. Myasnikov6, G.G. Denisov5, E.M. Tai6,
E.A. Solyanova6, SA. Malygin6
1Max-Plank-Institut für Plasmaphysik, EURATOM-IPP, Boltzmannstr.2, D-85748 Garching, Germany
2Forschungszentrum Karlsruhe,Institut für Hochleistungsimpuls- und Mikrowellentechnik,
D-76021 Karlsruhe, Germany
3Forschungszentrum Karlsruhe, Institut für Materialforschung, D-76021 Karlsruhe, Germany
4Institut für Plasmaforschung, Universität Stuttgart, D-70569 Stuttgart, Germany
5 Institute of Applied Physics, 46 Ulyanov St., Nizhny Novgorod, 603950, Russia
6 GYCOM Ltd, 46 Ulyanov St., Nizhny Novgorod, 603155, Russia
The first two-frequency gyrotron Odissey-1 has been installed and put into operation in the new multi-frequency ECRH system at the ASDEX Upgrade tokamak experiment. It works at 105GHz and 140GHz with output power 610kW and 820kW respectively for a pulse length of 10s. A further extension of the system with 3 more gyrotrons is underway. These gyrotrons will be step-tunable and operate at two additional intermediate frequencies between 105 and 140GHz. The variable frequency will significantly extend the operating range of the ECRH system, e.g. allow for central heating at different magnetic fields. Other experimental features, like the suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. The stabilization of NTM’s requires a very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this reason fast movable launchers have been installed.
The two-frequency GYCOM gyrotron Odissey-1 has a single-stage depressed collector. Therefore the beam voltage can be limited to a maximum value of 60kV. The maximum beam current is 40A. The operating modes are TE17,6 at 105 GHz and TE22,8 at 140 GHz. Here we make use of the 3l/2 and 4l/2 resonances (l is the wavelength) of the single-disk synthetic diamond vacuum window at these frequencies. The frequency can be changed between two ASDEX Upgrade pulses and requires an adjustment of the cryomagnetic field, the gun magnetic and the collector magnetic fields. Reliable operation was achieved, only limited by the available high power long-pulse load. Two modulation schemes have been tested with this gyrotron. A 100% power modulation with frequencies up to 1kHz was achieved by switching both, cathode and body voltage on and off. Higher modulation frequencies up to 25 kHz with a modulation depth up to 90% were achieved by a reduction of only the cathode voltage from 42kV to 25kV while keeping the body voltage constant. First plasma test shots were performed with a maximum power of 820kW at 140 GHz and a pulse length up to 0.8s. The total measured frequency variation during a gyrotron pulse was 140 MHz. Out of this, a drift of ~100MHz happened in the first 100ms of the pulse and repeatedly during modulation (Fig.1), very likely due to space charge effects. The remaining shift of 40MHz to steady state results from the thermal expansion of the cavity. The freezing of the inner gyrotron cooling circuits, caused by a magnet failure, led to a deformation of the cavity of Odissey-1. The gyrotron was returned to GYCOM for repair and will be replaced by the next two-frequency series tube Odissey-2. After repair, Odissey-1 will be equipped with an improved quasi-optical mode converter to further reduce the stray radiation in the tube. A tunable double-disc window will be mounted allowing the operation of Odissey-1 as a step-tunable gyrotron.
Fig.1: Measured frequency drift of gyrotron Odissey-1 during a modulated and a cw 140 GHz pulse.
Since the phase distribution and the azimuthal angle of the gyrotron output beam are different at different frequencies, a special pair of phase correcting mirrors is required for each frequency. The mirrors are mounted on turntables in the Matching Optics Unit (MOU) and automatically put into place when the frequency and therefore the operating mode changes. The transmission to the torus is in normal air, through corrugated aluminum waveguides with I.D.=87mm over a total length of about 70m. Since most part of the waveguide path is straight, the number of miter bends could be limited to 7 and 8 respectively. Calorimetric measurements in the MOU and at the end of the transmission line, next to the torus window, gave a total transmission loss of only 12% at 105GHz and 10% at 140GHz. A fast steerable launcher enables to steer the beam over the whole plasma cross section. In order to cope with thermal load, disruption forces and mechanical dynamics, the mirror is made out of copper-plated graphite. The toroidal angle can be varied between shots by rotating the launcher around its axis. A fast drive is used to control the poloidal launching angle during a discharge. The design goal of 10°/100ms was achieved during the tests.
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