The W7-X ECRH plant: status and recent achievements

V. Erckmann and  the W7-X ECRH- teams at IPP Greifswald¹, FZK Karlsruhe² and IPF Stuttgart³

¹Max Planck Institut für Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, D-17491 Greifswald, Germany

² Forschungszentrum Karlsruhe, Association EURATOM-FZK, IHM, D-76344 Eggenstein-Leopoldshafen, Germany,

³ Institut für Plasmaforschung, Universität Stuttgart, D-70569 Stuttgart, Germany

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7-X Stellarator (W7-X), which is under construction at IPP-Greifswald. W7-X (R = 5.5 m, r_eff = 0.55 m) aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters [1]. W7-X is equipped with a superconducting coil system and an actively pumped divertor for 10 MW steady state heat removal. A 10 MW ECRH plant with CW-capability at 140 GHz is under construction to meet the scientific objectives. The physics background of the different heating- and current drive scenarios is reviewed. The microwave power is generated by 10 gyrotrons with 1 MW each. The development of W7-X gyrotrons was successfully executed in Europe (FZK, CRPP, TED) and USA (CPI). Series production of gyrotrons has started with TED as industrial partner. The status of commissioning is briefly reported.

The operation of the TED gyrotrons was recently extended to a 2^nd frequency of 103.6 GHz at reduced output power and first results are presented. The standard vertical collector sweep systems of the TED gyrotrons inherently display a pronounced peaking of the collector heat loading, which limits the operation. We have developed an improved collector sweep system, which generates an almost perfectly smooth power distribution thus extending the safety-margin of present day gyrotrons. This system is of particular interest for next generation gyrotrons with higher power (2 MW) , because existing collector designs would comply with these demands.

The distinct microwave beams from each gyrotron are combined and transmitted to the W7-X Stellarator ports by an open quasi-optical transmission system with high transmission efficiency. The most-loaded section of the transmission-line passed integrated full power CW-tests and showed excellent performance. A versatile Fast Directional Switch (FADIS), which is based on fast frequency shift keying of high power microwave beams [2] was investigated and first experimental results are discussed. Such a beam switch/beam combiner is an attractive universal transmission component, which can be realized in waveguides or open systems with many possible applications (switch power between different ports, between different launchers for NTM stabilization etc.). The microwave power is launched to the plasma by in-vessel quasi-optical plug-in launchers. The front steering system allows for wide scanning angles (< 36 deg), first cyclic tests of a mock-up were performed. The integrated test-results may provide valuable input for the ITER ECRH-plant.

[1] V. Erckmann, et al., Proc.17th IEEE/NPSS Symp., Fus. Engin. (1997) Vol. 1, p.40
[2] W. Kasparek, et al.,Proc. 14th Joint WS on ECE and ECRH, EC-14, Santorini, Greece, (2006), pp. 424-429, ISBN 960-89228-2-8.