Design and Experimental Results of the 10 MW, 140 GHz ECRH-System for the Stellarator W7-X

G. Dammertz¹, H. Braune³, V. Erckmann³, G. Gantenbein⁴, W. Kasparek⁴, H. P. Laqua³, W. Leonhardt¹, G. Michel³, G. Mueller⁴, G. Neffe¹, B. Piosczyk¹, M. Schmid¹,

M. Thumm^1,2

¹Forschungszentrum Karlsruhe, Association EURATOM-FZK, Institut fuer Hochleistungsimpuls- und Mikrowellentechnik (IHM), Postfach 3640, D-76021 Karlsruhe, Germany,

²Universitaet Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik,

Kaiserstr. 12, D-76128 Karlsruhe, Germany

³Max-Planck-Institut fuer Plasmaphysik, Teilinstitut Greifswald, Association EURATOM,

Wendelsteinstr. 1, D-17491 Greifswald, Germany

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

e-mail: guenter.dammertz@ihm.fzk.de

The stellarator W7-X is a helical advanced stellarator with strongly varying plasma cross sections, and with a five-fold field periodicity. The radius of the torus and of the plasma are R=5.5m and a=0.55m, respectively. The stellarator will be built as a step towards reactor-like fusion machines. The quasi steady state operation requires a continuously operating heating scheme. ECRH was chosen as the basic heating system for W7-X. A heating power of about 10 MW is necessary to arrive at the physical goals of the stellarator. The 10 MW power will be achieved by 10 gyrotrons each with an output power of 1 MW, operating at 140 GHz in continuous wave (CW) operation.

A prototype gyrotron with an output power of 1 MW was developed in collaboration between European research laboratories and European industries. This gyrotron is equipped with a single-stage depressed collector for increasing the efficiency, an optimized quasi-optical mode converter and a CVD-diamond window. The prototype gyrotron has been successfully tested with an output power of 0.89 MW at a pulse duration of three minutes and an output power of 0.54 MW for about 15 minutes (energy content per pulse: 505 MJ). The specified values (1 MW output power for 30 minutes) have not completely been reached. The reason for the lack in output power is seen in the azimuthally inhomogeneous electron beam distribution, which leads to a saturation of output power with increasing beam current. The limitation of pulse length can be explained by a strong temperature increase and the related outgassing of the internal ion getter pumps.

At W7-X, the ten gyrotrons are arranged in two subgroups symmetrically to a central beam duct in the ECRH hall. The RF-transmission between gyrotrons and plasma torus is performed quasi-optically and consists of single-beam waveguide mirrors (SBWG) and multi-beam waveguide mirrors (MBWG). The gyrotron beams are matched by 5 SBWGs (matching mirrors, polarizers) for each beam to a stigmatic Gaussian beam with the correct beam parameters and combined to two beam lines each with a transmitted power of 5 MW. The mirrors of the MBWG are optimized with respect to low mode conversion of the on-axis as well as off-axis TEM₀₀ beams using physical optics and analytic calculations. Furthermore, the configuration of the mirrors is such that mode conversion, which is a general feature of curved surfaces, cancels at the end of the MBWG. The mirrors have been tested in low- and high-power measurements. The results for deformation, efficiency and so on agree very well with theoretical predictions.