Speaker
Robert Wolf
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I4.114.pdf
Electron-cyclotron-resonance heating in Wendelstein 7-X: A versatile
heating and current-drive method and a tool for in-depth physics studies
R. C. Wolf1, C. D. Beidler1, K. J. Brunner1, A. Dinklage1, G. Fuchert1, G. Gantenbein2,
M. Hirsch1, U. Höfel1, J. Jelonnek2, W. Kasparek3, H. Laqua1, C. Lechte3, S. Marsen1,
N. B. Marushchenko 1, B. Plaum3, T. Stange1, M. Thumm2, Y. Turkin1, M. Zanini1, and the
W7-X Team
1
Max-Planck-Institute for Plasma Physics, Greifswald, Germany
2
IHM, Karlsruhe Institute of Technology (KIT), Germany
3
IGVP, University of Stuttgart, Germany
Important criteria for the optimum heating mix of magnetic confinement fusion experiments
are heating efficiency, power deposition profiles or the ability to also drive plasma currents. In
particular, for stellarators, which need no or only small amounts of current drive, ECRH is a
promising heating method even for the envisaged application in a fusion power plant.
Wendelstein 7-X (W7-X) is equipped with a steady-state ECRH system consisting of ten
gyrotrons which operate at 140 GHz corresponding to the 2nd cyclotron harmonic at a magnetic
field of 2.5 T. The heating power available ranges from 7 to 9 MW which today is the largest
ECRH facility in operation. W7-X uses ECRH for plasma start-up, heating and current drive
and also plasma vessel conditioning. All ten gyrotrons are operational and already have
delivered 7 MW to W7-X plasmas. With the heating power available in the first campaign,
energy confinement times were achieved which were lying on the international scaling for
stellarator confinement (ISS04). First studies indicate the existence of a radiative density limit
with a Sudo-like power scaling.
The power of the ten gyrotrons is transmitted through air to the W7-X plasma vessel using a
quasi-optical mirror system. The overall transmission efficiency was experimentally estimated
to be 94%, which is close to the theoretical value. Front steering launchers direct the gyrotron
beams individually to the plasma. First current drive experiments revealed periodic internal
plasma-crash events, which can be explained by the appearance of low order rationals of the
rotational transform, ι, due to current drive close to the plasma center leading to an ι-change,
∆ι ∼ I/r2. In the current operation campaign of W7-X remote steering launchers (RSLs) were
tested for the first time. Without movable parts near the plasma, the remote steering technology
may be a possible solution for a power plant. Since the RSLs are located near the minimum of
the magnetic field, they also allow probing of the electron distribution function by changing the
amount of trapped and passing electrons.
The paper addresses plasma confinement properties specific to the application of strong electron
heating by ECRH, the heating of plasmas at high densities which requires a change from 2 nd
harmonic X-mode to 2nd harmonic O-mode, the possibility to test our understanding of electron-
cyclotron current drive, and heat-wave experiments delivering insight into the characteristics
of plasma transport.
