7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Overview of the design and first experimental results of the ICRH system for thelarge optimized stellarator Wendelstein 7-X
J.Ongena1, Ye.O.Kazakov1, K.Crombé1,2, D.Hartmann3, D.Castaño-Bardawil1, D.Lopez-Rodriguez1,2, I.Stepanov1,M.Verstraeten1, M.Vervier1, B.Schweer1, A.Dinklage3, T.Fornal4, M.Gruca4, K.P.Hollfeld5, J.P.Kallmeyer3,I.Ksiazek6, A.Krämer-Flecken7, M.Kubkowska4, Ch.Linsmeier7, F.Louche1, O.Neubauer7, D.Nicolai7,G.Offermanns5, G.Satheeswaran7, L Syrocki4, M.Van Schoor1, R.C.Wolf3, the TEC team1,7and W7-X team8
1

Laboratory for Plasma Physics, Royal Military Academy, TEC Partner, Brussels, Belgium
2
Department of Applied Physics, Gent University, Belgium
Max-Planck-Institut für Plasmaphysik, Wendelsteinstraße 1, D-17491 Greifswald, Germany
4
Institute of Plasma Physics and Laser Microfusion, 23 Hery Str., 01-497 Warsaw, Poland5
Zentralinstitut für Engineering, Elektronik und Analytik – Engineering und Technologie, (ZEA-1),
6Institute of Physics, Opole University, ul. Oleska 48, 45-052 Opole, Poland
7
Institut für Energie- und Klimaforschung / Plasmaphysik (IEK-4), Trilateral Euregio Cluster (TEC) Partner, Forschungszentrum
Jülich, D-52435 Jülich, Germany
8
See the author list of the paper by T.Sunn Pedersen et al., Nucl. Fusion 62 (2022) 04202
3e-mail (speaker): j.ongena@fz-juelich.deThe superconducting stellarator Wendelstein 7-X (W7-X) atthe Max-Planck-Institute in Greifswald has been operational
since 2015. One of the main goals of W7-X is to demonstrateefficient confinement of fast ions at high plasma densities. Toachieve this, W7-X was specifically optimized to handle
volume averaged beta values of up to 5%, which corresponds
to plasma densities above 1020 m-3. To simulate the behaviourof alpha particles in a future stellarator reactor, a populationof fast ions with energies ranging from 80-100keV isrequired in the core of high-density plasmas in W7-X. A
promising method to tackle this demanding task in W7-X isIon Cyclotron Resonance Heating (ICRH), using minority
heating of H in 4He and D plasmas and the 4He-(3He)-H or
D-(3He)-H 3-ion scenario.

allow realizing plasmas with higher beta-values at moderateheating power levels. In these experiments using the standard
magnetic configuration in W7-X the LCMS was located at 17cm
from the first wall and the antenna-LCMS distance was between
3 and 10cm. Up to 700kW RF power was delivered without
hitting an upper limit for the voltage or currents in the system.
Despite the unfavorable heating conditions (k|| ~ 0), a clear
increase of the plasma stored energy was seen at constantelectron density, pointing to an increase in the ion temperature.
The Faraday Screen is omitted in this antenna, based on
extensive experience on TEXTOR [2]. No edge interaction
could be observed and the impurity levels remained low when
ICRH was switched on top of ECRH (see Fig.2). In plasma
breakdown experiments at 1.7T plasmas were created for the full
duration of the ICRH pulse with about 300kW RF power.
After removal of the antenna from the W7-X plasma vessel onlya slight discoloration of the antenna straps and box was observed.

Figure 1. The ICRH antenna in the vacuum vessel of W7-X
for three radial positions.
The ICRH antenna for W7-X consists of two poloidal straps,
with each strap terminated by a pre-matching capacitor at one
end, short-circuited at the other and the RF power is fed at anintermediate position along the straps [1]. The shape of theantenna is matched to the 3D shape of the Last Closed Magnetic
Surface (LCMS) of the standard magnetic field configuration on
W7-X, leading to a variable curvature in toroidal and poloidal
direction over the plasma-facing surface of the antenna. Inaddition, the antenna can be moved radially over max. 35 cm(with a speed up to 6 mm/s), and a gas puffing system isincorporated to puff gas in the region between the scrape-offlayer (SOL) and the LCMS to locally improve the coupling. The
full system was commissioned on W7-X plasmas in Februaryand March of 2023. In these experiments only one of the two
straps of the antenna was powered, because of a faulty prematching capacitor (and vacuum feedthrough), leading tooperation with k|| ~0.
Two main milestones, operation at high power levels (above500kW) and plasma breakdown using ICRH only at magnetic
fields below the usual 2nd harmonic used by ECRH (2.5T), toFigure 2. No difference seen in impurity levels between
ECRH only and combined ICRH+ECRH heated phases in
W7-X pulses.
References:[1] J.Ongena et al., Physics of Plasmas 21, 061514 (2014)[2] R. Van Nieuwenhove et al., Nucl. Fusion 32 (1992) 1913