Speaker
Stefan Marsen
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1068.pdf
Optimization of ECRH operation at high densities in Wendelstein 7-X
S. Marsen1 , K. J. Brunner1 , H.P. Laqua1 , D. Moseev1 , T. Stange1 , W7-X Team
1 Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, Wendelsteinstraße 1, 17491
Greifswald, Germany
One of the major goals of Wendelstein 7-X is to achieve steady state operation (up to 30 min.)
at plasma parameters relevant for a future fusion reactor. This includes operation at plasma
pressures requiring a density above 1020 m−3 . The only steady state capable heating currently
available is ECRH using high power gyrotrons. Wendelstein 7-X is equipped with 10 gyrotrons
providing up to 7.5MW to the plasma vessel. The power is transmitted to the machine using
a quasi optical transmission line where the polarization and launching angle can be remotely
controlled by the central W7-X control system. Using X-mode polarization break down can
easily be achieved and the launched power is nearly perfectly absorbed (>99%) over a wide
range of plasma parameters. Here, the achievable density is limited by the X-mode cut-off
density of n = 1.2 · 1020 m−3 at the used frequency of 140GHz. In order to reach higher densities
O-mode polarization is necessary where the single pass absorption depends more sensitively on
the plasma parameters and is typically in the order of 50...80%. Plasma start-up in O-mode is
not possible because a target plasma with Te > 1keV is necessary to deposit enough energy to
sustain a hot plasma. During the first experimental campaigns of W7-X a scenario to achieve
ECR heated plasmas at densities above the X-mode cut-off was developed. The target plasma
was created using two or three gyrotrons in X-mode. After creating a low density target plasma
with ne ≈ 2 · 1019 m−3 the remaining gyrotrons were switched on in O-mode and the density was
ramped up to ne ≈ 6 · 1019 m−3 . While ramping up the denstiy the polarization of the start-up
gyrotrons was changed to O-mode. Thus, a purely O-mode heated plasma was created. Using
pellet injection the density could then be further increased. Line averaged densities of ne = 1.4 ·
1020 m−3 as measured by a single channel interferometer were achieved. The exceeding of the
X-mode cut-off was confirmed by ECE measurements where the signal was lost eventually. In
order to maximize the heating efficiency a three pass heating scheme was established reflecting
the beams through the plasma axis once at the high-field side of the vessel and once at the
low field. Thus, a total absorption of >90% could be reached as determined by stray radiation
measurements. Reducing the amount of non-absorbed microwave power in the vessel is not
only necessary to maximize the plasma performance but also in terms of safe machine operation
since in vessel components can suffer from heating up due to stray radiation. The stray radiation
levels measured around the machine were comparable to or even lower than expected levels.
