Speaker
Eric Nardon
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O3.110.pdf
Plasma breakdown in the WEST tokamak
E. Nardon1, H. Heumann2, J.F. Artaud1, N. Fedorczak1 and the WEST team
1
CEA, IRFM, F-13108 Saint-Paul-Lez-Durance, France
2
CASTOR, INRIA SAM, 06902 Valbonne, France & LJAD, Université de Nice Sophia
Antipolis, 06108 Nice Cedex 02, France
The WEST project involved important changes from Tore Supra in terms of magnetic
configuration, the main ones being two new in-vessel divertor coils surrounded by stainless
steel casings and two new copper stabilizing plates. These additional components had major
consequences on the magnetic field map at breakdown. In the initial WEST configuration,
breakdown could be achieved but the plasma current would not go above a few tens of kA.
This was attributed to induced currents in the stabilizing plates. It was therefore decided to
remove the lower stabilizing plate and, when this proved not sufficient, to also cut the upper
one (which was hard to remove). This finally allowed successful plasma breakdown and Ip
ramp-up, using a high electric field (~1V/m) and low D2 prefill pressure (~3mPa). Due to
these parameters, plasma breakdown is however often accompanied by runaway electron
(RE) formation but, counter intuitively, it has been found that reducing the prefill pressure
can help avoid RE. Extensive vacuum magnetic field modelling has been performed, for both
preparation and analysis of the experiments. For the preparation, inverse time evolutive
simulations with the FEEQS.M code have been used to calculate optimal premagnetization
poloidal field coils currents and voltages to be applied in the breakdown phase. For the
analysis, magnetic field maps have been reconstructed with two different approaches: direct
time evolution simulations with FEEQS.M (starting from measured coils currents and
applying measured voltages) and extrapolation from magnetic measurements with the
FREEBIE_ID code. FEEQS.M- and FREEBIE_ID-reconstructed field maps are consistent
with each other and with fast visible camera images, which allows for a precise assessment of
the necessary conditions in terms of magnetic field for plasma breakdown in WEST.
