Speaker
Peter Thomas Lang
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O2.103.pdf
Isotope mixture control in the high density regime by pellet injection at
ASDEX Upgrade
P.T. Lang1, A. Drenik1, R. Dux1, T. Jackson2, O.J.W.F. Kardaun1, A. Mlynek1, B. Ploeckl1,
M. Prechtl3, V. Rohde1, R.R. Ruess3, P.A. Schneider1, M. Schubert1, ASDEX Upgrade Team
1) MPI für Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany
2) CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK
3) Hochschule für angewandte Wissenschaften, Friedrich-Streib-Str. 2, 96450 Coburg
The all-metal-wall tokamak ASDEX Upgrade is equipped with a versatile pellet launching
system. Offering an injection set up similar to that foreseen for EU-DEMO, one of its major
tasks is to investigate reactor relevant aspects of core particle fuelling. A future fusion power
plant has to operate at high core densities with a D:T isotope mixture of about 1:1 in order to
harvest a maximum of fusion power. To access accordingly high densities beyond the
Greenwald density nGw fuelling by pellets, mm-sized bodies of solid fuel, is required. This
approach must happen in a controlled manner, raising the core density while keeping edge
density sufficiently low to avoid confinement degradation. Hence, the challenge of the task is
to develop both suitable plasma scenarios and effective tools capable of simultaneously
controlling the density profile and the isotope fraction in the core.
ASDEX Upgrade deploys a sophisticated control system, providing full feedback control of
the pellet launcher. The reactor relevant D/T scenario was mimicked by using H/D. To enable
for isotope fraction control, the pellet lauching system was modified to produce H2/D2 pellets,
delivering pellet trains with a constant H/(H+D) fraction of 0.5 ± 0.03. Pellet injection can
alter the isotope mixture in plasma as requested; after equilibration a 1:1 H:D ratio was
established in the plasma, as confirmed by spectroscopy and residual exhaust gas analysis. In
addition, pellet actuation allows for operation at high core densities.
Hence, our experiments proved pellet actuation can yield access to the high density regime
while simultaneously establishing and maintaining the requested H/D isotope ratio.
A database containing key parameters was created for the set of experiments dedicated to
pellet based isotope fraction control and their pure D reference discharges. It covers plasmas
with a H/(H+D) fraction in the range 0 – 0.8 and core densities up to 1.8 x nGw. For the energy
confinement, degradation with increasing core density was observed. An increasing H fraction
correlates with lower energy confinement. However, the latter correlation does not fit well to
the smooth transition with the average ion mass M as predicted by e.g. the scaling
H98(y,2) ∼ M0.19. Conversely, small fractions of H were found to cause a significant
reduction. This observation demands further consideration for its potential consequences since
e.g. for the engineering design of the H removal system in the EU-DEMO fuel cycle a 2 %
contribution of H to the plasma particles has been mooted as acceptable. From our data, there
is also strong indication of an increasing H content significantly reducing the particle
confinement. The same pellet actuation shows a pronounced lower density build up in cases
with a significant H fraction in comparison to their pure D counterparts. In addition, analysis
of the density evolution after pellet injection shows a distinct shortening of the pellet particle
sustainment time for the HD compared to the D pellets.
