Speaker
Umar Ahmed Sheikh
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I2.110.pdf
Pedestal Structure and Energy Confinement Studies on TCV
U. A Sheikh1 , L. Frassinetti2, M. Dunne3 , P. Blanchard1 , B. P. Duval1, B. Labit1 ,
A. Merle1 , C. Theiler1 , C. K. Tsui1 , the TCV Team1 and the EUROfusion MST1 Team4
1 Swiss Plasma Center, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
2 KTH Royal Institute of Technology, Stockholm, Sweden
3 Max Planck Institute für Plasmaphysik, Garching, Germany
4
Author list of H. Meyer et al., Nucl. Fusion 57 (2017) 102014
The high confinement plasma mode is characterised by an edge transport barrier leading to
strong Te and ne gradients at the plasma edge, termed the pedestal. The pedestal can be tailored
through fuelling and seeding to improve core confinement, enhance fusion yield and reduce
divertor power loads. Recent studies have focussed on the role of the pedestal position, partic-
ularly the location of the density gradient relative to the separatrix. In AUG, an outward shift
of pedestal position was correlated to a high density front in the high field side scrape-off layer
(HFSHD), resulting in reduced pedestal top pressure and global confinement[1]. Similarly, a
relative shift between the locations of the peak Te and ne gradients was observed in low triangu-
larity discharges at JET-ILW, although it has not been conclusively linked with the HFSHD[2].
TCV is an ideal machine to address these uncertainties as a HFSHD is not expected due to the
open geometry and carbon walls. Pedestal behaviour can therefore be investigated in conditions
that are not attainable in standard operation in AUG or JET. Furthermore, due to the TCV’s
present engineering characteristics and flexible shaping capabilities, experiments can shed light
on the effect of combinations of impurity seeding, fuelling and plasma shape.
A database consisting of 36 shots scanning plasma triangularity, fuelling and nitrogen seeding
rates in NBH heated ELM-y H-mode plasmas was constructed. Increases in pedestal top Pe
and stored energy of approximately 30% and 10% respectively were measured with increasing
triangularity. These were accompanied by a reduction in the relative shift between the locations
of the peak Te and ne gradients. Increased seeding at high triangularity led to an outward shift in
pedestal position and decreasing pedestal top Pe , but with increased total stored energy by up to
5% and core Pe by up to 15%. These observations suggest that the outward shift of the pedestal
and the relative shift between the locations of the peak Te and ne gradients are not necessarily
correlated with the HFSHD, but may include changes in pedestal transport. The relevance of
these results in relation to results from AUG and JET, will be discussed in comparison with
interpretive modelling carried out with an EPED-like code.
References
[1] Dunne, M. G., et al., Plasma Phys. Control Fusion, 59, 025010, (2017).
[2] Frassinetti, L., et al., Plasma Physics and Controlled Fusion, 59, 014014, (2017).
