Speaker
Hongjuan Sun
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I4.117.pdf
Correlation of the near SOL transport with plasma properties of the
confined edge region in ASDEX Upgrade
H J. Sun1, E. Wolfrum1, T. Eich1, B. Kurzan1, A. Kallenbach1, A. Scarabosio1, U. Stroth1,2 and
the ASDEX Upgrade Team
1
Max Planck Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany
2
Physik-Department E28, Technische Universität München, 85747 Garching, Germany
Improvements to the Thomson Scattering edge system have enabled, for the first time in
ASDEX Upgrade, the study of upstream SOL electron density (ne) and temperature (Te)
profiles and thermal transport over a wide set of plasma parameters and regimes, which
include H-, L-, and I- mode, as well as the high density limit (HDL). Results show that
parallel transport in the near SOL region is always dominated by Spitzer-Harm conduction in
both L-and H-mode, consistent with SOLPS modelling. The near SOL ne and Te profiles are
found to be closely coupled across different plasma regimes and divertor conditions, with
gradient ratio 1 ≤ 𝜂𝑒 ≤ 2. A study of H-L back transitions show that there is a bifurcation
between H- and L-mode plasmas in near SOL perpendicular transport, at the same global
parameters and heating power. An expression for the perpendicular thermal diffusivity,
𝜒 ⁄2 𝜒 𝜒
𝜒⫠ = 𝑐𝐿,𝐻 𝑛𝑒 −1 𝑇𝑒3 with 𝑐𝐻 �𝑐𝐿 ≈ 0.5, is derived from the wider database. Also the transition
from L-mode to I-mode shows a change in perpendicular thermal transport.
For H-mode plasmas under attached conditions, steep pedestal gradients and higher pedestal
top values of both Te and pe, which are beneficial for plasma performance, appear to correlate
with narrower SOLs, which are unfavourable for plasma exhaust. However, the same trend is
not observed for fixed plasma current. Here, the pedestal Te gradients can vary by a factor of
2 while the SOL Te decay lengths do not change. This suggests that the pedestal-SOL
correlation is a consequence of the pedestal and the SOL regions being influenced differently
by the same global parameters rather than a direct dependence of one on the other. This offers
the prospect of optimising the two regions separately. A good example is the increase of the
pedestal top pressure by N seeding while no influence on the SOL Te decay lengths is
observed.
Across the database of H-mode plasmas, the SOL MHD ballooning parameter, αsep , increases
almost linearly with separatrix density for 𝛼𝑠𝑠𝑠 <2, consistent with JET results. After 𝛼𝑠𝑠𝑠
reaches 2-2.5, it doesn’t vary with density which is consistent with the theoretically predicted
onset of ballooning modes. A confinement degradation caused by these modes may be the
mechanism for the HDL.
