Authors: R.M. Churchill, C.S. Chang,
R. Hager, R. Maingi, R. Nazikian, D.P. Stotler
Abstract: Simulations using the fully kinetic code
XGCa were undertaken to explore the impact of kinetic e ffects on
scrape-o ff layer (SOL) physics in DIII-D H-mode plasmas. XGCa is
a total-f, gyrokinetic code which self-consistently calculates the
axisymmetric electrostatic potential and plasma dynamics, and
includes modules for Monte Carlo neutral transport. Fluid
simulations are normally used to simulate the SOL, due to its high
collisionality. However, depending on plasma conditions, a number
of discrepancies have been observed between experiment and leading
SOL fluid codes (e.g. SOLPS), including underestimating outer
target temperatures, radial electric field in the SOL, parallel
ion SOL flows at the low field side, and impurity radiation. Many
of these discrepancies may be linked to the fluid treatment, and
might be resolved by including kinetic e ffects in SOL
simulations.
The XGCa simulation of the DIII-D tokamak in a nominally
sheath-limited regime show many noteworthy features in the SOL.
The density and ion temperature are higher at the low- eld side,
indicative of ion orbit loss. The SOL ion Mach flows are at
experimentally relevant levels (Mi 0:5), with similar shapes and
poloidal variation as observed in various tokamaks. Surprisingly,
the ion Mach flows close to the sheath edge remain subsonic, in
contrast to the typical fluid Bohm criterion requiring ion flows
to be above sonic at the sheath edge. Related to this are the
presence of elevated sheath potentials, (see PDF for formula),
over most of the SOL, with regions in the near-SOL close to the
separatrix having (see PDF for formula). These two results at the
sheath edge are a consequence of non-Maxwellian features in the
ions and electrons there.
Submitted to: Nuclear Materials and Energy
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