NSTX-U L-mode plasmas in support of transport and turbulence
validation Authors: W. Guttenfelder, S.M. Kaye, R.E. Bell, A.
Diallo, B.P. LeBlanc, and M. Podestà
Abstract:
A variety
of stationary L-mode plasmas were successfully developed during
the first run campaign of the National Spherical Torus Experiment
– Upgrade (NSTX-U) project to support numerous core, edge and
boundary research activities. The NSTX-U L-mode discharges span a
range of plasma current, Ip=0.65-1.0
MA, and line-averaged density,, using
a magnetic field Bt=0.63 T larger than previous NSTX
operational limits (£0.55 T). The higher density
L-modes were sustained with up to 3 MW of neutral beam heating.
Transport analysis shows that ion thermal transport approaches
neoclassical levels at the relatively high collisionalities
required to avoid transition to H-mode. Ion-scale turbulence
measurements from 2D beam emission spectroscopy (BES) show
significant fluctuation amplitudes. Initial gyrokinetic analysis
predicts that ion temperature gradient (ITG) modes are unstable
around normalized radii p
=0.6-0.7, although E×B shearing rates are larger than
the linear growth rates over much of that region. The electron
temperature gradient (ETG) instability at electron scales is
also found unstable and nonlinear ETG simulations predict
significant electron thermal transport outside p>0.5. Deeper in the core (p<0.6) of higher beta (βT≈4%,
βN≈2) L-modes, the electromagnetic
microtearing modes are also unstable, possibly contributing to
the anomalous electron thermal transport in those cases. In
contrast, at lower beta (βT≈2%,
βN≈1), the microtearing modes are
very weak and almost completely stabilized. These low aspect
ratio, modest beta discharges (R/a~1.6, bN~1-2) provide an experimental
target for validation and cross-code benchmarking that is
intermediate between high aspect ratio, low beta (R/a~3, βN 1-2) where the bulk of gyrokinetic
validation studies exist, and low aspect ratio, high beta (R/a~1.6,
βN~5) where gyrokinetic simulations
are less tested and challenged by stronger electromagnetic,
equilibrium, and non-local effects (at large p*=pi/a).
Submitted to: Nuclear Fusion
_________________________________________________________________________________________________
,