Perturbative momentum transport in MAST L-mode plasmas Authors: W. Guttenfelder, S.M. Kaye, and Y. Ren
Abstract:
Non-axisymmetric magnetic fields are used to perturbatively probe
momentum
transport physics in MAST L-mode plasmas. The low beta L-mode target
was chosen
to complement previous experiments conducted in
high
beta NSTX H-mode plasmas (β N=3.5-4.6)
where
an inward momentum pinch was measured.
In
those cases quasi-linear gyrokinetic
simulations of unstable
ballooning micro-instabilities predict weak or outward momentum
convection, in
contrast to the measurements. The weak pinch was predicted
to
be due to both electromagnetic
effects
at high beta and low aspect
ratio
minimizing the symmetry-breaking of the instabilities
responsible for
momentum transport. In an attempt to lessen these
electromagnetic
effects at low aspect ratio,
perturbative
experiments were run in MAST L-mode
discharges at lower beta (βN=2).
The perturbative
transport analysis used the
time-dependent
response following the termination of applied 3D fields that briefly
brake the
plasma rotation (similar to the NSTX H-mode experiments).
Assuming
time-invariant diffusive (
χϕ
)
and
convective (Vϕ) transport
coefficients, an inward pinch is inferred with
magnitudes,
(RVϕ/χϕ)
=
(-1)-(-9), similar to those found in NSTX H- modes
and
in conventional tokamaks. However, if
experimental uncertainties due to
non-stationary
conditions during and after the applied 3D field are considered, a
weak pinch
or even outward convection is inferred, (RVϕ/χϕ)
=
(-1)-(+5). Linear gyrokinetic simulations
indicate that for these lower
beta L-modes,
the predicted momentum pinch is predicted to be relatively small, (RVϕ/χϕ)sim
≈
-1.
While this falls within the experimentally
inferred
range, the uncertainties are
practically
too large to quantitatively validate
the
predictions. Challenges and implications
for
this particular experimental technique are
discussed, as well as additional
possible
physical mechanisms that may be
important
in understanding momentum transport in these low aspect ratio
plasmas.
Submitted to: Nuclear Fusion
_________________________________________________________________________________________________