From:
WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: ALcator
C-Mod Weekly Highlights
Approved: plasma@cs.uml.edu
Newsgroups:
sci.physics.plasma
Organization: MIT
Alcator
C-Mod Weekly Highlights
Oct
4, 1999
Last week began a maintenance period at Alcator C-Mod. No
plasma runs were
scheduled. This week will also be a maintenance week.
A detailed survey of TF magnet resistance measurements, using a
quiet 100A DC
supply, was carried out as part of the investigation of the
increased values
noted during operation the previous week. These
measurements showed a pattern
consistent with the post-shot scanner
readings, although the maximum observed
value of 2uOhm was somewhat lower.
A series of 50kA and 100kA TF pulses were
run, with instrumentation on 8
turns of the magnet in the vicinity of H-port,
where the elevated readings
occurred. The resistances measured during these
high-current pulses are
all similar, and within a range of 1.2 to
1.8uOhm. These measurements
remained essentially constant over the 3 second
long flat-top of the pulse
and showed no abnormal behavior during ramp-up and
ramp-down.
A
visual inspection revealed the presence of frost on several metal
fittings
inside the C-Mod cryostat. The inspection was limited to the area
visible
through the double plexiglass window at the top of the cryostat;
the magnet
itself is not viewable through this port. It is believed that a
leaky gasket
is responsible for allowing some moisture into the dewar. We
are presently
warming up the machine, and will inspect and repair any
leaky seals during the
present maintenance interval. Warming the entire
machine to room temperature
is expected to take of order one week.
Physics
& Analysis
-------------------
The x-ray emissivity shows a
pronounced pedestal in H-mode, which, at the
outboard midplane, is
typically located 5 mm inside the pedestals of electron
density and
temperature. This shift can be explained by a strong inward
convection of
impurities, localized to the edge of the plasma. Neoclassical
theory
predicts a strong inward pinch of impurities where the plasma density
gradient
is large, i.e. in the electron density pedestal
region. One-dimensional
simulations using the impurity transport code MIST
show that the strong
pinch must extend to roughly the top of the x-ray
emissivity profile but
not further inward than that, and that the width of the
x-ray emissivity
pedestal is mostly a function of D, the impurity diffusion
coefficient.
Using measurements of the top of the xray pedestal, it is found
that the
inward pinch region does coincide with the region of strong electron
density
gradients. D values inferred from the x-ray pedestal width are in good
agreement
with D values inferred from previous impurity injection experiments.
During several recent runs, CaF2 was
injected by laser blowoff. The edge
xray
arrays are sensitive to radiation from fluorine, and can therefore
detect the
CaF2 injection and observe its transport on a fast timescale
(12 us sampling).
During the past year, obvious asymmetries between the
top and outboard xray
pedestals have been consistently observed in
steady-state, and this is thought
to be due to the same neoclassical
return flow that was proposed to explain
the strong top/bottom asymmetry
of Ar17+ reported previously (Rice, et al.)
The recent CaF2 injection
experiments show a transient (200 us) burst of xrays
in the pedestal
region at the top of the plasma, but no simultaneous burst
from the
outboard pedestal. The injected
impurities pass through the views of
both xray arrays, but apparently the
flow quickly sweeps the impurities up to
the top of the plasma before they
begin to emit xrays. Given the
relevant
ionization rates, we find that the neoclassical flow must
therefore be of the
order of 10 km/s.
Central impurity toroidal
rotation was measured during a current scan of
plasmas with constant
target density and ICRF power at 2 MW. The range of
currents achieved was
from 0.4 to 1.4 MA. The ratio of the observed rotation
to the plasma
stored energy increase during the ICRF pulse was determined to
be a
decreasing function of plasma current. This is qualitatively similar to
the
predictions of neo-classical theory which has the toroidal rotation
velocity
inversely proportional to the poloidal magnetic field.
ICRF
System
------------
Based upon a cavity model, we have
proceeded to make a modification to FMIT#1.
This modification requires
replacing the bottom third of the cavity with a
3/8" greater radius
than the original. This change is
expected to result in
an ~30% decrease in plate impedance, allowing higher
power for a given
voltage. If this
modification works as expected, we will make the same
modification to
FMIT#2. Testing will begin this
week.
The modification to J-port resonant loops is nearing
completion. Modeling
indicates
that the addition of 1/2 wavelength to the loops, as well as
modifications
to the decoupling loop, will permit operation with (0,pi,0,pi)
phasing,
which should correspond to an improvement in the k-parallel spectrum
for
heating. The physical layout is also
ready to proceed with the
modification.
We expect to have the new J-port configuration assembled and
tested
before next plasma operation.
The low power RF chain in FMIT#3 and
#4 is also being upgraded. New
power
supplies have been obtained and are being installed to eliminate a
temporary
power supply. This
should increase the system's reliability and simplify the
overall
circuit.
Travel and Visitors
-------------------
Amanda
Hubbard, Martin Greenwald, and Joe Snipes took part in the IAEA
Technical
Committee Meeting on H-mode and Internal Transport Barrier Physics
in
Oxford, held 27-29 September. They also participated in the Confinement,
Threshold,
and Edge Pedestal database meetings at JET on 30 Sept- 1 Oct.