Subject: ALcator C-Mod Weekly Highlights
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
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.
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
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.