Newsgroups: sci.physics.plasma
From IRBY@CMOD2.PFC.MIT.EDU Tue Jun 4 17:33:00 1996
From: IRBY@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
June 4, 1996

The up-to-air continues. Progress has been made on the outer
divertor, diagnostics, divertor cryopump, and other engineering
systems.
The GH outer divertor module has been re-installed invessel over
the cryopump as part of the cryopump fitup. No problems were encountered
during the fitup, and the pump can now be returned to the vacuum test
stand for a final set of high pressure pumping tests and continued
PLC program development.
After a several month delay, the vendor has delivered the
laser for the new tangential interferometer. This laser is a frequency
doubled, CW device, producing beams at 0.532 and 1.064u. The laser is
currently being installed on the optical table and should be under test
within the next few days. The retro-reflector, mounted in a ceramic,
protective tube, is also now in-house.
All welding on the divertor components is complete. The final
machining operation, drilling holes in the inserts for the mother bolts,
should begin soon. The re-installation of completed modules should begin
this week.
All fiberoptic cables needed for new diagnostics such as the
edge Thomson scattering system have been installed. Termination of the
cables is proceeding.
Interface wiring for the upgraded TF cooling system is complete,
as is the new PLC programming. Changes to the Paragon programming are
underway. All LN2 plumbing is complete and the bus tunnel manifold
box can now be closed.
The TF and PF 13.8kV breakers are being upgraded and
serviced. The yearly servicing includes hi-potting, contact resistance
checks, calibration and mechanical adjustment.
All magnet supply DC bus components are back in place and ready
for operation. The aluminum to copper connections were all inspected,
cleaned, and replated before reassembly.
The insulators for the new glow discharge system have been
machined and are being sent out for coating. Prototype electrodes
have been made and a test fitup invessel has been scheduled. The
high voltage vacuum feedthroughs are now being welded to the flange.
Further analysis of our recent H-modes with high H alpha emission has
shown that these H-modes are in a high recycling regime. A sharp peak is seen
in the H alpha emission very localized to the bridge of the inner nose of the
divertor. These high recycling H-modes (HRH-modes) are characterized
by high plasma pressure in the core ( > 4 atm), moderate divertor
plate temperatures (10 - 20 eV), high divertor densities ( > 10^21 m^-3),
low main chamber radiated power, low molybdenum emission, and high confinement
(H factors up to 2). Because the HRH-modes reach a steady state in density
and confinement, they can be maintained for the length of the ICRF pulse
without degradation. The longest HRH-modes exceed the length of the ICRF
pulse and last more than 0.7 sec. Some HRH-modes exhibit very small chaotic
ELMs that do not fit the standard ELM classifications, though many show no
signs of any ELM activity. This HRH-mode regime may be a good compromise for
ITER operation in that it combines good steady state confinement with
reasonable divertor heat loads, no large ELMs, and no impurity accumulation.
A systematic study of the effect of substantial edge current density
on EFIT flux reconstructions of C-MOD H-mode plasmas has been carried out. The
minimum in chi-square (magnetic) is obtained with an edge J close to that
inferred by our standard between shot analysis. A slightly higher edge current,
which also gives an acceptable chi-square, may give more consistent flux
mapping for divertor probe data near the strikepoints. Variation of the
(imposed) edge current density in the reconstructions can lead to significant
variation in the inferred location of the x-point and LCFS.
The C-MOD and DIII-D groups have been collaborating on "identity"
experiments in which discharges in the two machines are prepared with all
dimensionless quantities made nearly equal. In the initial experiments, which
were designed to compare L-mode plasmas, each machine encountered H-mode
transitions; since the methodology involved fine-scale power scans, accurate
values for the power threshold were obtained. These discharges have been
analyzed to test the hypothesis that the H-mode power threshold depends only
on non-dimensional plasma physics parameters, in which case the threshold
powers should be observed to scale as R^(-3/4). However, the power
threshold in the C-MOD discharges was was found to be about a factor of
two lower than would be predicted by scaling from the DIII-D threshold
observed in these experiments (2.7MW observed vs 5.45MW predicted). All the
other global non-dimensional plasma parameters were well-matched, so
this result implies that the H-mode threshold contains non-plasma
physics dependences, such as a dependence on atomic physics
(neutrals, radiation, ...) for example. Note that if the dimensionless
scaling arguments were comprehensively true, the L-mode profiles would have
to be identical across the whole plasma. The apparent lack of scaling we see
could be direct (by influencing the L-H transition physics), or indirect
(by changing the L-mode profiles near the edge). We should eventually be able
to provide some insight on this issue as well.