Alcator C-Mod Weekly Highlights
		         March 23, 1998
 


	The removal of the cylinder last Tuesday allowed us to see
the cause of the TF short for the first time.  The arc appears to
have started on the TF vertical leg at or near the first-to-last turn
location and exhausted out radially between the leg and the cylinder.
The damage is localized to the leg and upper arm, which can readily
be replaced because of the jointed design.  Fabrication of
a new TF leg is already well underway.  New copper plates are being
machined and should be in-house next week, tooling for assembly is being
set up, and processing for the new feltmetal is moving along rapidly.
Complete documentation of the fault site has been carried out to
assist diagnosis and correction. Additional inspection and measurements
of the rest of the TF have also been undertaken.  We have also begun the
disassembly of the magnet so that all arms and legs can be inspected,
cleaned, and the feltmetal repaired where required.
	Work continued on the DNB last week.  A test fixture was completed
for checking fiber optic links.  The HV cable tray has been run in the
diagnostic labs.  PLC analog output wiring to the waveform generators has
been completed.  The slow thermocouple chassis has been designed, documented,
and built.  We have begun testing and calibration of the fiber optic-based
v/f and f/v links, and fault circuitry for the arc/fil/snubber voltage
feedback, monitoring and fault systems.  Fiber connections have been made,
the cabling dressed, and we have started to test and calibrate the
required 36 channels.
	New bolometer measurements have been made in the vicinity of the
X-point in high-powered H-mode discharges with nitrogen impurity seeding.
We are able to experimentally discriminate between power loss from
electromagnetic radiation and that due to neutral particles using a
novel `gas filter'.  The gas filter blankets the bolometer detector with low
pressure gas, hence filtering away neutral particles through neutral-neutral
scattering collisions and passing only the electromagnetic radiation.
Analysis of the bolometer data indicates that the majority of
the power loss near the X-point is due to electromagnetic radiation,
with only a small fraction (< 10%) due to neutral particles.  It is too early
to say whether this is a general feature of discharges in C-Mod, e.g. there
is evidence from other diagnostics that high density detached discharges
have a significant fraction of power loss through the neutral particle
channel.  More extensive measurements are required in the future.
	We have been analyzing data regarding the geometry and characteristics
of a midplane MARFE in C-Mod.   Using a CCD camera, filtered for D_alpha, we
have found that the MARFE emission is primarily from INSIDE the separatrix.
The existence of the MARFE on closed surfaces has been shown previously as
well, but through other means.  We are also able to determine the density
(2-3 x 10^21 m^-3) and Te (~0.7eV) in the MARFE and the local recombination
rate.  All of these observations point to the current models of the MARFE
as not being complete.
	Dr. Asakura (JT-60U) communicated to the Alcator C-Mod edge 
group some recent results he has obtained from a midplane scanning
Mach-probe diagnostic on JT-60U which are very similar to results 
previously reported by B. Labombard on C-Mod: (1) Reverse parallel plasma flows 
are measured in the outer divertor leg for the normal magnetic field direction 
(BxGradB towards lower x-pt). (2) The magnitude of these flows decrease
as the plasma density is raised and divertor detachment sets in.
(3) With the magnetic field reversed, the parallel flows also reverse,
flowing towards the divertor along field lines. (4) A strong correlation 
is observed between the parallel flows and the inside/outside divertor 
asymmetries.  Dr. Asakura also obtained some important new results in 
neutral beam heated plasmas, showing that the direction of toroidal momentum
input to the core plasma does not affect the direction of the edge
plasma flows. Dr. Asakura is planning on visiting MIT for two weeks in
May to collaborate with the edge group on these and other edge plasma 
topics.
        On 3/16-17 we received a visit from Howard Scott and Alan Wan from
LLNL.  This is part of a collaboration jointly with the PSFC edge theory
group on the use of the CRETIN code for modelling of radiation transport in
dense plasmas.  We are interested in using this code for modelling of the
transport of deuterium radiation (particularly the Lyman series) and the
effects of radiation transfer on the ionization/recombination balance in
our edge plasmas.  We now have the code and some instruction in its use.
A theory post-doc and graduate student will be working with Jim Terry and
Bruce Lipschultz on this code.
	Martin Greenwald, Amanda Hubbard, Rejean Boivin and Bob
Granetz attended the Transport Task Force workshop in  Atlanta last week.
Two oral presentations were made, and one poster was presented.
Bob Granetz spoke on  "Characterization of Edge Pedestals in Alcator
C-Mod with X-ray Imaging" and Amanda Hubbard discussed "Comparisons of
Edge Parameters on C-mod with Numerical Simulations of Rogers and Drake."
Rejean Boivin presented a poster on "Investigation of the role of neutrals
in Alcator C-Mod Plasmas."  Martin Greenwald was elected chairman of the
TTF during this meeting.
	Milos Porkolab attended the Special ISCUS-SWG1 meeting in San Diego
on the Advanced Tokamak version of ITER.  He gave the presentation and lead
discussion on  "Importance of Shaping in AT Regimes".  He also contributed
to the discussions on the ISST Tokamak, a new initiative from MIT on an
"Inertially Shielded Superconducting Tokamak".