From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights


Alcator C-Mod Weekly Highlights
September 22, 2003


Reassembly of the MIT alternator began last week. Work continued on
Lower Hybrid and ICRF systems. All vacuum work was completed, and
preparations are being made for plasma operations.

Physics
-------

Catherine Fiore is working to determine the position of the ITB foot
by two methods: by fitting a function to the bremsstrahlung data
profile (the sum of a Gaussian and a quadratic form) and taking
derivatives to obtain the inflection point and second by calculating
rho*p (defined as the ration of the Larmor radius at the ion sound
speed to the pressure gradient scale length) and determining the point
where this exceeds 0.014, a value established for JET ITB plasmas and
verified on other tokamaks. Both methods determine the foot position
to be at the same location. This location, when plotted as a function
of magnetic parameters of the plasma, shows that the ITB foot location
moves inward with increasing magnetic field and increasing q95. It
possibly decreases with increasing plasma current. The ITB foot
location occurs at a q value (as determined by EFIT) between 1.1 and 1.3.

Run Planning
-------------

The first follow-up meeting to the C-Mod 2004 Ideas Forum was held on Friday
to prioritize Burning Plasma Support experiments to be carried out during the
Fall-Winter 2003 campaign. In addition to on-site personnel, collaborators
from PPPL and U. Texas participated in the discussion by video and audio
links. The group reached agreement on the six highest priority experiments
(out of 20 considered), and designated individuals who will be principally
reponsible for writing the detailed MiniProposals for these. The other task
forces and topical science groups will be holding similar prioritization
meetings during the next two weeks.

Operations
----------

On Monday and Tuesday tooling was developed and then used with a
borescope and video cameras to remove a thermocouple cable that had
come free from the outer vessel wall during the last run campaign.
Following repair of two gate valves and installation of new diagnostic
flanges and gate valves for the boron injector and a new particle
detector, the vessel was pumped down on Wednesday. The vessel
heater and cryogenic systems were brought back on-line on Thursday
and Friday. A vessel bake is planned for next week. The Hybrid
Computer has been tested and initial tests of the data highways
have begun. A new set of 14 linux workstations are being installed
in the C-Mod control room and old VMS systems are being retired.


Alternator
----------

GE returned last Monday and began reassembly of the alternator while
working double shift days. By Friday all major work was complete with
just a few minor tasks remaining to be completed early this week.


Lower Hybrid Systems
--------------------

Considerable progress was made on the lower hybrid launcher this week.
Emphasis was placed on coupler repair, i.e. removal, cleanup, and
re-brazing of windows, and elimination of ceramic fractures. The
couplers are undergoing a braze etching process to remove the alumina
windows. Currently, all visible alloy has been removed, but the
bricks are not yet free from the titanium coupler walls and etching is
continuing. This process typically takes over 48 hours. Both MIT and
PPPL engineers have agreed to modifications to the coupler design that
will reduce the titanium wall thickness on the outside of the
outer-most coupler windows. This modification will reduce mechanical
stresses on the ceramic windows induced by the brazing process. In
order to reuse the present titanium coupler grill, another
modification will move the ceramic bricks away from the sidewall
scalloping that resulted from cleanup of the original braze joints.

The first six of a completely new set of alumina windows have arrived
at PPPL from the vendor for dielectric constant measurement at 4.6
GHz. These measurements will determine the specification for grinding
the windows to the proper thickness.

The vendor for window re brazing has performed 18 braze coupon tests to
investigate the choice of brazing alloy, brazing temperature, and the
capability to wet or restrict wetting of the ceramic.

Measurements of the coefficient of thermal expansion of the ceramic at PPPL
agrees within 5% of the measurement performed for MIT by an outside firm,
and is close to the published value. Measurements of the titanium CTE
disagrees with MIT's outside measurement by about 15%, with both
measurements being at least 30% higher than the published value. These
discrepancies are continuing to be investigated. Model calculations performed
by Bill Beck of MIT using the measured CTE values will guide changes to the
coupler design.

Nevell Greenough, PPPL, spent the week at MIT, performing high-power
RF tests of one of the two forward wave guide stacks with Monty
Grimes and Dexter Beals of MIT. The nominal power level through one
channel is calculated to be ~15 kW when the lower hybrid waves are
launched into the C-Mod plasma. Tests were performed on 25 of the 48
channels, with the achievement of 25 kW for 2.5 sec, 70 kW for 0.5
sec, and 100 kW for 10 msec. The latter values tested voltage holding
with a strong mismatch. The wave guide test was highly successful,
and the unit will be returned to PPPL on 9/23 for reassembly
completion.


ICRF Systems
------------

The J-Port transmission line has been modified for 50 MHz operation. The
system has good decoupling and will allow co- and anti-current drive and
symmetric phasing. In addition, the dummy load transmission line has been
modified to allow for 50 MHz operation.

Travel and Visits
-----------------

George Tynan, UCSD, spent a day at the PSFC to discuss data analysis
techniques for C-Mod fluctuation data. The goal is to study
non-linear wave coupling dynamics in the plasma edge and its relation
to the L/H transition. He also delivered a seminar titled "Inverse
Energy Transfer and Shear Flow Formation from Collisional Drift
Turbulence"

Nevell Greenough, PPPL, visited all week working on the high power
tests of the forward wave guide stack.

Prof. Riccardo Betti of the University of Rochester visited the Plasma Science
and Fusion Center on Friday 19 September to give a seminar on the
"Hydrodynamics of ICF Implosions". He also visited with a number of C-Mod
staff to discuss theories and their comparisons with experimental results on
plasma rotation.



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