From: IRBY@CMOD.PFC.MIT.EDU
Date: Tue, 28 Apr 1998 10:07:48 -0400
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980428100748.29600b4c@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT Plasma Fusion center


Alcator C-Mod Weekly Highlights
April 27, 1998



Work on the TF magnet last week included a promising first test
of an inductive heating technique that we hope will simplify the replacement
of feltmetal on the TF horizontal arms and vertical legs. Copper plates for
the new TF vertical leg have been machined, checked, and are in-house.
Procedures for cleaning and re-plating the central column fingers are being
developed.
Work is proceeding on the FMIT#4 tunable transmitter as we
continue to investigate the crowbars occurring above 500 kW at 78 MHz. We
have convinced ourselves that the problem is not in the crowbar trigger circuit
but is in fact a real arc. We plan to test the driver into the dummy load
directly. This test should indicate whether the arc is in the driver or
a following stage. We have also been in contact with PPPL to ask them
about their experience with the units near their upper frequency limit.
Control system tests of FMIT#3 have begun and testing of the control boards
have continued. Careful documentation of the MIT antenna top protection
tiles has begun. Both melting and tracking damage has been found. Some
impurity injection events measured by the bolometers were correlated to
events seen on the upper antenna protection tiles by the LANL fast camera
which had a view of the top half of D-port antenna.
Randy Wilson visited last Tuesday for the DoE Quarterly Review
after which plans for the PPPL antenna were discussed. The Faraday
screen problem (see previous updates for details) is being addressed
by changing the TZM threads with inconel. Before the fix is tested
mechanically, the screen will be assembled on the antenna and the
antenna electrical characteristics will be measured. The electrical
characteristics are needed to complete the resonant loop design.
Following the electrical tests, deflection (equivalent to torques
expected from disruption forces), shock, and thermal tests will be
done. In parallel, quotes will be sought on copper coated inconel rods
in case the TZM Faraday shield fails. We also looked into designing
antenna current monitors for the new antenna.
We continued to make progress on the DNB. Construction of the chassis
for the arc/filament/snubber interface to the MCL was started. Design of the
required logic boards for this interface is underway. The modified clock
board for the MCL was successfully tested. Hardware and software for the
shakedown of the new CAMAC timing system and its interface to the MCL were
completed and tested. Conditioning of the oil in the DNB Mod/Reg isolation
transformer was completed, and in final tests, the breakdown voltage was
observed to be 35kV. Designs for the Mod/Reg screen supply board and the
grid drive board were completed and are ready for prototyping. Work on
fault circuitry calibration for the arc/filament/snubber voltage feedback
and monitoring systems continued.
Steve Wolfe, Martin Greenwald, Joe Snipes, and Amanda Hubbard took
part in the Combined Workshop of the ITER Confinement and Transport Expert
Group and the Confinement Database and Modeling Expert Group, held at
PPPL April 20-25. Joe Snipes presented H-mode threshold power predictions
for ITER using results from all machines from the ITER database. Corrections
for the absorbed ICRF power in C-Mod have been included. Analysis of L-Mode
discharges by Steve Wukitch has shown that the absorbed power decreased as
density to the -0.6 power for densities above about 10^20 m^-3,
reaching absorptions of about 50% by 1.3 x 10^20 m^-3. Absorption at 8 T was
also shown to peak with around 5% He3 concentration at 70-80% absorption.
At higher He3 concentrations, the absorption drops gradually and at lower
concentrations, the absorption drops dramatically. Including these absorption
factors in the H-mode threshold database significantly reduced the density
dependence of the threshold in C-Mod from being nearly linear to being close
to a square root. Because C-Mod has the highest densities
and fields in the database, these corrections also reduced the density
dependence of the power threshold using all tokamaks in the database. The
latest scaling Snipes found was

Pth = 1.35 * nebar^0.64 * B^0.76 * R^1.13 * a^0.74 * kappa^0.22,

which predicts 82 MW for ITER at a target density of 0.5 x 10^20 m^-3.
The kappa dependence was found not to be statistically significant. If
kappa is removed, the exponents on the other parameters change only
slightly as does the multiplying coefficient.
Dr. David Newman from ORNL visited last Friday. He gave a general
seminar titled Self Organized Criticality as Paradigm for Turbulent
Transport. He also gave a more specialized seminar on transport
barrier formation.
Last week, on April 21, an Alcator C-Mod Quarterly review was
held via a teleconferencing link with DOE. Recent physics results,
collaborations, and status of the machine were discussed. Randy
Wilson represented PPPL, and Bill Rowan represented U. Texas at the
review.