From: IRBY@CMOD.PFC.MIT.EDU
Date: Tue, 30 Jun 1998 9:05:32 -0400
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980630090532.25a0efae@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
June 29, 1998


Engineering:

Very good progress is being made on the TF refurbishment. The
upper TF core cleanup has been completed and the core is now ready for
plating and electroforming. All TF arms, both upper and lower, have
completed the cleanup process and are being inspected in preparation for
plating. The plating activity will be started early this week. Silver
plating samples have successfully completed cooldown cycle tests to LN2
temperatures, and vacuum bakes and UT tests to reveal any voids between
the copper and plating surface.
The inductive heating process has produced very good temperature
distributions and full scale soldering tests have begun. Careful
adjustments of the inductive heating power and cycle time, and modifications
to the heat sink were required to achieve these results.
We have assumed in the past that measurement of the gap between
TF fingers was an adequate measure of feltmetal pad pressure. New
diagnostics are being developed to directly measure this pressure
during machine reassembly. In a related effort, we are looking at better
designs and materials to be used in making the spring plates that
apply the pressure to the pads.
We have continued to work on the control system for FMIT#3 and #4.
We are waiting for parts to finish the repair job on #4's input cavity.
One of the two EIMAC tubes which have been used in FMIT#4 has been sent
back to the company for repair. The tube holds DC voltage but breaks down
with application of RF power.
We continued to make progress toward the initial conditioning of the
DNB plasma source. The Arc/Filament/Snubber interface chassis which links the
supplies with the master control logic and the PLC was successfully tested.
Remote control and monitoring of the primary power for the
Arc/Filament/Snubber supplies was tested. After correction of a problem in
the monitoring instrumentation, and tests of the power and control phasing,
we will begin pulsing the arc and filament supplies into the dummy loads.
The dummy loads for the Arc/Filament/Snubber power supplies were installed
last week.


Physics:

We have experimental evidence of the formation of an internal
transport barrier during off-axis D(H) minority heating. The primary
evidence is a ~2.5-fold increase in neutron rate over the H-mode only phase
with an associated 25% increase in central density. Analysis of the x-ray
brightness profile also indicates a central peaking (amplitude
increase by 50% and the profile width decreases by 50%). In addition,
the radiation profile suggests both a core and edge barrier. As the
barrier forms, the sawtooth amplitude decreases and the sawteeth are
eventually completely stabilized. This behavior is reminiscent of
PEP-mode, but without pellet injection. Preliminary analysis suggests the core
barrier is formed following strong Elmy phases (the neutron rate increases
following two Elm bursts and a dithering L-mode phase) and the radiated
power profile also suggests a growing core barrier. The mode is terminated
by MHD activity.
RF sheaths have been detected on magnetic field lines connecting the
D-port RF antenna and the AB limiter. The RF voltage driving the sheaths is
deduced from the decrement during RF to the floating potential measured by
a Langmuir probe on an antenna side protection tile, and found to be of the
order of 100V. Using the ion saturation current density profile measured
by the A-port scanning probe, it is found that on order of 1% of the net RF
power is lost to acceleration of ions into the antenna and limiter by the RF
sheath voltage.
The 1998 Alcator C-Mod Ideas Forum is being planned for August 19-20,
1998. The forum, to be held at MIT, will consist of short (five minute)
presentations proposing experiments to be carried out on Alcator C-Mod during
the next run campaign. Details concerning the forum will be posted on the
C-Mod Web page (www.pfc.mit.edu/cmod/) shortly, or can be obtained by
contacting Steve Wolfe at wolfe@psfc.mit.edu.


Travel and Visits:

Dr. Gary Hallock from U. of Texas visited last week to
discuss PCI (phase contrast interferometry) diagnostic upgrades and the
fluctuation data collected thus far by Alex Mazurenko. We agreed that
there are two possible modes of operation for the system: heterodyne
RF mode and plasma fluctuations mode. The former is the primary
objective for the upcoming campaign because of its potential in
providing long sought after core RF wave information. The fluctuation data
can be taken when the diagnostic is not in use specifically for the
RF. Until we have a fuller complement of diagnostics to help
determine where the fluctuations are located, interpretation of the
results will be difficult. We will be adapting amplifiers built for
the TEXT-U PCI system for the planned upgrade to 32 channels at C-Mod.
We expect to have 12 amplifiers and spares ready by the fall for the
upcoming campaign. Dr. Hallock also provided some important insights
concerning the existing data and established an initial direction for us
to proceed. This appears to be a very interesting research development.
Gary Taylor of PPPL visited to work on the 19 channel GPC2 instrument.
Modifications to CAMAC modules were made and tested. Progress was also made
on analysis of data from the last campaign. Calibrated temperatures and
radii are now available in the MDSPlus tree for selected shots.