From:
irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Approved:
plasma@cs.uml.edu
Newsgroups: sci.physics.plasma
Organization:
MIT
Subject: Alcator C-Mod Weekly
Highlights
Alcator C-Mod
Weekly Highlights
September
28, 2001
The inspection of Alcator C-Mod continues. Progress was also made on
LH, ICRF, and
divertor systems.
Physics
-------
Data from
power ramp experiments, including some new results from August
2001, have
been analyzed in collaboration with Ben Carreras of ORNL. In
these experiments, edge parameters
are measured with high time and spatial
resolution throughout a controlled
L-H-L hysteresis cycle. An effort
has
been made to cast measurements in terms of flux vs gradients, in the
forms
suggested by models of the H-mode as a critical transition. The variation
of thermal conductivity
chi, in L and H mode, as a function of local
gradients can then be
directly assessed. Strongly non-monotonic
flux-gradient
relationships are indeed found. Results
are consistent with
a dependence of conductivity on temperature gradient,
having a very sharp
decrease above a critical value, which is independent
of density. Other
flux-gradient
relations are also possible. These
results were summarized
at a recent C-Mod group meeting and presented at
the recent IAEA TCM in
Japan. It
is hoped that they will stimulate further discussion with other
theorists.
Analysis
of the edge turbulence data continues (MIT/PPPL collaboration).
Using the
Princeton Scientific Instruments "ultra-fast" camera, that takes
12
sequential 4 usec images in D_alpha light from the separatrix region,
we are
able to follow the evolution of the "blobs" of emission
that have been seen
previously in the C-Mod edge plasma. We believe that they reflect the
"blobby"
nature of the density there. The
characteristic size of the
brighter "blobs" is slightly less
than 1 cm in both the radial and poloidal
dimension (they have much longer
characteristic wavelengths along the field).
The characteristic lifetime
of the brighter "blobs" is 10-50 microseconds.
The
time-sequenced images show clearly that these "blobs" move both
radially
and poloidally. Outside
the separatrix the dominant radial propagation
direction is outward. Characteristic outward speeds for the some
of the
brighter "blobs" are as much as 400 m/s, which means that
they move a distance
similar to their size during a lifetime. In contrast, some "blobs" move
very
little during their lifetimes.
The poloidal velocities for the features that
move poloidally are
also as much as a few hundred m/s.
Eddy-like motion of
some features has also been observed. Examples of data from this
diagnostic
may be viewed at:
http://www.psfc.mit.edu/people/terry/edge-turbulence-movies.html
A
sawtooth heat pulse propagation study has shown that plasma discharges
(off-axis
ICRF heated) with an internal transport barrier (ITB) may have a
notch-shaped
transport coefficient profile (Chi_e). The notched region
corresponds to a
discontinuity in the time-to-peak profile as shown in soft
X-ray data. A
code has been developed to calculate the Chi_e profile by
solving a
simplified 1-D transport equation. The ITB region also seems to
move
inward during the first 10-20 msec of the discharge, which may give a
hint
of the trigger mechanism. Further study
is underway to check the
ballistic effects in the heat-pulse study.
Inspection
----------
All
upper horizontal TF arms and vertical legs have been removed from the
machine. The upper wedge and mounting plates have
also been removed, giving
full access to the upper OH coaxes. A detailed
inspection of the TF joints,
spring plates, and feltmetal is in
progress. No significant problems
have
been found. Inspection of the
OH coaxes will begin next week.
Lower
Hybrid MIE Project
------------------------
The HVPS has
arrived in port (New Jersey). Shipment
to Cambridge should take
place next week.
We continue to prepare the control system, dummy load,
cabling,
data links, interlock circuits, and other equipment needed to test
the
supply.
Work continues on the TPS (Transmitter Protection System)
for the LH
klystrons. All HV
faults are now indicating properly. HV
enable outputs have
also been verified for proper operation. The enable outputs have been shown
to
properly drive the fiber-optic transmitters.
The control PLC ladder
logic program has been modified so that it will
communicate properly with
the HVPS PLC when it arrives.
We continue working with the
contractors on the layout and specifications
for the cooling water
manifolds. The contractor is making progress
on the
cooling water piping.
Wiring of the EMI/RFI control and
protection chassis is proceeding as is
the wiring for the transmitter
control and supply cabinets.
ICRF Systems
------------
We
continue to test mockups of the new phase demodulators. A system
using a downconvertor with an
IF at 10 MHz has been shown to provide
very good linearity. Modeling of the antennas and transmission
system
is proceeding.
Inner Divertor
--------------
Installation
of the divertor components invessel revealed a .020"
interference
between the c-plates. We are currently
making modifications
to the plates, and expect to be ready for another
trial fitup next week.
Stud welding tests, and preps to tile support
plates are moving forward.
Leadership Award
----------------
Ron
Parker has been awarded the Fusion Power Associates 2001 Leadership
Award. The citation reads:
Ron Parker,
former director of the MIT Plasma Fusion Center and former
leader of the
ITER co-center in Garching, Germany, is recognized for his
many scientific
contributions to fusion research, his dedication to the
production of high
performance fusion conditions in the laboratory and his
leadership and
vision to the cause of developing practical fusion power."
His awards
states, "You have provided inspiration and technical guidance to
the
tokamak fusion program and leadership to those seeking to expedite the
development
of a practical fusion power system."
Trips and
Visits
----------------
Miklos Porkolab, Ron Parker, and Earl
Marmar attended the Fusion Power
Associates annual meeting in Washington,
Sept. 25,26. Ron presented a talk
entitled
"Comparative Overview of Burning Plasma Experiments", and Earl
presented
a talk on the "Alcator C-Mod National Facility Advanced Tokamak
Program."
Stew
Zweben was at MIT 9/24-26 for discussions on the analysis of the
Gas Puff
Imaging diagnostic results. Gerrit
Kramer, Raffi Nazikian
and Jay Kung came up on 9/28 to present their
latest thoughts on upgrades
to the C-Mod reflectometer diagnostic which
would extend the
measurement of fluctuations to the plasma core; several
issues remain
to be resolved. Gerd
Schilling was at MIT 9/25-28 for discussions on
programmatic issues, ICRF
antenna modeling and the remaining 4-strap
antenna modifications, and to
participate in the reflectometer
upgrade discussions.
Bob
Harvey from CompX visited the PSFC (Sept. 24-28). Bob worked with Paul
Bonoli
on theoretical and computational issues related to coupling the
TORIC ICRF
field solver to the 3-D Fokker Planck code CQL3-D. They also
carried out
simulations of lower hybrid current drive for C-Mod using plasmas
typical
of the density barrier mode produced with off-axis ICRF. The
calculations
were done with CQL3-D and the GENRAY ray tracing code using an
EQDSK MHD
equilibrium reconstruction for an actual C-Mod ITB discharge.