From:
IRBY@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod
Weekly Highlights
Newsgroups: sci.physics.plasma
Organization:
MIT
Alcator C-Mod Weekly Highlights
July 24, 2000
The last
full week of invessel activity has been completed. Alcator
C-Mod will be pumped back down early this week. Modifications to all the
antennas will
be completed by 7/25 as will numerous cleanup and repair
activities
associated with invessel diagnostics.
Maintenance and calibration
activities continued on power and cryo
systems.
Invessel Work
-------------
Work
continued this week on rf shielding of the perimeter of the PPPL
ICRF
antenna. The last measurements
performed at the end of June
supported
the idea that the low heating efficiency was due to rf power
leaking around
the back of the antenna
and coupling to plasma surface waves.
The top and
bottom of the
antenna have now been enclosed with stainless steel shim stock
and
spot welded to the antenna frame and vacuum vessel wall. The left and
right sides of the
antenna frame could not be directly grounded to the
vessel wall because
eddy currents generated during disruptions would have been
too large. Instead, rf bypass capacitors have been
installed between the
antenna and the wall. These capacitors are essentially shorts at rf
frequencies,
but appear as a relatively high impedance to disruption currents.
All
molybdenum protective tiles on all antennas have been replaced with
boron
nitride tiles. In addition, a new set
of boron nitride tiles has
been installed between the D and E antennas to
better protect their
Faraday screens.
The viewing telescope,
optics, and coherent fiber bundle for the new
PPPL 2-D turbulence imaging
diagnostic have been installed inside the
vacuum vessel. A test target at the position of the gas
puff was
clearly imaged onto an external video camera.
Invessel
calibration and alignment of the F-top and F-side optical systems
used
for viewing "plumes" generated by gas injection from the F-bottom
scanning probe took place during the recent C-Mod opening. Both a
relative
pixel calibration and an absolute brightness calibration were
obtained, the
latter using the Labsphere. A back-illuminated aluminum
plate with holes
drilled on a precise grid pattern was used to determine
pin-cushioning and
other distortion effects. Absolute alignment was
performed by viewing a small
light bulb placed at various known locations
in the vessel, e.g. in grooves
between divertor tiles and at the tip of
the scanning probe when inserted to a
known vertical position. The system
is now ready for operation during the
upcoming run campaign.
Other
Engineering Activity
--------------------------
Work continued
last week on an upgrade to our glow discharge system.
Besides a general
cleanup of the layout and improvements to the PLC control
electronics, we
are also adding fast solid state controllers to the current
going to the
glow paddles. These controllers will be
able to rollback
the current to the electrodes in a few microseconds and
limit the energy
dumped into arcs to a few mJ. This energy will be insufficient to sputter
material from
the vessel walls.
Our 13.8 kV current and pulse transformers have
been re-calibrated. Results
indicate
both current and voltage measurements were accurate to within
+-2%. Fuses and breakers associated with C-Mod
power systems
have been inspected.
A fuse for the A-phase of the FMIT#1 transmitter
had clearly been
arcing at some point since the last inspection. This
condition may explain some of the transient fault
conditions experienced
over the last few weeks of operation.
Preventative
maintenance to the CRYO system proportional valves continued
last
week. Work on the new cryostat exhaust
valve also made progress.
All CYRO system filters have been cleaned or
replaced. The LN2 pump has
been
removed from the sump and is undergoing an inspection.
Quarterly
Review
----------------
A C-Mod Quarterly Review was held on
July 20th. Rostom Dagazian and
Warren
Marton from DoE were at MIT for the review.
Presentations by MIT,
University of Texas at Austin, and PPPL
personnel were given. The agenda
and
the presentations can be viewed in PDF format at
http://www.psfc.mit.edu/cmod/sciprogram/q_rev_jul00/
Travel
and Visitors
-------------------
Stewart Zweben from PPPL was
at C-Mod for the week, working with Jim
Terry on the turbulence imaging
installation and presenting a talk at
the Quarterly Review.
Miklos
Porkolab attended the Fusion Power Associates Meeting on July 17
in San
Diego and made the following presentation: Alcator C-Mod Program and
Recent
Results. The following 2 days he attended the FESAC meeting at General
Atomics
where the recent draft of the Integrated Program Planning Activity
document
was discussed. Earl Marmar also
attended the FESAC meeting,
as well as the first meeting of the Next Step
Options Program Advisory
Committee.