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 31, 2000
Invessel work on Alcator C-Mod was
completed the middle of last week.
After
final cleanup, documentation, and diagnostic alignments the
vacuum vessel was
closed up and pumped down on Friday. Also on Friday, the vessel heaters
were
turned back on and a GN2 purge was begun of the cryostat and bus
tunnel.
A leak on a newly installed flange was found and fixed on
Saturday, and more
sensitive leakchecks were performed today. No
additional leaks have been
found.
We plan to begin LN2 cooling of the machine structure and a 120C
bake
of the vessel later this week.
Invessel Work
-------------
The
J-Port antenna is now rf shielded top and bottom, and rf currents are
bypassed
through ceramic capacitor arrays on the left and right sides. These
changes should considerably
reduce or eliminate the parasitic power losses
to plasma surface waves,
which we think have been responsible for the
previously observed low
heating efficiency. Both the J-Port
antenna and the
two older antennas at D- and E-Port have also had their
molybdenum protective
tiles replaced by boron nitride to reduce metallic
impurity influx to
the plasma.
Alignment of the tangential
two-color interferometer and new charge
exchange analyzer, replacement of
mirrors boronized during the last run
period, extensive documentation
using digital and 35mm cameras, vacuuming
and wipedown, and final
inspections were also completed last week.
Pictures documenting the
invessel status at pumpdown can be found at
http://www.psfc.mit.edu/cmod/operations/EngImages/CmodImages.asp?ref=/INVESSEL/2000/Close_Up_Survey/
Other
Engineering Activity
--------------------------
Nine klystrons
needed for our lower hybrid experiments which had been on loan
to PPPL
were returned to MIT last week. All 16
tubes from the Alcator
C Lower Hybrid experiment are now back on site at
the PSFC. The first of the
nine
returned tubes is now being prepared for installation into our klystron
test
stand. The first of two delrin
insulators needed for repair of two other
klystrons is now ready for
installation.
The J-Port external resonant loop, coupling the
antenna to the transmitters,
has
been reassembled and checked for resonance in the two 80 MHz phasing
configurations:
(0,180,180,0) and (0,180,0,180). It has
also been
tested for operation at 70 MHz.
Operation of the D- and E-Port
antennas at
their nominal 80 MHz combined with J-Port at 70 MHz will allow
us to form
the highly peaked discharges discovered in June by ICRF heating
off-axis,
while simultaneously using the lower frequency power for on-axis
heating near
the density peak.
Chris Brunkhorst (PPPL) came to
MIT 7/24-27 and worked on optimizing the
tuning of the #3 and #4 ICRF transmitters that are coupled
to the J-Port
antenna for operation at 70 and 60 MHz. 70 MHz operation is needed
for the off+on-axis
heating scenario described above, while 60 MHz is
intended for current
drive experiments.
Work continued last week on the DNB and
associated diagnostics. The
alignment
of the beam was measured and adjusted.
The field generated by
the DNB dump magnets was measured. The transmission of the F-Top CXRS
window
before and after cleaning was measured.
The tangential CXRS optics
were backlighted to assure that no
mis-alignments had occurred during the
several months since
installation.
The upgrade to the glow discharge and boronization
system is moving very
quickly and should be ready for operation within two
to three weeks.
Upgrades to the power system instrumentation in
preparation for long
pulse operation are also progressing well.
Physics
-------
Very
low density H-modes produced in June as part of an exploration of the low
density
H-mode limit have recently been analysed.
Most of these discharges
were
ELM-free, with high pedestal temperatures and modest density rises. The
edge collisionality was
correspondingly lower than in most EDA H-modes. It is
possible that this prevented access to the EDA
regime. However, the
discharges
had q95 ~ 3.5-3.7, which is on the low
side for EDA access, so this
result is not conclusive. Several discharges showed isolated ELMS,
which
occurred at fairly high edge pressure and temperature, unlike our
usual
Type III ELMs. These may
have been Type I ELMs, which are seen regularly on
other machines but not
normally on C-Mod. Further experiments
to clarify the
ELM type and the effect of density on the EDA/ELM-free
boundary are
proposed for September.
Travel and
Visits
-----------------
Rejean Boivin travelled to
INRS-Energie et Materiaux, located in Varennes
Quebec, for the thesis
defense of Irina Condrea, who did research on rotation
measurements in L
and H modes. He also attended, along
with Martin
Greenwald, the APS program committee meeting in Washington for
the upcoming
APS-DPP meeting in Quebec.
Chris Brunkhorst
visited from PPPL to work on improved tuning procedures for
the #3 and #4
transmitters.
Tom Fredian visited PPPL to analyze the use of the
MDSplus data
system on the NSTX experiment and make suggestions for
improved performance
and functionality. The system is installed and
working well. A few fine
tuning suggestions were made.
Gabriele
Manduchi from Padova (one of the original authors of MDSplus)
visited Tom
Fredian, Josh Stillerman and Stuart Sherman to discuss new
MDSplus
features and future development plans.
David Winslow (UT-FRC) visited
the PSFC during the vent to remove the Texas
probe and get an absolute
position calibration. After modifications and
prior to beginning of the
next campaign, the probe will be reinstalled to
take data during the
remainder of the campaign.
Pablo Acedo (Carlos III University,
Madrid) has returned to MIT for
6 weeks to continue our collaboration on
interferometry.
John White, Juergen Alex, and Walter Kaufmann from
Thomcast Radio Systems,
Inc. visited last week for two days of discussion
on the high voltage
power supply/modulator Thomcast will supply for our
Lower Hybrid experiment.
Items covered included contractual clarification,
and technical issues such
as electrical interfaces, testing procedures,
protection circuits, and PLC
software.