From:
wolfe@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator
C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization:
MIT
Alcator
C-Mod Weekly Highlights
June
4, 2001
Plasma operations continued at Alcator C-Mod last week. Three
run days were
scheduled, but only two were completed, due an unscheduled
"clean" vent to
retrieve a detached viewing dump. A total of 14
plasma shots were obtained,
with a startup reliability of about 65%.
Progress
was also made on the Divertor upgrade and Lower Hybrid projects.
Plasma
operation is scheduled to continue this week. An extra run day has been
scheduled
to make up for the time lost to the vent.
Operations
----------
ECDC
and a low temperature (60C) bake were carried out over the long weekend
and
during a maintenance day on Tuesday.
Wednesday's run was devoted to
continuation of machine clean-up and
conditioning and RF conditioning of
the D&E port dipole antennas. The density
was increased to ~1.6e20/m^3
with flattop currents of 800kA, at our standard
operating field of 5.4T.
At least as long as divertor detachment did not
occur, the density was
maintained by feedback on the gas puff, rather than
being dominated by
wall fueling as had been the case in the previous week,
indicating the machine
was cleaning up. The H/D ratio was between 0.3 and 0.6,
a considerable
improvement over the previous week, when it was over
2. Between-shot
vacuum conditioning of the J-port antenna was carried out in
preparation
for operation into plasma. Development of a lower triangularity
shape
suitable for a planned experiment matching ASDEX-Upgrade non-dimensional
parameters
was begun.
Late in the run on Wednesday a viewing dump for the
visible bremmstrahlung
array became detached from the outer wall and came
to rest in the divertor
between B and C ports, where it was observed on
one of the divertor camera
views. The run was halted, and on Thursday
morning the machine was vented to
helium and the dump, a small piece of
black-passivated stainless steel shim
stock, was removed using tooling
manipulated from outside C-port. The system
was then pumped down again
and, following overnight ECDC and a 60C bake,
operation resumed at noon on
Friday, despite rather high H2O levels on the
RGA. Not surprisingly, the vent resulted in
significantly degraded machine
conditions. While the startup reliability
was high (90%), the plasmas were
resistive, and short-lived. Discharges
were limited to currents less than
about 500kA and pulse lengths of about
.25 seconds. The machine conditioning
program has therefore been set back
several days.
Additional baking and ECDC were carried out over the
weekend, and operation
will be resumed on Monday.
Physics and
Diagnostics
-----------------------
A comparison was made
between the collisionality calculated at the q=1.5 and
q=2 surfaces in
C-Mod with several scalings for Neoclassical Tearing Mode
(NTM) thresholds
in DIII-D and ASDEX-Upgrade for a large number of high beta
shots
(1.2<beta_N<1.7). In a couple of
these shots, large low frequency MHD
modes
were observed that
increase in amplitude with increasing beta during the
discharge and then
finally lock at an amplitude of about 50 G measured at the
wall and lead
to a beta collapse. Most of the C-Mod
data lie at substantially
higher collisionality than the DIII-D and
ASDEX-Upgrade NTM data, but the two
discharges with large MHD modes fell
close to each of the
DIII-D and ASDEX-Upgrade scalings in terms of
dimensionless parameters
betan/rhostari and nustari. The C-Mod rhostari values for these
discharges
fell in the range of 0.004 to 0.007 and the collisonalities
ranged from 0.01
to about 2.0.
While the proximity of the threshold from these other
machines to
these two shots with large MHD does not prove they are NTM's, it
is
further evidence suggesting that these higher beta discharges at low
collisionality
are at least approaching the NTM threshold.
Initial data was
collected from the in-vessel Penning ionization gauges during
the May 25
startup discharges. These cold cathode ionization gauges,
mounted to the
vacuum vessel wall at four locations, use the ambient magnetic
field
inside the tokamak for operation. Three gauges are positioned in the
main
chamber to record toroidal and poloidal variation in the neutral
pressure.
One gauge is located in the lower divertor to record fast changes in
the
divertor neutral pressure when the divertor bypass flaps are opened. The
main-chamber gauges appear to be
operational. No-plasma calibration shots
indicate that their signal is
proportional to the torus neutral
pressure. Initial data during plasma
operation indicate weak toroidal
variation in the neutral pressure (less
than a factor of 2) and strong
poloidal variation (up to a factor of
5). In order to avoid a saturated
signal,
the divertor gauge must be operated in a reduced bias mode where a
non-linear
pressure response might be expected. Nevertheless, this gauge
appears to
provide information on the rate of change of pressures in the
divertor
when the bypass flapper is opened.
Initial results suggest that the
transient neutral exhaust rate
through the C-divertor flapper is small (~1
torr-liter/s). Further
experiments are planned.
All channels of the FRC ECE system are
working, and calibration is in
progress.
Some shots were lost because of CAMAC problems. These
problems are being
resolved.
ICRF System
------------
The J-port
antenna was vacuum conditioned to ~32 kV and the first (of
three) tuning
calibration factor, DC1, was obtained in preparation for
plasma
operation. The remaining 2 calibration numbers require plasma
discharges
to obtain. We have continued to cross
check power, voltage
and current measurements to properly monitor and
characterize the
system performance.
D and E-port antennas have continued to operate up
to 0.5 MW each
when plasma operation has permitted.
Diagnostic Neutral Beam
Systems
-------------------------------
The DNB was baked for
approximately 40 hours. In the past, a
bake
preceded the lowest water concentration, but several additional days
of beam shots were also necessary.
A small number of DNB shots
are lost each day because of nuisance faults that
prevent a beam from
being generated, but which are spurious.
One of these
incorrectly indicates an absence of the diverting
magnet current. The
diagnostic for
the magnet current is slow and sometimes does not indicate that
current is
present until the time is past for generation of a beam. This
diagnostic is being replaced a
faster system. Design is complete
and
fabrication is in progress. A
second nuisance fault is related to the "false
start fault"
which when operating properly, indicates that the control system
has
received multiple start pulses.
Diagnostics have been installed to search
for the source of this
spurious fault condition.
Inner Divertor Project
----------------------
Ceramic-coated
tungsten probes to be used in the new inner divertor probe box
were
received and test-fit in an aluminum tile prototype. Existing molybdenum
tiles
will be modified to accept the probes. A new stud face-off tool is
complete,
and will be tested in the mock-up.
A detailed schedule for
finishing the machining of the Rear Girdle plates has
been developed with
the vendor. All of these plates are to be ready for silver
plating at the
beginning of next month. Machining of the Upper Section of the
first
C-plate is nearly complete.
Lower Hybrid MIE Project
------------------------
The
TPS (Transmittter Protection System) Backplane board assembly is
complete.
Parts to assemble TPS and CPS (Coupler Protection System) circuit
boards
began to arrive. Work was begun on the overall Lower Hybrid wiring
interconnection
schematic.
Work continued on the SFOL (Serial Fiber Optic Link)
circuits which will be
used for communications in the LH control system.
Progress was also made in
development of communications software for the
active controller. The data
compression and decompression programs in
Labview RT are now working. Tests
were performed in which analog outputs
were wrapped back into the analog
inputs on the controller chassis to
check for sampling problems; the data is
being evaluated.