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
April
20, 2001
Invessel
installation of the modified J-Port ICRF antenna is expected to begin
this
weekend. Invessel work on DNB related
diagnostics was completed, and
we continue to work on the LH MIE Project
and power systems.
Physics
-------
We continue
to analyze the dynamics of off-axis ICRF-induced core transport
barriers.
In these discharges, the barrier formation follows the transition
into an
EDA H-Mode, at slightly reduced toroidal magnetic field, which places
the
hydrogen minority resonance for 80 MHz ICRF to the high field side. In
typical
cases, the density begins to peak, with a strong gradient in pressure
approximately
out to the half minor radius. The peaking of both majority and
impurity
densities can lead to radiative collapse and subsequent disruption. In
a
series of experiments performed in December, 2000, we used our tunable RF
sources
to apply heating at 70 MHz from the 4-strap antenna, while using the 2
dipole
antennas at 80 and 80.5 MHz respectively. After the formation of the
core
transport barrier, application of the lower frequency RF, for which the
minority
resonance is close to the magnetic axis, can lead to the arrest of the
density
and impurity buildup. In some cases, the peaked pressure profiles are
maintained
over many confinement times, while the discharge continues to
exhibit
sawtooth activity. An MPEG movie of the density profile evolution,
which
demonstrates a number of the interesting properties of one such
discharge,
can be downloaded from
www.psfc.mit.edu/people/marmar/denmovie2.mpg
The
movie shows the density profiles at 1 msec time intervals, along with the
ICRF
powers, in MWatts, at the two different resonance locations. TRANSP
analysis
of these discharges, including the measured ion and electron
temperature
profiles, is ongoing.
Recombination
profiles in the Alcator C-Mod divertor have been investigated
using the
tangentially-viewing divertor CCD camera, the visible
spectrometer, and
the flush-mounted and scanning Langmuir probes. It has been
observed that during a density scan, the
recombination rate on the outer
divertor leg remained small until it
detached, and then increased and remained
constant as the density was
increased beyond the detachment threshold.
The recombination rate in the
private flux zone increased until detachment of
the outer leg, when the
recombination rate was reduced to near negligible
levels and continued at
that level as the density increased.
The
recombination rate on the inner leg decreased until the core
density was
approximately 1.7 X 10^20 /m^3 (which is before the outer leg
detaches)
when the recombination rate increases with increasing density.
After comparing
the 2D recombination profiles from various densities, it is hypothesized
that the cause of
the plasma in the private flux zone and density dependence
of the
recombination rates in the three regions, may be due to an ExB drift
caused
by a parallel Grad(Te) term. Total particle current entering the
private
flux zone through the outer divertor leg estimated by recombination
profiles
and probe measurements, and the expected particle current due to the
ExB
drift quantitatively agree to within the experimental error. The
recombination profile of a high
density, reversed field discharge yields
qualitative agreement with the
hypothesis of the ExB drift.
Further
investigation is continuing using the B2-Eirene code.
ICRF
Systems
------------
The striplines, which were baked over the
long Holiday weekend, were removed
from the oven on Tuesday. Following a final polish and cleanup, they
are
now ready for installation invessel.
Work concentrated this week on the
installation of 0.03"
copper plated steel covers needed as part of the
rf current return
path. These covers were e-beam welded
to the Faraday
screen modules and then polished to eliminate any chance of
arcing in this
critical area. This
process, together with some other small tasks, took more
time than
expected, and we have lost some ground on our schedule. We expect
to complete antenna
installation, and invessel cleanup and documentation in
the coming
week.
Gerd Schilling has taken the old stripline and septum support
components
back with him to PPPL.
Randy Wilson plans to assemble them in the PPPL
rf test stand and
collect data on rf breakdown effects.
They will also
be able to apply a magnetic field during the tests
using a set of
PLT magnets. These
tests are aimed at improving our understanding of the
behavior of the
original feedline configuration in C-Mod.
The new J-Port pressure
gauge and mount has been installed. The
gauge will
be ready for operation following installation of the controller,
cable, and
link to CAMAC.
Work continued on the phase fault
system. All the needed arc/phase
detector
modules have been upgraded with new faster phase detector
chips. System testing and calibration of the phase
balance boards with
their associated arc/phase demods are ongoing.
Diagnostics
-----------
Both
of the CXRS shutters have been tested by manually pulling on the shutter
control
wires. All in-vessel fiber optics and
guide tubing for the shutter
actuation wires have been spot welded in
place and clamped down. The
exterior
vacuum vessel work is also nearly complete. Only the attachment of the
wires to the
feedthroughs remains before all flanges
can be torqued down.
We have begun an upgrade to our ratiomatic
gauge controllers used for
pressure measurements on C-Mod. A prototype was tested on C-Mod during
the
last run period. A new PC board
incorporating the changes is being
being developed.
Lower
Hybrid MIE Project
------------------------
The detailed
electrical design for the transmitter cart and equipment racks
has been
completed. Work can now begin on the
waveguide components.
The first TPS (transmitter protection system) PC
board has been released
for bids.
Design has begun on the safety key-lock interlock system.
A
test stand for simulating arcs on the lower hybrid coupler became
operational
this week. A 3kW magnetron source at
2.45 GHz drives a
waveguide arc that will be used to test arc detection
electronics.
One of our new serial fiber optic links (SFOL) has been
packaged and is
ready to undergo testing in the LH phase and amplitude
control system.
Programs have been written to do open loop tests with the
SFOL, I/Q
detector, and vector modulator, after which the closed loop
tests will
begin.
Power Systems
-------------
New
CAMAC and current and voltage transformers have been added to our
hi-yard
diagnostics. These signals will allow
us to better monitor
our power systems, particularly the new lower hybrid
power supply.
Travel and Visits
-----------------
Ed
Synakowski from PPPL came to C-Mod on 4/20 for discussions on the off-axis
ICRF-stimulated internal transport barrier discharges and to present
the PSFC Seminar on "The NSTX Program and an Initial Look at Local
Physics."
Gerd Schilling (PPPL) spent the week here
working with us on the J-Port
antenna modifications.