From:
irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod
Weekly Highlights
Organization: MIT
Newsgroups:
sci.physics.plasma
Alcator
C-Mod Weekly Highlights
August
17, 2001
Disassembly of Alcator C-Mod continued this
week. Equipment on
and around the
machine was removed. The next major
operation will be
the removal of the cryostat. Work continued on the LH MIE Project and
ICRF
upgrades.
Operations
----------
Diagnostic
equipment on the east side of the cell diagnostic stand has
been removed
in preparation for installation of the new mezzanine. The
mezzanine will be used to support the LH klystrons and
associated equipment.
Power distribution transformers and ICRF
transmission lines have also been
relocated. All diagnostic and support equipment mounted on the igloo
has
been removed, allowing the upper igloo covers and 1st and 2nd igloo
tiers to
be removed from the cell.
The cryostat is now in full view for the first
time in nearly three
years. Disassembly of the cryostat will
begin next week.
Physics
-------
Analysis of
the total particle inventory and main-chamber
fueling for a series helium
discharges has been performed.
In these discharges, a constant plasma
density was maintained
after the helium gas puff was turned off, as one
would expect for a
100% recycling gas species. The measured amount of
helium injected
into the vessel roughly agrees with total inventory of
helium ions in
the plasma and helium neutrals in the main chamber and
divertor.
The agreement was found to persist over the full set of
discharge parameters.
These results suggest that the present array of
neutral pressure
measurements roughly captures the full inventory of
neutrals in the vessel.
Also during these discharges, a set of 3 outer
divertor flappers was
repetitively opened and closed at 100 msec
intervals. From the resulting
rate of plasma and neutral density rise in
the main-chamber,
the transient neutral flow rate through a single flapper
when it was
initially opened appears to be about 3 Torr-l/s. This flow
rate is
about an order of magnitude lower than that expected from the
vacuum
conductance and the pressure under the flapper at the time of
opening.
These results suggest that plasma-neutral and/or plasma-wall
interaction at
or above the flapper opening may limit the effective
neutral conductance.
Further experiments have been done using the
li-pellet ablation trail technique
to measure internal magnetic field
pitch angles. The pellet position is
monitored using a downward imaging
photodiode array located at the same
toroidal location as the pellets (C
port), which gives the radial position as a
function of time. The images
of the Li II ablation trails are taken with the
PSI-3 ultrafast camera, in
collaboration with PPPL and Princeton Scientific
Instruments, which
provides 12 sequential 2-d snapshots (64 x 64 pixels each)
with time
resolution as fast as 0.5 microseconds. For the pellet images, 30
microsecond
imaging was used, which yields a spatial resolution of about 2 cm
in the
major radius direction. Several techniques are being investigated to
measure
the angle of the ablation trail, which is aligned with the local total
magnetic
field, and to evaluate the uncertainties. One technique, which appears
to
be very promising, involves taking the 2-d Fourier transform of the
image,
and then measuring the angular properties in k-space. A typical set
of images
from one discharge, along with an inferred pitch angle profile,
can be seen at
http://www.psfc.mit.edu/people/marmar/li_theta_1010731004.pdf
Typical
error bars range from +/-0.15 degrees to +/-0.4 degrees. These
measurements
can be used to constrain the EFIT reconstructions and thus yield
important
q-profile information.
Lower Hybrid MIE Project
------------------------
We
continue assembling the TPS PC boards.
PLC development also continues.
Modification of drawings for the
rear panels of the EMI/RFI equipment rack
have been released to the
machine shop.
Site modifications for the project are moving
forward. The contractor
for the
water cooling system has been on site several weeks now running
the
cooling lines. The concrete slab that
will support the high voltage
power supply will be poured next week after
several weeks of prep work.
ICRF Systems
------------
The
E and J-Port antennas have been removed from the vessel. The new J-Port
strip lines are in
excellent condition (no indication of arcing on the
strip lines, vacuum
feedthroughs, or vacuum vessel).
Changes made to J-Port
during the last up-to-air were all very
successful. However, some arc
marks
were discovered at the ground bridge of the J-Port straps where
the
voltage exceeded 15 kV/cm in an E||B location. The bolts holding down
some of the
plasma facing boron nitride tiles also showed some arc marks.
These
locations are now being carefully modelled and changes will be made
during
the machine inspection period to eliminate these effects.
Modelling
of transmission line components suggests that we could
use a double stub
system to match the plasma load to the transmitters.
This design could
allow us to test real time matching using fast ferrite
stub tuners (from
AFT) during plasma operation.
Improvements in the matching
network operation could also be made
and are under investigation.
Work continues on development of the
new phase detectors. The latest
prototype
is together and under test. The design
of new equipment racks
and the layout of the associated equipment
continues.
Inner Divertor
--------------
The
vendor for the rear backing plates is still on schedule for an
early
September delivery of all plates. We
continued work developing
a more compact stud welding system.
DNB
---
The
DNB did not operate due to disassembly of C-Mod. Analysis of
the diagnostic calibration data taken during
the previous week
continued.
Travel and Visits
-----------------
Stewart
Zweben visited from PPPL this week to work on the fast
imaging
camera.