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
Jan 8, 2001
Last week the C-Mod vacuum
vessel was subjected to a standard pre-entry
de-boronization procedure
consisting of one night of glow discharge cleaning
followed by backfilling
and an overnight purge with room-air. No diborane was
detected at any
point during the up-to-air process, and invessel work began on
Thursday
morning with an inspection and photographic survey. As expected,
the D and
E-port ICRF antennas, wall components, and most diagnostics were
found to
be in good condition. Details of
observations made of the J-port
antenna and MSE diagnostic collection optics
are discussed below.
Work continued last week on the Lower Hybrid
system, upgrades to the RF
control system, and general maintenance of
engineering systems.
Physics
--------
More
analysis has been carried out on the data from the two-frequency ITB
experiments
carried out on Alcator C-Mod on December
20, 2000.
In order to reliably produce the density barrier mode
with off-axis ICRF
heating only, it was found that a high enough plasma
target density was
essential, in this case a core nl = 1.0X10^20. Using
the higher target
density produces a sustained EDA H-mode phase with an
early L->H transition
at (0.7-0.8) s, and void of any back (H->L)
transitions. It has been
suggested
that the higher target density effectively dilutes the plasma
impurities,
leading to less radiation, and thus avoiding a collapse of the
H-mode phase.
The density increases were quite strong in many of these
higher target
density plasmas with ne0 (from Thomson scattering) rising to
8 X
10^20. The density profiles as measured from visible
bremsstrahlung
also exhibited clear peaking.
Successful on-axis
heating was also carried out with the 70 MHz J-port
power during the
density barrier phase, at different rf power levels
and at different
times. Increases in the neutron rate
and central Ti
were produced by injecting J-port power late in the
discharge (1.25 -
1.5 sec) after the density barrier formed. The density
increase during
barrier formation was found to be arrested in some shots
by the
injection of the J-port power. Interestingly enough, on other shots
the
density increase was also found to continue during J-port
heating.
Thus it is possible that the on-axis heating causes the
density barrier to
"leak" under certain plasma conditions. A scan of
the radial
position of the HIREX diagnostic was also carried out to
get data for Ti
and plasma rotation profiles during on-axis heating
of the density barrier
mode. The data obtained during this day of
running should be adequate to
carry out a careful transport analysis
in order to determine if heating
inside a thermal energy barrier was
occurring in these plasmas.
Mode-converted
ion Bernstein waves (IBW) near the ion-ion hybrid layer
between Hydrogen
and Helium-3 were again observed with the Phase
Contrast Imaging (PCI)
system on the 22 December 2000 run. Plasmas
were run with high field (5.9
T), low density (ne0 ~ 1 10^20) and low
current (400 kA), and a comparable
mix of H, He-3 and D in order to have
mode conversion occur near the
plasma center. Strong signals (5 to 6
times background) were seen for both
the D and E port antenna launch (the
PCI is directly in front of the E
port antenna). rf wave code results
from TORIC suggest that the
mode-conversion layer and associated
electron Landau damping should be
more aligned along a vertical chord
at lower plasma current, hopefully
simplifying the PCI observation,
which vertically integrates along 12
channels. Preliminary results
indicate that there is still IBW signal in
several distinct major
radial locations, but possibly more localized than
in the high current
case from 23 June 2000.
General
Engineering
-------------------
Following warm-up of the
machine last week, maintenance of the cryo
system was begun. LN2 control valves are being reworked,
filters will be
checked and cleaned, and the LN2 sump will be opened up
and inspected.
The LN2 manifold that distributes the liquid nitrogen to
the magnets has
been checked for loose connections with nothing
found.
Invessel video is now available via the web. During manned entries either
the wireless
cameras or the wide angle view can be found at:
http://www.psfc.mit.edu/cmod/online/control_room_rv1.html
or
rtsp://www.psfc.mit.edu/encoder/cmod.rm
Pictures
from the invessel survey can be found at:
http://www.psfc.mit.edu/cmod/operations/EngImages/CmodImages.asp?ref=/INVESSEL/2001/Up_to_air_survey/
ICRF
System
-----------
Arc sites found on the J-port (four-strap)
antenna were consistent with
along-field-line paths between exposed metal
components. This observation is
also consistent with expectations based on
video images of the antenna during
plasma operation. Changes now being
implemented to the boron nitride and moly
tiles on the antenna should
ameliorate this problem. Impurity
injections from
these sites had limited coupled rf power during four-strap
operation
to <= 2.5 MW if plasma disruptions were to be avoided. This level of
operation was much better
than obtained before the July modifications to the
antenna (<=1.3
MW). We are hopeful that changes made
during the current
up-to-air will allow us to step up to somewhat higher
levels of reliably
coupled rf power without disruptions.
Work
was begun last week on installation of digital panel meters for the
transmitter
tuning circuits. A layout for the
interface
board required has been produced and should be ready to send
out for
fabrication soon. Work was
also started on design of the remote control and
instrumentation of the
coax switches and dummy loads. Work
continued on
the design of the new rf demodulators. Design of a prototype pc
board layout
will begin soon. The
National Instruments PXI hardware for the Fast Ferrite
Tuner Control system is now in-house. This equipment will be
mounted in a
temporary rack early next week to allow setup and software
development to
begin. Assembly of some of the patching
facilities for the
test setup have been started.
Lower Hybrid System
-------------------
Work
continued last week on the electronics needed for the LH system.
PXI
controller equipment ethernet connections have been established and
Labview
RT as been downloaded to the PXI embedded controller. A test
frequency generator virtual instrument has been
downloaded and tested
in the controller equipment. Various test patching facilities for the
PXI
controller are being built to facilitate testing of analog and digital
input
and output circuitry. A test
panel has been designed to allow
characterization of the vector modulator.
An rf test circuit will be built
for the vector modulator and other
components, and we will use
the NI test panels to help characterize its
operation. A VSWR circuit for
protection
of the LH sources was breadboarded and measurements made. These
measurements
agreed well with what was expected.
Work is progressing on the
design of the new two channel serial
fiber optic transmitter/receiver boards
for analog signals to be used in
the control system and elsewhere.
DNB and Related
Diagnostics
---------------------------
Glass dust from at
least one of the optical components of the MSE diagnostic
invessel
assembly was found at the bottom of F-port. This damage is under
evaluation;
some repair work for this diagnostic will be required during the
present
opening.
Visitors and
Travel
-------------------
Ian Hutchinson has begun a 5 month
long sabattical at the Austalian National
University. Earl Marmar has taken on his
responsibilities while he is
away.