From:
WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Organization:
MIT
Newsgroups: sci.physics.plasma
Subject: Alcator C-Mod Weekly
Highlights
Alcator
C-Mod Weekly Highlights
May
17, 1999
Last week was a maintenance week at Alcator C-Mod. No
plasma operation was
scheduled. Plasma operations will resume this week.
Physics and Analysis
--------------------
The
impurity injection system is fully operational and running reliably.
CaF2
is now the material of choice, with several central and edge charge
states
available in the VUV and x-ray spectral regions. MIST has been updated
to
include calcium, and modelling of x-ray spectra shows good agreement
with
the observations. Time histories of VUV and x-ray lines are also in
good
agreement with simulations for L-mode discharges.
Comparison of the
Edge Thomson measured density profiles and scanning F-probe
profiles shows
good agreement. Some modifications to
the Edge TS system are
being made. The width of spatial observation range
is being increased to get
better coverage of temperature pedestal. In
addition, the spatial resolution
in the density pedestal region will be
improved to about 1 mm. The upgraded
system should be operational this
week.
Data obtained with the AXUV edge array have been examined for
the various
experiments of this run campaign. The array measures the
radiated power
profile at the edge with a nominal 2mm resolution, and its
coverage has been
changed from a previous campaign to look further in the
plasma. Results now
show a
pedestal in edge radiation profile during H mode. Standard hyperbolic
tangent
fit show a full width of 4-5 mm, with a midpoint present approximately
8
mm inside the reconstructed separatrix. Further analysis is proceeding to
compare
this data with other high resolution pedestal diagnostics, such as the
X-ray,
edge Thomson, and visible bremsstrahlung emission.
ICRF
Systems
------------
We have implemented changes to FMIT#3 and
FMIT#4 DC power supply circuit as
recommended from the review of FMIT
transmitters (review was conducted in
collaboration with PPPL). This review was undertaken to analyze the
safety of
the high power vacuum tubes.
The recommendations included installation of a
second soft start
resistor, adding a screen crowbar and low energy storage
grid bias
regulator, reducing a 150 ohm resistance to 40 ohms, adding an
ignitron
holding anode power supply, and additional instrumentation.
A testing and debugging phase is in
progress. Remaining tests include
measurement
of the anode bus voltage during fast RF interrupts, and operation
of the
transmitters into a dummy load.
Conditioning of the new RF antenna can
then proceed. In parallel,
parts required for FMIT#1 and #2 have been ordered.
Ross dividers for the
new instrumentaiton upgrade are being provided by PPPL
until new ones are
received.
Visits and Travel:
------------------
Marco
Brambilla (IPP-Garching) ended his one month visit at the PSFC last
Friday
and has returned to Garching. His trip was very successful given that
we
implemented a new version of the TORIC ICRF code on the C-Mod computer
cluster.
This code computes the electron Landau damping of ion Bernstein waves
(IBW)
without the use of a numerical broadening technique that was used in the
previous
version of TORIC. The code was also coupled to an ICRF current drive
package
and an IDL post processor was written for use both at MIT and in
Garching. An interesting prediction of the code for
off-axis mode conversion
in D(He-3) plasmas in C-Mod is that some ELD can
be expected near the plasma
center in addition to the ELD that is expected
off-axis near the ion-ion
hybrid layer.
This occures because an IBW is also excited on the tokamak LFS
where
f_o < 2f_cd. A preliminary inspection of old MCEH data by Eric Melby
indeed
shows some evidence of this secondary on-axis peak in electron damping.