From:
"E. Marmar" <marmar@psfc.mit.edu>
Subject: Alcator C-Mod
Weekly Highlights
Organization: MIT
Newsgroups:
sci.physics.plasma
Alcator C-Mod weekly highlights June 16,
2003
Alcator C-Mod completed 3.5 out of 4 runs scheduled last week.
Plasma
operation will continue this week.
Operations
__________
The
four scheduled miniproposals last week were: MP 303 High
Performance
ITB's; MP 318 High Performance at 5.4 Tesla; MP 282 Elm
Studies; and MP
332 Edge Fluctuation Studies. Thursday's run was
delayed by a computer
hardware problem, which prevented communications
with the hybrid computer.
Reconfiguring one of the other vms machines
allowed us to start the run in
the afternoon. As a result, Friday's
run was split between two
miniproposals, and both were able to get
useful data.
We
produced a total of 87 plasma discharges last week, with startup
reliability
of 91%.
Physics
________
BOUT simulations by Xu and Nevins predict that
the Quasi Coherent Mode
(QCM) in EDA H-mode results from nonlinear mode
coupling of a drift
Alven mode at 1.4 MHz, an electron geodesic acoustic
mode (e-GAM) at
1.5 MHz, and a resulting beat frequency mode at 100 kHz
that would be
the QCM. During a
recent low density high RF power EDA H-mode run, we
sampled magnetic
pickup coil data at 5 MHz to look for these high
frequency modes. The QCM was observed on coils close to the
plasma
with a frequency dropping from 200 kHz to 70 - 80 kHz during
EDA
H-mode. The lower GH Active
MHD antenna was used only passively for
this run to measure mode
activity. The QCM was not observed on
the
antenna, probably because such a short wavelength mode is averaged
out
by the large size of the antenna.
At higher frequencies, a cascade of
what appear to be Alfven
eigenmodes in the range of 600 - 1200 kHz
were clearly observed on both
the small pickup coils on the outboard
limiter as well as on the passive
MHD antenna. These modes had
toroidal
mode numbers in the range of 5 < n < 10.
In addition, a mode
was found with a slowly varying frequency from
1.8 MHz to 1.93 MHz;
this mode was visible on the pickup coil signals, and
showed up more
strongly on the passive MHD antenna. The n number was difficult to
determine
for this weak mode, but it appeared to be low, with n < 3.
While this
mode could be the long wavelength mode predicted by the
BOUT simulations,
the other high frequency short wavelength mode was
not observed.
Subsequent analysis indicates that the high frequency
mode is more likely
associated with a beat between the QC mode, and
the 2 MHz mode generated
by the beat between the RF generators at 78.0
and 80.0 MHz.
The
idea of the MP 332 run on edge fluctuations was to make a
simultaneous
spatiotemporal measurement of plasma density and plasma
potential
fluctuations. This is done by establishing a magnetic
mapping between the
observation region of the GPI system and the
F-probe. This mapping occurs
when Bt=5.4 T and I_p=610 kA. We measured
cross-correlation between I-sat
and V-floating signals of the F-port
probe and the light fluctuations on
the radial array of GPI views over
a L-mode density scan. We also took
movies of GPI viewing region using
the PSI camera. We also tried to look
during EDA H-mode, but were
unsuccessful in making an EDA H-mode. We
successfully completed the
L-mode density scan. The somewhat surprising
result is that the
probe-diode correlation DECREASED to small values at
line averaged
densities below 1.1e20. Reasons for this are still not
clear. We found
that at line averaged densities below 1.1e20 the plasma is
created by
recycling off of the probe body in the far-SOL maps (with
some
cross-field spreading) along the local flux tube. The
observational
evidence for this was the appearance of plasma in the
far-SOL regions
viewed by the diodes when the probe is inserted, but only
when the
probe maps magnetically to the diode-viewing region. The
probe-diode
correlation is 30-45% at line average densities between 1.6
and
2e20.In this range the probe-diode cross-correlation has a
"dipole"
structure, positive for negative lag times and negative
for positive
lag times.
The run on Thursday afternoon and
Friday morning, MP 282, ELM studies,
was used to investigate the effects
of edge current density on ELM
triggering during H-modes. Current ramps
were used to transiently
increase the edge current density, and
comparisons were made with
similar shots which had no ramp. There were
reproducible differences
in ELM behavior, which are being analyzed.
Lower
Hybrid Project
___________________
We are running CST simulations of the
planned configuration of the
target chamber/absorber for the high power
lower hybrid launcher
tests. We received RF hardware from PPPL, including
50dB directional
couplers and RF sniffers. Cabling and instrumentation
work for the
klystron control systems continues.
ICRF
Systems
____________
The
ICRF systems were operated in support of all 4 runs this week,
with up to
6 MW total coupled through the 3 antennas.
A problem which
had been affecting the D-port power feedback was
resolved by retuning
the FPA stage. Reliable operation under feedback
control is expected
for the coming week.
Power Systems
_____________
Work on the second
TMX power supply for the non-axisymmetric control
coils is underway. The transformer-rectifier chassis has been
cleaned
and all wiring has been rung out. The main contactor unit has
been
rebuilt and reinstalled. The main AC breaker has been ordered
and
should be here by the end of next week. Detailed wiring diagrams
are
being produced for the installation of the power supply, including the
plc
and camac interface and the interconnection of the two power
supplies.
Travel
and Visits
________________
Ken Hill, PPPL, was at C-Mod
6/11-13 to take and analyze data from the
NSTX imaging X-ray crystal
spectrometer. A problem was noticed in the
x-position (spectral dimension)
encoding of the data from the KBSI 2-d
imaging X-ray detector. Since the
y-position (spatial dimension)
appeared to be normal, the x and y cables
were swapped, allowing the
first observation of argon spectra from
the
spectrometer. Neutron/gamma/hard x-ray shielding is being designed
for
the detector.
Gerd Schilling, PPPL, was at C-Mod for the
week, participating in the
ICRF operation and programmatic discussions
with C-Mod physicists.
Sarah Messer, of Univ. Maryland, visited for
two days and gave a PSFC
seminar on the Maryland Centrifugal
Experiment.
--
Earl Marmar
MIT PSFC
marmar@psfc.mit.edu
office
617-253-5455
control room 617-253-5391
fax 617-253-0627
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