From:
WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator
C-Mod Weekly Highlights
Organization: sci.physics.plasma
Newsgroups:
plasma@cs.uml.edu
Organization: MIT
Alcator C-Mod Weekly
Highlights
August
23, 1999
Plasma operations continued on Alcator C-Mod last week.
Five run days were
scheduled. The run on Thursday was terminated after one
plasma shot due to
malfunction of a 13.8kV breaker; this was repaired and
normal operation
resumed on Friday morning. The machine operated well, with
72 plasma shots
produced with typical currents of 1MA and toroidal fields
of 5.1 to 5.4T. The
main focus of operations continued to be commissioning
of the J-port ICRF
antenna. In addition, a series of ohmic H-mode shots
were run in support of
several experiments, including Edge Fluctuation
imaging, Puffing probe plumes,
and Omegatron; on these shots, the RF was
pulsed at the end of the current
flattop.
Physics and
Analysis
--------------------
On Wednesday's run, the F-port
and A-port scanning Mach probes were configured
to record cross-field
profiles of floating potential and ion saturation
current fluctuations
during L and H-mode phases of ohmic H-mode discharges. In
previous
attempts to monitor ion saturation currents with these probes, the
Isat
probes were held at a constant bias.
This caused a runaway heating and
arcing condition. For this run, a
voltage sweep was maintained on the "Isat"
probes. During data
analysis, the Isat portion of the sweep will be extracted
for fluctuation
analysis. Preliminary results suggest
that the 6 mm poloidal
spacing between the "north" and
"south" probes is too large to reliably record
poloidal E-field
fluctuations. This is necessary to allow an estimate of the
local
cross-field fluctuation-induced fluxes. However, good correlation is
seen
in floating potential fluctuations between the "North" and "East
(or
"West") probes which have a 3 mm poloidal separation. A
redesign of the high
power-handling Mach probe head with closer probe
spacing is being considered.
Such a design would allow both
fluctuation-induced particle fluxes and
parallel plasma flows to be
recorded up to the separatrix.
The Beam Emission Spectroscopy
receiver was used to look at fluctuations in
D-alpha emission. The views
were essentially toroidal. We hoped to
see
correlations and phase shifts between poloidally separated views, with
the
goal of determining the radial electric field. Instead, we saw highly
correlated
fluctuations (for frequencies below ~50 kHz) between *radially*
separated
views across almost the entire plasma.
In contrast, poloidal
correlations rapidly dropped off with
separation of the views. In both
cases
there was little (poloidal) or no (radial) indication of
propagation.
We interpret these results as due to contamination by
D-alpha emission from
the near- and far-fields of the views. We will next try a couple of
modifications
of the system:
i) reduction of
the collection angle of the optical system to
minimize D-alpha collected
in the near and far field (by stopping down the
light cone exiting the
optical fibers),
ii) detection
of bremsstrahlung emission, which peaks closer to the
foci of the
views.
ICRF System
------------
We continued
J-port antenna conditioning and testing into 5.2 T plasmas (H
resonance is
on-axis at 78 MHz), and were successful in demonstrating heating
with the
J-port antenna. Scanning different
relative antenna strap current
phases, the best heating was observed with
[0,pi,pi,0]. Other phases did not
result
in H-mode and had signficantly more impurity influx, particularly Ti,
Mo,
and C. The H/D ratio was increasing
with RF power and throughout the day.
The hydrogen source may be the TiCN
coated Faraday screen. We found that
full
current disruptions could be avoided by lowering the RF power. A small
density scan suggested that the
impurity injections were insensitive to
density. With an outer gap of 8-10mm the antenna loading was high (above
10
Ohms). The antenna operated with lower loading (more like normal D and
E-port
loading values) with an outer gap at 1.2 cm. Operation this week
will
determine if more conditioning of the antenna can reduce the impurity
source.
In FMIT#3 and #4, the low power RF amplifiers and control
system have been
further investigated.
An upgrade to power feedback circuit has been
implemented and
tested into dummy load, vacuum, and plasma.
It functions
better than its predecessor and will undergo further
refinement. Although the
reason is
unclear, a 56 kHz amplitude modulation in the 78 MHz signal was
found to
be related to the presence of a splitter between the preamp (10 W)
and the
Kalmus amplifier (<500 W). A new set
of splitters was installed and
the amplitude modulation was
eliminated.
The Eimac 2274 tube from GA arrived, and, following
inspection (including
photographs), cleanup, and hi-potting, was installed
in the FMIT#2
transmitter. FMIT# 2
will be tuned and tested into dummy load at power levels
up to 2 MW. The cavity high voltage connection was
upgraded as well, and was
high potted to 30 kV in situ. We anticipate operation with this tube
into
plasma (E-port antenna) this week.
Travel and
Visitors
--------------------
Miklos Porkolab attended the
FESAC Panel meeting in Knoxville, Tenn, from
Aug. 18 through Aug. 21.
Ron Bravenec (U. Texas) was at C-Mod last week looking at D-alpha
fluctuations
with the BES system optics.
Perry Phillips
continued his stay working on the UT ECE system. Software
was developed to look for changes in local Te
gradients, the slow ECE
channels were reconfigured to improve signals, and
a new temperature
fluctuation IF system was added.