From:
wolfe@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator
C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization:
MIT
Alcator
C-Mod Weekly Highlights
June
24, 2002
Plasma operations resumed on Alcator C-Mod last week. Four
run days were
scheduled and completed. A total of 23 plasma shots were
produced, with a
startup reliability of 27%, which is low but not atypical
for the first week
of operations after a major vent. The operation was
devoted to tokamak clean-up
and conditioning; several diagnostic systems
were also brought online.
Work also continued on ICRF, DNB, and
Lower Hybrid Systems.
Plasma operations are planned to continue
this week.
Operations
----------
Following a week
of vessel conditioning (bake and ECDC), plasma operation
resumed last
Tuesday. The purpose of the week's runs was to begin the process
of
discharge conditioning of the tokamak in preparation for physics
operations.
We also began discharge development for several new equilibrium
configurations,
including higher triangularity and near-double null plasmas,
called for in
the experimental plans for this campaign. In addition, we began
bringing
diagnostics online, and verifying control and data system operation.
We
also began to gain experience with the new Linux-based computer systems
under
actual runtime conditions.
As is typical of plasma operations after
a major vent, the plasmas produced
early in the week were short,
disruptive, and characterized by high
resistivity and radiated power
fraction, with frequent injections. Density was
typically higher than the
programmed value (~1e20/m3), with most of the source
coming from the walls
rather than the gas valves. As the machine began to
clean up, 800kA
discharges were produced that lasted until their programmed
rampdown, and
control of the density improved.
The initial discharges were
programmed to be "standard" lower single null
divertor
configurations. Once full-length plasmas were obtained in this
configuration,
we began to modify the equilibrium toward a double null and to
increase
the triangularity, taking advantage of the new inner divertor
assembly
which allows more range for the lower x-point position. The purpose
of
these equilibrium changes was both to begin development of new discharge
shapes
required for upcoming physics experiments and to enhance the clean-up
of
the vessel by moving the strike points onto previously unscoured tiles.
By
the end of Friday's run, full-length discharges were produced in a
near-DN
configuration (SSEP~5mm). The H/D ratio, one important indicator
of machine
condition, was down to ~0.3 or less. However, progress was
limited by rather
poor startup reliability. This may simply be due to
inadequate wall
cleanliness; we are also investigating the possibility
that the "sparker", an
invessel spark-gap system in
line-of-sight to the tokamak chamber which
provides some initial
ionization to assist breakdown at low loop voltage, may
have
malfunctioned.
ICRF System
-----------
New RF
Cell rack cabinet wiring was verified.
Some minor changes are being
made for PLC operation of stub
tuner/phase shifter. Stub tuner/phase
shifter
controllers are both working with local control. PLC operation for #1 has
been
done. PLC operation for #2 will be
completed soon.
Final jumper configuration of the new fault system
was done. We have started
on J
port fault circuit testing and entered and tested proper setpoints for
phase
balance. Voltage limit settings were
calculated and entered and will be
tested. Power threshold settings for J port were tested. Delay from phase
balance fault to fault
system output (in cell) was measured for several
channels and is less than
2 uS. J fiber optic receiver boards in
power room
electronics were modified to support failsafe signal operation
from cell
electronics and tested.
PLC to fault board trip signals were tested and
verified. Several problems were found and
corrected.
Software modifications associated with use of the new
cPCI data system for RF
operations are ongoing.
Lower
Hybrid MIE
----------------
Work continued on improving the
response time of the body current protection
circuits for the LH
klystrons. Tests of a new breadboard circuit indicated
less than 2usec
delay. New, faster response, current transducers have been
ordered.
DC
power supplies for the flow-meter instrumentation have been
ordered.
Mechanical work is proceeding on carts 1 and 2.
DNB Systems
-----------
The
beam was operated throughout the week during the conditioning discharges.
Beam
conditioning shots were run between C-Mod shots. The beam was synched
with the firing of C-Mod shots. A recent survey spectrum of the beam
showed
lines from hydrogen only, attesting to its cleanliness. The MSE system has
acquired data from
both beam-into-plasma and beam-into-gas shots, and the
initial results
look very encouraging. A set of optical
channels is being
configured for the MSE to look at the background plasma
emission
(i.e. non-beam-induced) to improve the signal-to-noise even
more.
Diagnostics
-----------
The two new
off-axis tangential xray spectrometers were successfully tested
during
plasma operation.
The Electron Cyclotron Emission diagnostics,
including Michelson, GPC and
GPC2, are all operational and taking data,
but not yet recalibrated. The
FRCECE diagnostic also observed ECE signals
during several shots.
Work continued on the amplifier and power
supply system for the Active MHD
Spectroscopy Antenna.
Travel
and Visitors
-------------------
Ian Hutchinson, Joe Snipes,
Miklos Porkolab, Eric Nelson-Melby, Remi Dumont,
Martha Redi, and Stewart
Zweben attended the Conference on Plasma Physics and
Controlled Fusion of
the European Physical Society in Montreaux, Switzerland,
last week. Hutchinson presented a paper "Plasma
Mach-probe with Unmagnetized
Ions"; Snipes presented a paper
"Structure of the Quasi-Coherent Mode in
Alcator C-Mod";
Nelson-Melby (now at CRPP, Lausanne) presented "Phase Contrast
Imaging
of Mode-Converted Ion Cyclotron Waves in the Alcator C-Mod tokamak";
Dumont
(PPPL) presented a paper titled "ICRF wave propagation and
absorption
in plasmas with non-thermal populations"; Redi (PPPL)
presented a paper titled
"Gyrokinetic calculations of
microinstabilities and transport during RF
H-modes on Alcator C-Mod";
and Zweben (PPPL) presented a paper titled "Edge
turbulence imaging
on NSTX and Alcator C-Mod."
David Mikkelsen (PPPL) visited
C-Mod for discussions with Amanda Hubbard,
Perry Phillips, and Alan Lynn
about using ECE Te diagnostics and Bo
sweeps to determine temperature
gradient scale lengths.
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