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
July
23, 2001
Plasma operations continued at Alcator C-Mod last week. Five
run days were
scheduled and four and a half were completed. A total of 64
plasma shots were
produced with a startup reliability of 80%. Runtime was
devoted to ITB and
Double Barrier studies, non-dimensional similarity
experiments to be
coodinated with ASDEX-Upgrade, and turbulence imaging;
half a run was also
devoted to MSE calibration shots requiring DNB pulses
into gas with controlled
toroidal and vertical fields, but no
plasma.
Plasma operations are scheduled to continue this week. We
are also considering
extending the campaign into next week, which had been
designated as a
contingency option.
Physics
-------
The
first three run days last week were in support of MP#289A, "Double
Barrier
Studies". This MP is a multi-run experiment to investigate
the physics of the
internal barrier formation previously reported in EDA
H-mode plasmas with
off-axis ICRF heating. While previous results were
reproduced, and some
additional data on the evolution and possibility of
control of the core
barrier by on-axis heating and field ramping were
obtained, these experiments
were limited by the high hydrogen fraction and
by control circuit problems
withe the 70MHz ICRF system. Additional
experiments in support of this
MiniProposal are planned for this
week.
Thursday's run was in support of MP287, "ASDEX-Upgrade
Similarity
Experiments". The goal was to make high density, high
power EDA H-modes in a
shape identical to that in ASDEX-Upgrade 'Type II
ELM' discharges. This
regime,
which was only recently obtained on AUG, appears rather similar to
that
obtained on C-Mod when power is increased into EDA H-modes. At reduced
powers, AUG sometimes sees a
'quasi-coherent' mode. We would like to
see if
the same behavior is observed on the two machines with the same
dimensionless
parameters. We will
obtain edge profiles both at threshold and in the steady
H-mode
period. AUG will then try to match the
dimensionless parameters
(nu*,beta, rho*) of these discharges. The shape development part of the run
went
quite smoothly, and we did get shots which were a very close match to
ASDEX
Upgrade shapes. Compared to our usual operation, these plasmas are quite
low
triangularity and very close to being double null. As expected, it was
more
difficult to get steady EDA H-modes in the lower triangularity
ASDEX-Upgrade
shape, and the reduced absorbed power because of the high H/D
ratio was an
additional impediment. More run time with more favorable machine
conditions
would benefit this experiment.
The first half of the 7/20/01 run
was for edge turbulence imaging measurements
using the Princeton
Scientific Instruments Ultra-Fast Camera (PSI-3), which
can take 12
successive frames up to 1 million frames/sec.
Discharge
conditions were set to match a baseline case for which
edge profiles are
available and theoretical simulations are being done by
Hallatschek and
Rogers. The camera
view is essentially tangential to the field near the
outboard midplane at
the plasma edge, and illumination is provided by a gas
puff, either of D2
or He, from a capillary located at a gap in an outboard
protection limiter
assembly. Excellent images were obtained showing the
evolution and motion
of turbulent structures ("blobs") in the poloidal
plane. Images
obtained with D2 puffs and a D-alpha filter showed structures
near the
separatrix and extending into the SOL, while images formed with a
helium
puff and a He-II (4686A) filter showed emission further inside the
LCFS,
with smaller features and apparent rotation in the opposite sense to
those
at larger radius.
In reviewing magnetics fluctuation data it was
noted that, under some
conditions with small outer gaps <= 1 cm and at
relatively low density, the
poloidal magnetic field pick-up coils located
underneath the tiles on
the outboard AB and GH limiters have sufficient
signal from the Quasi-Coherent
Mode (QCM) in EDA H-mode to be able to make
out the toroidal mode number. At
least
a dozen shots have been found with reasonable signal levels and the
resulting
toroidal mode numbers of the QCM range from n = 15 - 19 for ICRF
heated
EDA H-modes. The coils beneath the
tiles are typically only 2.5 cm
from the LCFS whereas the coils on the
sides of the limiters and on the inner
wall are typically 5 cm from the
LCFS. Because of the high mode numbers
of
the QCM, the rapid radial decay of the mode amplitude makes the signal
too
small to be observed on the coils that are 5 cm from the LCFS. These toroidal
mode numbers are
somewhat smaller than the calculated n~30 from the radial
decay of the QCM
from the A-side scanning magnetic probe measurements in Ohmic
EDA H-mode,
but still indicate a rather high toroidal mode number.
Operations
----------
The
machine ran well last week, but the H/D ratio during high-power RF
operation
was higher than optimum, rising above 0.1 during typical H-modes,
and as
high as 0.2 during some shots. As a
result, the ICRF minority heating
efficiency degraded significantly during
the pulse, compromising some planned
experiments. Additional glow
discharge cleaning (GDC) in helium was carried
out from Friday evening to
Sunday afternoon (7/20-7/22), followed by a fresh
boronization on Sunday
evening. It is hoped this procedure will reduce the H/D
to acceptable
levels for this week's experiments.
Wednesday's run was delayed in
order to remove a shunt from the OH2L buss. The
shunt had been observed to
be giving high readings during Tuesday's run,
indicating that its
resistance had increased by about a factor of two from its
nominal value.
The shunt was replaced by a length of busswork, and is being
shipped back
to the manufacturer for tests and analysis. The buss shunts are
not
presently used for any control or feedback functions, and primarily
provide
redundant information with the buss rogowski loops.
For several
weeks we have noted signs of damage in snubber resistors in the
bypass
circuit of the chopper supply used for fast vertical position control;
we
have been replacing these resistors during maintenance, and the problem
has
not resulted in any loss of run-time. Analysis by the resistor
manufacturer
indicates that the resistors suffered from overheating as a
result of too much
power dissipation, but our initial calculations
indicate that they were being
operated within their specification. We are
now testing a new set of
resistors, with higher power/energy dissipation
rating and better temperature
capability, in the snubber circuit. Comparison of waveforms from this
campaign
with earlier run periods does indicate we have been drawing somewhat
higher
average currents from this supply than in the past. It is not known
whether
this is related to the resistor damage. Last week the programming for
this
supply was changed to reduce the average current back to its historical
value,
which has the benefit of increasing the dynamic range available for
feedback
stabilization of the vertical instability.
ICRF Systems
------------
The
J-port antenna operation at 70 MHz made good progress towards reliable
operation. By the end of the week, 1 MW was coupled to
the plasma without
faulting. The
decoupling stub was optimized and the vacuum and plasma
operation became
more stable. A reflected power signal
critical to the
accurate prediction of antenna match was found to be
malfunctioning; this was
repaired, allowing efficient matching to plasma
loads. The antenna phase
determined
from the voltage probes was found to be unreliable and resulted in
false
phase balance faults. From current
probes at the same location, the
phase is as expected, 0 deg, and
stable. We concluded that these probes
are
located near a voltage minimum at 70 MHz and the voltage phase
measurement is
difficult due to the small signal level. The current probes will be used for
the
phase balance fault arc detectors for the remaining 70 MHz operation.
Diagnostic
Neutral Beam Systems
-------------------------------
The beam
operated at full voltage and current throughout the week. We
continued with diagnostic
development. The arc power supply was
modified to
allow the arc pulse to be increased from a maximum of
approximately 300 ms to
a maximum that is at least 700 ms. This is to facilitate conditioning of
the
arc. On Friday, we completed a
beam into gas experiment with various toroidal
and vertical magnetic
fields to calibrate the MSE diagnostic.
Lower Hybrid MIE
Project
------------------------
The reflected power probes of
the lower hybrid launcher have been tested
successfully at PPPL by Stefano
Bernabei. At MIT, work continued on the TPS
chassis wiring and timing
circuitry.
Inner Divertor Fabrication Project
----------------------------------
The
vendor responsible for fabrication of the Rear Girdle Plates has informed
us
that they will be unable to meet their previously agreed schedule, and
proposed
a new delivery date two and a half months in the future. In view of
the
delays already experienced at this vendor, we have decided to transfer
the
remaining work on these components to the shop which previously
produced and
delivered the C-plates; the latter vendor has agreed to
complete the work on a
schedule compatible with our requirements.
In-house
work on the probe box and mock-up parts is proceeding. All short
studs have been welded on the
mock-up wall. Diagnostics were mounted on the
wall as well. A plastic Rear Girdle Plate, Inconel
C-Plate, Tiles, Transition
keys were mounted on the Mock-up wall. Two
limiter backing plates with tiles
were placed directly above the new Inner
Divertor and found to fit perfectly,
confirming that there should be no
installation difficulties provided the Rear
Girdls Plates are within
specs. Furthermore, there was very
little work
required to align the Girdle Plates with the Limiter Backing
plates and
interlock them with a transition key.
Travel and
Visitors
-------------------
Stewart Zweben was at MIT for the
week, continuing to obtain high-speed, high
sampling rate
"movie" images of plasma edge turbulence with the gas puff
imaging
diagnostic.
A quarterly review was held on Monday, July 16. Rostom
Dagazian attended from
OFES. The viewgraphs from the presentations can be
found at
www.psfc.mit.edu/cmod/sciprogram/Q_rev_Jul01/C-Mod_Quarterly_Rev_July_2001.pdf