From:
"Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To:
wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion
Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroup:
sci.physics.plasma
Alcator
C-Mod Weekly Highlights
June
23, 2003
Plasma operations continued on Alcator C-Mod last week.
Four run days were
scheduled and completed. Experiments included
MiniProposals from the
Transport, MHD, and Divertor/Edge research
topics.
This week will be a maintenance week. C-Mod research
operations are scheduled
to resume next week.
Operations
----------
A
fresh boronization was carried out over Monday night, prior to the week's
operation.
A total of 97 tokamak plasma discharges were produced last week in
support
of four principal experiments. Startup reliability was over 80% for
the
week. All tokamak systems performed well.
A newly calibrated control
relay was installed in the TF#2 breaker
on Thursday, replacing a unit which
was suspected of causing spurious
trips. The new unit is performing
nominally, with no faults
observed.
In a variation from our standard practice, overnight
electron cyclotron
discharge cleaning (ECDC) was not carried out prior to
runs on Wednesday and
Thursday. Perhaps in consequence, startup
reliability during Thursday's run
was poor, with a large fraction of
fizzles. ECDC was carried out over Thursday
night.
The air
conditioning in the cell and cooling water source for equipment in the
diagnostic
labs was offline on Thursday and Friday due to an unannounced
service
interruption by MIT Physical Plant. Operation of some data acquisition
systems
and diagnostics was impacted by the outage.
Physics
-------
During
a 4.5 T run day two weeks ago, several FRCECE (electron cyclotron
emission
radiometer) channels experienced cutoff as the plasma density
increased. During the brief time interval while the ECE
signals were
transitioning from above cutoff to below cutoff, broadband
high-frequency
fluctuations (delta f ~ 50 kHz, peak f ~ 135 kHz) were
observed on a set of
four 1 MHz digitizer channels located within 5cm of
the plasma center on the
high-field side.
We speculate that these may be density fluctuations, as they
only
appeared very near cutoff, when the FRCECE system is sensitive to changes
in
density due to refractive effects.
On Wednesday, an experimental
study (MP#339) of mode locking at lower plasma
current was conducted,
using the new A-coils to apply non-axisymmetric
perturbations. In low density plasmas near the threshold
for locking by the
intrinsic error field, locked modes were induced by
applied fields at a
variety of mode phases, mapping out the regions of
locking and
unlocking. These data established the phase and magnitude of
the equivalent
intrinsic error field for these plasmas: a (2,1) amplitude
of about 0.35 mT
(about Bt/10000) and also the threshold field for locking
the mode. Prior
scaling fits of
the magnitude of the threshold field predict that C-Mod should
require
approximately a factor of 10 higher value for locking. Therefore, our
experiments,
which observe locking at much lower values than predicted,
contradict
these earlier scalings and consequently improve prospects for
avoiding
locked modes in ITER, because they disprove the previous strong
inverse
scaling of threshold with size.
Thursday's run was in support of
MP#335: "Edge plasma flows in upper versus
lower x-point
discharges". The primary purpose of these experiments is to
investigate
the sensitivity of the magnitude and direction of the parallel
plasma
flows (routinely observed in the edge plasma) to the magnetic geometry,
in
particular upper, lower and double-null configurations. Results were
limited
by poor startup reliability on this day. Nevertheless, strong parallel
plasma
flows were detected on the inner wall probe (ISP), always directed
toward
the inner divertor strike point (down for SNL, up for SNU), while the
outer
midplane probe (ASP) detected little or no flow at nebar~1.7 (SNL or
SNU)
and 1.4e20/m^3 (SNU) and a flow in the co-current direction for
nebar~1.0e20
(SNU). The latter co-current flow for low density has been seen
previously
in SNL discharges, suggesting that the mechanism responsible for it
is
independent of x-point location. The Chromex spectrometer saw some
significant
doppler shifts in the HeII line (He was puffed from NINJA on inner
and
outer midplane) from both inner and outer fiber views, corresponding to
Mach
numbers in the ~.2 range. This should allow us to get an independent
estimate
of the local flow velocities. Blobs seen on the PSI camera appeared
to
drift downward in SNL discharges but upward in some SNU discharges,
suggesting
that the flow fields (across B and/or parallel to B) had changed in
a way
that influences the blob motion.
ICRF Systems
-------------
The
ICRF system operated nominally last week, providing heating power in
support
of experiments as required.
Lower Hybrid System
-------------------
At
PPPL, the forward waveguide stack has been disassembled and is being
prepped
for replating. It is hoped that 25 of
the 50 plates will be
through the plating process by early July, after
which they will be baked
at MIT to assure the quality of the
plating.
At MIT, similations of the coupler during high power testing
are
underway. Initial results
indicate steps will have to be taken to
reduce the effects of the undriven
(passive) guides during the tests.
Work continues on the lower hybrid
active control system. Boards are
being
designed that will allow automatic testing and calibration of
this complex
system.
Travel and Visitors
-------------------
Stew
Zweben (PPPL) was at MIT 6/16-20 for edge turbulence measurements with
the
Gas Puff Imaging diagnostic. Good edge
turbulence imaging data was
obtained on about 50 C-Mod shots. These images showed poloidal and
radial
turbulence propagation with increased "blob"
intermittency nearer to the outer
wall.
Data was also taken during the up/down separatrix experiment and
the
fast H-L transition experiment.
Manfred Bitter and Ken Hill
came 6/18-20 and made significant progress on
investigating the problems
with the imaging X-ray crystal spectrometer data
processing
electronics. A complete checkout of all
electronic modules with
help from Willy Burke (MIT) and review of computer
settings with Korea
resulted in proper acquisition of signals from an
external X-ray source.
Argon spectra are now being obtained from C-Mod
plasmas. Detector and/or
electronics issues encountered at high count
rates are being investigated.
Gerd Schilling was at MIT 6/17-20 to
participate in ICRF operation. He
also
learned the procedures and computer settings for accessing the new
C-Mod
LINUX cluster remotely.
Douglas Loesser, PPPL, was at MIT
on Monday. He delivered two
stainless
steel forward waveguide plates to a local vendor for
stripping and copper
plating. After being returned from the
vendor,
the plates were baked in our oven over the weekend to verify
the
quality of the plating.
Monty Grimes was on travel last
week to San Antonio, TX, to inspect the
waveguide needed for the high
power launcher tests. He also
traveled
to San Jose, CA, to visit the vendor conducting the refurbishment
and
upgrades to two of our klystrons.
E3-I: This message has been scanned for
viruses and dangerous content by UML's antivirus scanning services.