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
9, 2001
Plasma operations continued at Alcator C-Mod last week. Three
run days were
scheduled, but only one-and-a-half were completed, due to a
problem with
switchgear in the RF power system on Tuesday, and a broken
in-vessel
instrumentation cable which necessitated an up-to-helium vent
on
Thursday-Friday. Eighteen plasma shots were produced with a
startup
reliability of 75%.
Plasma operations are scheduled to
resume this week.
Physics and Operations
----------------------
Tuesday's
run was intended to carry out MP#246, to measure profiles of
toroidal
rotation in high-power ICRF-heated EDA H-mode plasmas. The start of
the
run was delayed by nearly six hours because of a failure of a mechanical
component
in the switchgear providing AC to the FMIT#4 ICRF transmitter, and
by an
overtemperature indication in an alternator bearing, subsequently traced
to
faulty instrumentation. Once these problems were remedied, a few useful
shots
for this MP were obtained, but the MP was not completed.
The run
scheduled for Thursday was in support of MP#291A, which aims to
determine
whether the spontaneous internal transport barrier (ITB) formation
observed
with off-axis ICRF heating can also be produced when the ICRF
resonance is
on the low-field (outboard) side of the axis. A previous attempt
to
generate an ITB with a low-field-side resonance was unsuccessful, but
there
was speculation that the available RF power at that time may have
been
insufficient. The experiment began with a repetition of the
conditions under
which an ITB was produced with high-field-side ICRF
resonance, and the
phenomenon was again observed. However, the shots were
plagued by disruptions
and impurity injections. After a number of such
disruptive discharges, the
problem was identified using the wide-angle TV
view: a broken cable was moving
into the plasma near the outer divertor,
resulting in the impurity injections
and disruptions. The cable was
identified as being one of a set connected to
the new B-dot probes on the
J-port antenna.
The vessel was back-filled with helium, and a
"clean" vent carried out on
Friday. The broken cable, and two
others in the same set, were removed. Two
additional cables, and a
remaining broken-off segment of a third, which could
not be removed, were
secured and left in place. The vessel was pumped down on
Friday afternoon
and a bake to 120C carried out over the weekend. Glow
cleaning was begun
on Sunday.
Operation will resume this week with clean-up and
recovery discharges. We
anticipate that several days of such operation
will be required, after which
physics experiments will be resumed.
Piggyback
experiments last week continued to exploit the capabilities of the
new
ultra-fast Princeton Scientific Instruments camera. We are using it to
make
two important, but quite different, measurements. First, it is being used
to
measure the radial profile of the poloidal magnetic field, where the
changing
tilt of the "cigar-shaped" Li^+ ablation cloud is imaged as the
pellet
traverses the plasma. (The Li^+
ablation cloud is constrained to be
parallel to the local magnetic field.)
Based on the images, a preliminary
profile of the field pitch angle has
been contructed, and we are now
attempting to measure the pellet position
more accurately.
The second measurement we have undertaken with this
camera is to follow the
evolution of turbulence found in the SOL. We have
found previously that the
edge turbulence is "bursty", that is,
occurs with a higher incidence of large
density regions than would by
expected from a normal distribution. These
bursts have a lifetime of
~10-50 microseconds. With the camera we are able to
observe the growth,
movement, and disappearance of the emission from the
high-density
clumps. To date we have done this using
a radial view where they
appear as toroidal filaments. We find that at
least some of the filaments move
poloidally with a speed of ~200 m/s. We
will soon move the view so that we can
resolve both poloidal and radial
motion.
ICRF System
-----------
With the
exception of the mechanical failure in the FMIT#4 switchgear which
contributed
to the delay of the run on Tuesday, all four ICRF systems
functioned well
during physics experiments last week. Total power levels up to
3.5MW were
provided. All antennas heated well, and excellent EDA H-mode
discharges
were produced.
Following the discovery of the broken coax line on
Thursday, it was decided
that, in view of the short amount of time
remaining for this experimental
campaign, the frequency changeover from
78MHz to 70MHz for the FMIT#3 and 4
transmitters and the transmission line
to the J-port antenna should begin
immediately. This decision was taken to
provide the maximum opportunity to
carry out the planned experiments which
require the lower frequency heating
component, particularly double barrier
experiments with combined on- and
off-axis heating.
Re-plumbing
of the J-port transmission line for 70MHz is substantially
completed.
Re-tuning of the transmitters has begun, and will continue early
this
week, with the assistance of a PPPL engineer.
Diagnostic Neutral
Beam System
-------------------------------
A beam-into-gas
experiment was completed early in the week.
The gas pressure
in the C-Mod vessel was scanned to 1 mT, a
pressure predicted to induce at
least 60% reionization of the beam. The experiment will be interpreted for
diagnostic
calibration and as a reionization benchmark.
The beam was
instrumented to look for the cause of short arcs, one
of the nuisance problems
which does not prevent beam operation, but does
result in loss of data.
Inner Divertor Fabrication
Project
----------------------------------
All 12 Inconel
C-plates are now in-house. Work is proceeding on the mock-up,
and on
developing installation procedures.
Lower Hybrid MIE Project
------------------------
The
Transmitter equipment rack High-Voltage Control sub-chassis wiring has
been
completed. Work continues on the Transmitter Protection System (TPS) rack
wiring.
PLC I/O rack modules have been ordered in preparation for HVPS
acceptance
tests in October/November. Equipment to
make the User Interface
Control Station was specified and will soon be
ordered. This will also be
needed
for the HVPS tests. Work began on the
User Interface Control Station
screen display software.
Visitors
and Travel
-------------------
Howard Yuh attended the
PPPS-ICOPS meeting in Las Vegas from
June 20-22 to present an overview
oral on the C-Mod EDA mode
focusing on the modelling efforts and progress
made with the
new C-Mod Beowulf parallel Linux computer cluster.