From:
irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod
Weekly Highlights
Newsgroups: sci.physics.plasma
Organization:
MIT
Alcator
C-Mod Weekly Highlights
May
4, 2001
Alcator C-Mod is currently undergoing a 120C
bake that is expected to last into
early next week. Work continues on the ICRF, Lower Hybrid,
DNB, and diagnostic
systems.
Physics
-------
Significant
progress has been made in the past two weeks analyzing the
"two-frequency"
ICRF heating experiments of internal transport barrier (ITB)
modes in
C-Mod. These experiments were analyzed using the TRANSP code and
effort
was concentrated on a shot on which 0.6 MW of 70 MHz heating
power was
used to heat inside the transport barrier and also to control the
barrier.
Off-axis heating was provided by 2 MW of ICRF power at 80 MHz.
In this
shot, peaking of the density profile and the accumulation of impurities
were
found to be arrested by application of the central heating power.
TRANSP
analysis of this discharge indicates
the formation of a transport
barrier in Chi_eff at r/a < 0.45, during
the off-axis ICRF heating phase.
After turn-on of the 70 MHz ICRF power,
an increase in Chi_eff was found,
concomitant with the abatement of the
density peaking. However, a clear
barrier
in Chi_eff still persisted with Chi_eff remaining well below its
pre-ITB
value within the barrier region. The
ICRF power deposition and
energetic minority (H) tail evolution were
computed in TRANSP using the
FPP - TORIC ICRF module. The results of these
analyses will be reported
at the upcoming RF Topical Conference in Oxnard,
CA and the Transport Task
Force Meeting in Alaska.
We have
analyzed the ideal ballooning stability of the C-Mod edge in H-mode
using
kinetic EFIT runs based on Thomson scattering measurements of edge
Te and
ne profiles. This analysis is part of a
concentrated effort to
analyze stability of high and medium n ideal
ballooning, peeling modes, and
resistive ballooning modes, to understand
the physics of edge fluctuations in
the H-mode edge. For this analysis,
accurate reconstruction of the equilibrium
in the edge region is
important. Since measurements of edge
ion temperature
and current density are not yet available, we study the
effects of these
parameters on stability by using various assumptions to
model the edge profiles
for EFIT reconstruction. Thus, for ion temperature we use profiles with the
built in
assumption that Ti=Te and profiles which have no change in the Ti
gradient
in the pedestal region relative to the core region. For edge
current we use the profiles based only on
neoclassical plasma resistivity and
the measured loop voltage
("ohmic" profiles) and profiles with bootstrap
current
calculated with and without collisionality effects. Analysis of
infinite n ideal ballooning mode stability
carried out using BALOO code
showed that the pedestal pressure gradient is
generally unstable if no
bootstrap current is included in analysis.
However, taking into account the
bootstrap current driven by the pressure
gradient stabilizes the edge even
when the current is reduced
substantially by finite collisionality effects.
It is interesting to note
that two extreme assumptions for ion temperature
gradient lead to very
similar stability pictures. A reduced ion temperature
gradient leads to a
small reduction of total pressure gradient at the
edge (less than 20%
drop) and, since pressure gradients are normally
about 1.5 - 2 times
higher than first ideal stability limit, does not
affect the overall
results of the analysis.
Jim
Terry reported at the weekly Physics Meeting on his observations at the
April
meeting of the APS, which took place April 28 - May 2 in Washington D.C.
This
event is a general meeting of the APS. Jim presented an Alcator C-Mod
overview
poster entitled "Experiments in Plasma
Physics and Fusion
Science on the Alcator C-Mod Tokamak". It was one of six other
fusion posters
at the meeting. There was one invited talk from the fusion
community on
self-organized criticality in plasma transport. The participation
at this
meeting was an "outreach" by the fusion community to the rest
of
the physics community.
Operations
----------
The
C-Mod heater system was brought back online early this week in
preparation
for the 120C bake which began on Friday.
The CRYO system for
controlling liquid nitrogen to the magnets was
also brought online early
in the week.
The magnets are now cooled down to -25C to protect them
during the
bake. The ECDC system was brought back
into operation
briefly to check the gas control system and the magnetron
source. ECDC wall
conditioning
will begin early next week. The pulsed
gas system will be baked
out over the weekend and be ready for CAMAC cycle
tests early next week.
ICRF Systems
------------
We
continue to upgrade and test phase detectors and fault reporting
electronics
for the transmitters.
Delays are being measured on all fault signals to
ensure the
transmitters can be shut down in less than 10 us. Fast current
monitors for the transmitters have been
tested. S-Parameter measurements
of
the J-Port antenna indicate very little change from previous
measurements.
These measurements confirm our attempt to maintain a 50 Ohm
transmission line
while eliminating along field arc paths. Both the 78 and 70 MHz feedline
and
matching components have been fitup in the cell and tested successfully.
The
J-Port MKS pressure gauge cabling has been installed and the data
acquisition
tree prepared to read the new signal.
The response time of the
gauge and its proper operation will be
tested during the CAMAC cycle and
gas puff tests planned for early next
week.
All four FMIT transmitters have been tested into dummy loads
and are
ready for operation.
Chris Brunkhorst (PPPL) has sent
us his latest tuning models for the FMIT#3
and FMIT#4 transmitters.
Lower Hybrid MIE
Project
------------------------
Bids have been received by MIT
for the Lower Hybrid site modifications, and
they are in line with
expectations. Work is expected to begin
late summer
following the next run period.
We continue to work
on the control program for the active
controller. Parts
of the
program are being rewritten to improve
performance. Testing of the
new
arc detection concept continues.
DNB
---
Conditioning
of the DNB began this week with two days of arc only shots.
Arc currents of 150 Amps were quickly achieved
in an emission limited mode.
This amount of arc current is required for
full beam current operation.
One day of beam shots achieved beam currents
in the range of 4.5 A.
Inner Divertor
--------------
We
expect all inner divertor tiles to be in-house by late next week. Vendors
continue to work on the
divertor C and backing plates. The
inner divertor
mockup is proceeding quickly.
Power
Systems
-------------
All Alcator C-Mod power supplies are
ready for operation. The
Alternator
will be brought up to speed next week and new instrumentation
and sensors
will be tested.
Diagnostics
-----------
The
impurity injection laser has been replaced by Perry Phillips and Bill
Rowan
(UT-FRC). The replacement laser has
been tested to 2.4 J. This new
laser
will allow us to continue laser ablation of impurities for cold pulse,
impurity
transport, and spectroscopic experiments.
Travel and
Visits
-----------------
Martin Greenwald travelled to
Washington DC to attend a meeting of
the ESnet Steering Committee where he
presented a talk on Office of
Science programs and their network
requirements to a group of DOE
program managers.
Eric
Nelson-Melby traveled to the CRPP-EPFL in Lausanne, Switzerland
and gave a
seminar on C-Mod RF PCI results entitled "Phase Contrast Imaging
of
Ion Bernstein Waves in Alcator C-Mod."
Ron Parker, Miklos
Porkolab, Joe Snipes, and Spencer Pitcher attended the
Burning Plasma
Workshop II at General Atomics on May 1-3.
Options for a
future burning plasma experiment - Ignitor, FIRE, and
ITER-FEAT, were
discussed.