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
Mar
15, 1999
Last week began a scheduled maintenance interval at Alcator
C-Mod. No runs
were scheduled. The maintenance period continues this week.
Preparations for a quick clean vent, scheduled for this week,
continue. We
will be installing a detector array designed to measure
deuterium Lyman alpha
emission with high spatial resolution. It will have a tangential view,
slightly
below the midplane with 20 channels, spanning 6 cm, with a nominal
radial
resolution of 3mm. This will help in diagnosing neutral density
and
ionization profiles near the separatrix.
We plan also to verify
alignment of the Omegatron, located on a vertical port,
with modifications
to its support to allow for small rotations about its
supporting
axis. Additional work includes
inspection of invessel components,
such as the new RF antenna and
limiters, with a boroscope.
Physics Analysis:
-----------------
New
high spatial resolution magnetic fluctuation measurements of Type III ELM
precursors
have verified that the toroidal and poloidal mode numbers are
typically n
= 10, m >~ 16 with oscillation frequencies of 160 kHz. The modes
are strongly ballooning in
the sense that they are visible only on the outboard
coils and not on the
inboard coils even though the rational surface is closer
to the inboard
coils than the outboard coils. It is
now possible to also
compare outboard coils that are beneath molybdenum
tiles just like the inboard
coils and the ballooning of the ELMs is still
present, so the lack of
observable oscillations on the inboard coils is
not an effect of attenuation at
high frequency due to the molybdenum
tiles.
The tangentially viewing divertor camera system is installed
and operating.
The new camera system contains two cameras viewing the
divertor tangentially,
each filtered for a different deuterium Balmer
spectral line (i.e., D_alpha
and D_gamma). Images from recent shots have been analyzed and compared to
the
previous campaign. Most
inversions of images show features very similar to
those from the last
campaign, although the spatial resolution from the recent
inversions is
now approximately 5mm, a factor of about 2 better than the
resolution in
previous inversions. The typical
features are: strong emission
on the inner divertor ``nose,'' and strong
emission along the outer divertor
leg when the outer divertor is detached.
However, images recorded during Ohmic
L-mode shots imply a plasma in the
private flux region of sufficient
temperature and density as to dominate
the deuterium emission. This
observation
is confirmed by the divertor Langmuir probes showing a higher
plasma
density below the inner strikepoint than nearly on it. Analysis of the
mechanism generating
the private flux plasma is continuing.
RF Systems:
-----------
J-port
ICRF system:
FMIT Transmitter #4 has now produced 2.2 MW at 78MHz
for 1 sec into a dummy
load. The
tuning was done with the essential help of Chris Brunkhorst and
Gary
D'Amico from PPPL. Work began on
retuning FMIT#3 to reduce the reflected
power levels between the driver
and final amplifiers. An ignitron
was
replaced in FMIT#3's crowbar cabinet.
The 78 MHz resonant loops have been
installed along with the
decoupler loop. The stub dimensions
have been
determined and the stub is being fabricated. The resonant and decoupler loop
dimensions
for 60 MHz have also been determined.
The decoupling at 60 MHz is
more sensitive than 78 MHz and will
require more investigation. The first
operation into plasma is planned to
be at 78MHz. Remote operation of FMIT#3
and #4 was re-established after
considerable effort. The remaining
critical
path items for this week are retuning FMIT#3 and installing the
9" coax in the
cell.
D- and E-port ICRF system:
An
inspection of the 9" coax revealed that a number of elbows had poor
contact
between the anchor bullet finger assembly and center conductor due
to
oversized teflon supports. This
poor contact resulted in arcing at an elbow
that was near a high current
location. An inspection of the D-port
phase
shifter revealed that it was in good condition. Several factors could have
contributed
to the observed arcing in the D-port stub tuner. First the
push/pull rod hair pin assembly had been counter
bored leaving a sharp edge
that would enhance the local electric
field. Second the gap between the
rod
and copper conductor was occupied by a dielectric having a lower index
of
refraction than the rod, resulting in a locally enhanced electric
field.
Finally the loss tangent of the rod may have resulted in signficant
local
heating during vacuum conditioning.
The proposed solution is to install a
corona ring in the counter
bore that should reduce field enhancement due to
sharp edges and remove
the gap between the rod and conductor.
The rep rate of
vacuum conditioning will also be reduced. The tuner repair is awaiting the
soldering
of a new corona ring. The problem with
E-port's PLC has been
rectified. A
more robust design will be installed in the near future to make
the system
more reliable.
Diagnostic Neutral Beam:
-----------------------
Progress,
as measured by the target schedule, continues to be satisfactory.
The beam
pulsed high voltage supply has been tested and operated at full
performance
and is ready for operation. Testing of
high voltage crowbar
controls has been completed. Cleaning, replacement, and testing of
dielectric
oil for the arc notcher transformer has been completed. Problems were
encountered with the PC
board routing software, and the layout of the tube
regulator monitor
circuit boards has been delayed.
Travel and Visitors:
--------------------
Chris
Brunkhorst and Gary D'Amico from PPPL were onsite last week working on
tuning
of the ICRF transmitters.
Dave Mikkelsen (PPPL) spent the week at
MIT becoming familiar with the MDSplus
tools for generating and displaying
TRANSP runs based on C-Mod data. These
will be used as the starting point
for comparisons with various transport
models.
Paul Bonoli
attended a workshop at General Atomics last week (March 9-11,
1999) on the
Physics Requirements for Advanced Tokamaks. Paul was one of the
meeting
organizers, along with Ed Synakowski (PPPL) and Alan Turnbull (GA),
and
gave a talk on "Lower Hybrid Current Profile Control Studies in
Alcator
C-Mod".
Miklos Porkolab also attended the
"Physics Requirements for Advanced Tokamaks"
workshop on March
9-11 at General Atomics, where he made the following
presentations:
1.
Overview of the C-MOD AT Program
2. ITB formation Experiments in C-Mod:
PEP Mode During Current Ramp-Up
and ITB Induced by H/L Transitions.
He
also led the Discussion Session:"What Issues Remain for MHD Stability
?"
Miklos also had discussions with Ed Synakowski(PPPL) and
Keith Burrell(GA)
about the role of sheared electric fields and flows in
the termination phase
of past PEP experiments with ICRF and pellet
injection. A new interpretation
of the collapse of the PEP has emerged
from these discussions, which may be
tested in future experiments on C-Mod
and DIII-D. If the interpretation is
correct, it may be possible to extend
the PEP duration by spinning the plasma
in the counter current direction
with either beams (DIII-D) or mode converted
IBW (C-Mod).
Ian
Hutchinson participated in the Fusion Facilities Coordinating Committee
(FFCC)
meeting in San Diego, and also attended some sessions of the AT
Workshop.