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
Newsgroups:
sci.physics.plasma
Alcator
C-Mod Weekly Highlights
May
12, 2003
Plasma operations continued at Alcator C-Mod last week,
with four run days
scheduled and completed. A total of 56 plasma
discharges were produced, with a
startup reliability over 80%. The H/(H+D)
ratio is below 10% and the RF
systems have been conditioned up to 4.5MW
total power (through three
launchers). One run was devoted to initial
studies of locked mode generation
and suppression with the new
non-axisymmetric control coils. Progress also
continued on the DNB and
Lower Hybrid Systems.
Plasma operations are scheduled to continue
this week, following the first
boronization of the campaign. Three run
days are planned.
Operations
----------
The C-Mod
tokamak operated for four days last week. Runs on Tuesday, Thursday
and
Friday were primarily devoted to RF conditioning and continuation of
clean-up
discharges. Tuesday's run was scheduled to start late to allow
completion
of work on the RF systems, and was further delayed by a computer
outage
and a TF instrumentation problem. Several shots last week were
terminated
prematurely due to noise on bus instrumentation resulting in
spurious
fault indications. Following two such events on Thursday, the run was
suspended
and a series of power system tests were run in an effort to
determine the
source of the noise; these tests, which continued on Friday
morning, were
inconclusive. Plasma operation was then resumed. The origin of
this
intermittent noise is still under investigation.
Physics
-------
Stewart
Zweben has begun to analyze the motion of the plasma edge turbulence
in
high speed C-Mod videos taken with the Princeton Scientific Instruments
cameras. He has also made a new movie showing 15 of
the best shots from the
2001 run.
The movie can be seen at:
http://www.pppl.gov/~szweben/C-Modvideo/cmod_video.html
Wednesday's
run was primarily devoted to MP#331, an initial study of
error-field
effects using the non-axisymmetric control coils (A-coils). Locked
modes
were induced in a 1MA, 2e20/m^3, 5.4T discharge using a
"quadrupole"
configuration of four control coils, which produces
the maximum ratio of 2/1
to 1/1 perturbation at the q=2 surface of about
0.8. The threshold amplitude
for production of a locked mode in this
configuration was determined to be
0.4e-3<|B_r21|<0.55e-3 tesla,
uncorrected for any background non-axisymmetry
arising from PF coil
misalignment. The same A-coil configuration operating
with the opposite
polarity was successfully used to suppress a "naturally"
occurring
locked mode in a low-density 1MA discharge. Additional experiments
with
different helical field configurations and toroidal phase were not
observed
to produce locked modes in the 2e20/m^3 target discharge. Detailed
analysis
of these results is underway.
ICRF Systems
-------------
Conditioning
of the antennas, particularly J-port, into plasma discharges
continued
last week. The main purpose was to
increase the power handling
under L-mode conditions. The conditioning was slowed by a problem
with J3
stub tuner that was rectified on Friday. Digitizer problems also slowed the
conditioning process. Nevertheless, by end of Friday J-port was
operating
reliably at 2 MW and had short pulses up 2.5 MW.
A
control problem with D-port limited its availability. The directional
coupler at the transmitter output, DC0,
appears to intermittently under report
the actual power. The DC0 is used in the power feedback,
causing the output
power to exceed the intended demand. During testing of this system, a rf
switch
was observed to arc at high power. The
arcing was eliminated after
re-seating the switch.
DNB
Diagnostic Systems
----------------------
The diagnostic
neutral beam continues to perform well, firing on nearly all
C-Mod shot
cycles for the Tuesday-Thursday runs (and also between shots) at
49-50 kV
and 4.0-4.7 A.
It is now realized that the primary source of pitch
angle variation in the
MSE data is actually due to a windowing apodization
issue in the analysis
methods, and not to variation in the measured
signals. Improvements in
the
analysis methods are being investigated to avoid this issue, which
should
yield much less variation in the pitch angle results.
Lower Hybrid
System
-------------------
Testing of the 12th klystron was
completed last week. The first of the
external circulator waveguide
assemblies is being readied for installation.
Two additional
klystrons, which will be used as spares for the initial phase
of LH
operation, have been shipped out for refurbishment and an upgrade to the
cathode
performance.
A meeting was held at MIT Tuesday-Wednesday to discuss
the Lower Hybrid
launcher status; participants were Stefano Bernabei, Rich
Hawryluk, Joel
Hosea, Doug Loesser, Gerd Schilling, and Randy Wilson from
PPPL and Bill Beck,
Bob Childs, Monty Grimes, Amanda Hubbard, Ian
Hutchinson, Jim Irby, Earl
Marmar, Ron Parker, Miklos Porkolab, Rui
Vieira, and Steve Wukitch from MIT.
A list of observed issues was
generated, and tentative schedules for
addressing these and conduct of
pre-installation testing was developed.
An engineer from OPAL-RT was
onsite all last week consulting on interface
issues for the LH Control
System and training C-Mod staff on the OPAL-RT
system. Programming
modifications to allow direct communication between the
OPAL-RT system and
Mdsplus were carried out and tested successfully.
Travel and
Visitors
-------------------
Stefano Bernabei, Rich Hawryluk,
Joel Hosea, Doug Loesser, Gerd Schilling,
and Randy Wilson were at MIT
5/6-7 for discussions on the Lower Hybrid
launcher.
Miklos
Porkolab, Martin Greenwald and Joe Snipes participated in the US ITER
forum
at the University of Maryland, May 8-9. Greenwald presented a proposal
for
US leadership in data system software for ITER, and Snipes presented a
proposal
for an Active MHD Spectroscopy system.