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
March 20, 2000
Work
continued last week on the Lower Hybrid, ICRF, DNB, and Power systems.
We
are now beginning pre-operations checkouts on all the major machine
systems
in preparation for power supply tests into the magnets scheduled
to begin
this week. ECDC continued
all week to prepare plasma facing components
for operation.
Engineering:
The
klystron test stand is coming together rapidly. Wiring of the
Modulator/Regulator for the klystron is nearly
complete. Power has been
applied
to the control rack and testing has begun. Fabrication and wiring
is complete on the transmitter
protection system (TPS). The first
klystron
to be tested has been loaded into the test stand socket and water
cooling
lines have been connected.
The high voltage control rack wiring is complete.
The high voltage
power supply is ready to be wired into the test stand
and checked
out. Components from the Alcator C
lower level 4.6 GHz RF
system have been checked out and those useful for
the new system identified.
On Thursday and Friday of last week we changed
the bus configuration from
normal to reversed field. We are starting out with reversed field
this
campaign so that the initial startup time time can be combined
with
development of a robust reversed field startup. Other work on the power
systems included
a general cleanup of the bus and cabinets, the addition
of four new
signals to better instrument the alternator exciter supply, and
re-calibration
of hi-yard current and voltage monitors.
A new set of digitizers has
been installed in the magnetics rack.
We hope
these new units will improve the reliability of the
magnetics signals which
were on some occasions lost because of CAMAC
problems during previous
campaigns.
Work continued on the DNB
last week. To obtain somewhat more
filament
current the filament bus leads were increased in size and the
filament
power supply was adjusted for maximum voltage. Low filament current
had been limiting
the filament emission during DNB operation in the
Cell. The DNB will operate with the C-Mod shot
cycle this week during
power tests.
We continue to bring up the
ICRF systems for the next campaign.
Vacuum
conditioning of the antennas should begin this week. During his visit
last week Chris
Brunkhorst gave us the spreadsheet he uses for tuning
the PPPL transmitters. Using this program has allowed us to improve
the
tuning of all the transmitter stages.
FMIT#3 is the only transmitter
awaiting final tuning.
Physics:
Calibration
of the integrator circuits for the main magnetics signals
indicated
systematic changes in the time constants in the range of
1-3% (over the
past five years), with higher gain (lower RC) circuits
exhibiting the
bigger changes. The effect of applying
the revised
calibrations to EFIT analysis of several shots from the last
campaign was
evaluated. Use of the new values moves the outboard midplane
location of
the LCFS in by 2-3mm relative to the uncorrected analysis; the
change in
the inferred boundary is largest at the outboard side near the
midplane,
and essentially vanishes near the x-point and on the inboard
side. These
results tend to bring
the EFIT reconstructions into somewhat better
alignment with edge
diagnostics such as the A-side scanning probe,
although discrepancies of
3-5mm remain unaccounted for.
A new head for the A-side fast
scanning probe is being made with a poloidal
field pick-up coil mounted
inside the probe head so that it will be very near
the plasma at full
insertion. The probe head will be made
from boron nitride
to allow high frequency field fluctuations to penetrate
to the coil. The coil
is about the
size of a pencil eraser and is ceramic coated nickel-clad copper
conductor
wound onto a boron nitride spool. The
coil will be positioned about
9 mm back from the tip of the probe. We expect the new head to be close
enough
to measure the 100 kHz fluctuations that are observed in EDA H-mode on
the
Langmuir probes, reflectometer, and PCI diagnostics.
Being within about one wavelength of the pedestal, magnetic
fluctuations, if
present, will be observable on the magnetic pick-up coil.
The oscillations are
localized in the density pedestal and k_r is measured
to be approximately
5 cm^-1. A mode with properties like the one observed
has been seen in
electromagnetic gyrofluid simulations by Rogers and
Drake. The modes seen in
the simulations have a significant magnetic
component, so this coil should
provide an important experimental test of
the theory. The coil has already
been
made and absolutely calibrated. The
coil area is about one third of the
area of the existing pick-up coils
mounted on the outboard limiters.
Initially, the probe will be inserted
into Ohmic EDA H-modes to avoid the
higher heat flux associated with RF
heated H-modes and allow deeper
penetration of the probe. In a second stage, the probe will be inserted
into
RF EDA H-mode to also look for the high frequency (~600 kHz) modes
observed
on the outboard limiter pick-up coils to determine their radial
decay and
whether or not they are resonant at the edge of the plasma.
Last
week the physics meeting was broadcast using RealPlayer
(
http://www.real.com ) streaming video.
This technology gives
collaborators
access to the meeting using standard WWW tools.
Tune in to future meetings
with RealPlayer at
rtsp://www.psfc.mit.edu/encoder/nw17.rm
Travel
and Visits:
Chihiro Takahashi from the NIFS in Japan visited Tom
Fredian and Josh
Stillerman to discuss the use of MDSplus on the CHS
experiment. They hope to
migrate from their current data system completely
to MDSplus by the end of
this year.
Miklos Porkolab attended
the US-Japan RF Workshop in Princeton,
on 3/14/2000. Paul Bonoli also attended (3/14-3/16) and
gave a talk
entitled "Advanced tokamak physics studies in the Alcator
C-Mod device".
He also spent one day working with PPPL staff on
full-wave ICRF modelling
of high harmonic fast wave heating in the NSTX
device.
Chris Brunkhorst, from PPPL, was at MIT from 3/14-17 and helped with the ICRF
transmitter
tuning and checkout in preparation for the next run campaign.
Gerd
Schilling spent last week at PPPL, finishing the C-Mod
collaboration
Field Work Proposal and presenting the C-Mod
contribution to the US-Japan
RF Workshop, March 14-16, "Upgrades and
Recent Results from the
Alcator C-Mod ICRF Program."