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
Apr
5, 1999
Alcator C-Mod resumed plasma operations last week. Three runs
were scheduled,
but only one was completed. The scheduled runs were
primarily dedicated to
check-out and conditioing of the ICRF systems.
During the run on Wednesday, it
was determined that an arc had occurred in
the E-port transmission line, as
described below, and the Thursday and
Friday runs were cancelled so that the
line could be repaired.
Aside
from this arcing problem, the run on Wednesday was successful and
productive.
This was the first day of plasma operations following several
weeks of
maintenance activities, during which a "clean" vent of the
vacuum
system was performed. The machine condition was good. Startup
reproducibility
was high, and impurity levels, including Mo, were
low. EDA H-modes were
readily
obtained at about 1MW of RF power. The RF power was brought up to
1.7MW,
mostly from the D-port antenna.
Several piggyback experiments were
carried out. The FSP fast scanning probe
was observed on the video system,
in preparation for plume imaging
experiments. Ron Bravenec (U. Tx)
completed installation and first operation
of the BES system by observing
continuum emission to the red of D-alpha.
All
eight channels produced signal with low noise. Edge-peaked fluctuations were
observed
above 50 kHz. Data was obtained on the Omegatron edge plasma
diagnostic,
which had been aligned during the maintenance period. This probe
combines
a gridded energy analyzer and an ion mass spectrometer. Analysis of
these
data is proceeding.
Physics & Analysis:
-------------------
First
results from the new Kaiser spectrometer have been obtained. The Kaiser
system consists of 14
spatial chords looking tangentially at the plasma
boundary (spatial
resolution ~ 3 mm). In addition to
observing Halpha and B
II profiles, the spectrometer views three He I
lines emanating from a helium
gas jet which is fixed to the side of the AB
split limiter. Using the ratio
of
the three He lines, it is possible to derive the local ne and Te
profiles.
This work is in collaboration with PPPL (which has loaned the
spectrometer)
and TEXTOR (which has supplied the theoretical line
ratios).
First ne and Te profiles are very encouraging. Full profiles from several
centimeters
outside the separatrix to approx 1 cm inside have been obtained.
There is
reasonable quantitative agreement with the ASP (probe at
approximately the
same location), particularly with respect to the density
profile.
We
will continue to obtain data with the new system and hope to provide some
scalings
for separatrix values of ne, Te and their gradients.
ICRF
Systems:
-------------
Initial testing into plasma indicated an
arc in the E-port transmission line
between the transmitter and stub tuner
that prevented increasing the power
beyond ~400 kW. The initial damage was probably done during
testing with an
improper switch set ting.
Upon disassembly, an elbow assembly was found to
have arced. The elbows were replaced with spare elbows
and tested under
matched and unmatched vacuum conditions. To avoid a repeat of this problem, a
proceedure
will be put in place requiring visual inspection of switch
positions. A
lockout and monitoring system will also be implemented. On
FMIT#1 and #2, the FPA screen
current limit was changed from 4 A to 7.2 A
(recommended screen current
limit). This change should eliminate
some screen
overcurrent crowbars that occurred during the last
campaign. With the help of
Chris
Brunkhorst and Gary D'Amico (PPPL), a general survey of FMIT#1 and #2
were
conducted to establish documentation of present system and to identify
systems
that limit output power.
The J-port 9" coax has been
reassembled, including the phase shifter
#3. The phase shifter required reconditioning of the push/pull
rod
connection. The system was
evacuated and is currently pressurized to
5 psi N2 (15 psi will be the
initial operating value). All the
Mega
coax elbows have been inspected and those that failed were
modified.
Vacuum tuning and testing will begin shortly.
Travel
and Visits
------------------
Gary Taylor returned from PPPL to
work on the 19-channel grating
polychromator. He modified the filters on the fast channels, which gave
improved
signal to noise. During the maintenance
break, he completed
analysis of electron power deposition in some
previous RF modulation
experiments, which showed good agreement with TORIC
and other RF codes.
Tom Fredian has returned from his trip to the
Australia National University in
Canberra, Australia, March 8-19 to
install MDSplus for use on the H-1 Heliac
experiment.