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
August
2, 1999
Plasma operations on C-Mod were interrupted last week to repair
an internal
gas feed line. Tuesday's run experienced a high incidence of
current-rise
disruptions, accompanied by unusual levels of iron influx as
determined by
spectroscopic diagnostics. The run was halted, and visual
inspection revealed
that a stainless steel gas line had broken and was
sagging into the discharge
region. On Wednesday the vacuum vessel was
vented to helium and the broken
line was removed, using special tooling
introduced through a gate valve at
K-port. Following a borescope inspection
to verify that no other internal
components were compromised, the machine
was pumped down and Electron
Cyclotron Discharge Cleaning (ECDC) was
begun. Following one day of ECDC and
low-temperature (55C) bake, plasma
operation was attempted on Friday. However,
vacuum conditions were not yet
satisfactory, and only short, resistive plasmas
were obtained. ECDC was
resumed and continued through the weekend.
Plasma operations are
planned to resume on Tuesday this week.
Once reliable
discharges are obtained, the primary focus will be on
ICRF conditioning.
Physics and Analysis
--------------------
Operation
of the omegatron probe during recent tokamak discharges has shown
that it
is still possible to collect impurity spectra during ICRF heating,
despite
a general increase in the edge plasma temperature and density at the
omegatron
location. The M/Z species present in
the spectra do not change
before, during, and after ICRF. The absolute currents of resonant ions
increase
during ICRF. The fraction of current in
the omegatron that is
resonant appears to decrease during ICRF, but this
may be due to decreased
collection efficiency. Efforts to optimize the component biases to reduce
non-resonant
ion collection during ICRF heating are in progress.
C-Mod abstracts
for the upcoming APS-DPP meeting (4 Invited talks, 11
Contributed Orals,
and 35 Contributed Posters) are now available on the Web at
http://www.pfc.mit.edu/cmod/aps99/abstracts.html
ICRF Systems
------------
All four transmitters
were prepared for plasma operation with the caveats that
FMIT#1 is limited
to 0.6-0.8 MW (anode dissipation limits total output power),
and FMIT#2 is
limited to 1.2 MW for reliable operation at the present voltage
tap
setting; FMIT#3 and #4 are ready for further conditioning of the J-port
antenna. In addition, several engineering issues were
addressed. To limit
the anode
dissipation in FMIT#1, we set the FPA anode current overload to 70A.
We
repaired and upgraded the screen crowbars in #3 and #4; the units now have
a
limiting 1 ohm resistor in series with the screen.
We began
two additional projects: installing overload relays for FPA grid bias
and
driver screen current and creating as-built schematics of the breaker
control
wiring.
A problem with FMIT#3 interfering with FMIT#4 was traced to
the antilog
converter that provides a voltage to the power feedback
circuit that is
linearly proportional to output power. Modifications were made and testing of
the
feedback circuit is underway. In
addition, FMIT#3 and #4 power meters
(the most accurate power measurement
device available) are now read and
processed in a normal shot cycle. FMIT#1 and #2 power meters are
undergoing
testing. To increase
the output power in FMIT#2, the high voltage needs to be
raised. The high voltage connection needs to be
improved before this can be
done reliably. We have begun the design process for this upgrade.
Travel
and Visitors
-------------------
Bruce Lipschultz went to the
APS DPP 'sorters' meeting where the program for
the November meeting was
organized (Thursday/Friday).
Josh Stillerman visited EPFL in
Lausanne, installing logbook/database software
and discussing general
MDSplus issues.
Dr. Yuichi Takase is visiting from Tokyo
University. He is primarily
here
to discuss ICRF modelling with Dr. Paul Bonoli.