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
August
14, 2000
The 120 C vessel bake ended on Tuesday of last
week after one week at
temperature.
Leakchecks were completed Wednesday morning, and ECDC was
started
by the afternoon. The vessel is
cleaning up well, and we plan to
complete power tests and begin plasma
operation by the end of this week.
The Alternator and RF systems are also
being brought back online.
Engineering
-----------
Pre-operation
tests and calibrations of the magnet power systems were
completed
successfully last week. They are now
ready for operation. We have
also
begun testing and bringing up alternator systems.
All klystrons for
the Lower Hybrid Project have now been tested.
We have
fourteen tubes qualified for operation, giving us two spare
tubes.
Two out of sixteen tubes showed a similar type of filament
fault.
These tubes will not be needed for the initial LH 3 MW operation
scenario,
but will have to be repaired for 4 MW operation.
All
four FMIT transmitters are delivering more than 1.8 MW of power into
a
dummy load, and are ready for plasma operation. The transmission lines
are
reassembled and ready to be pumped down and purged as soon as new
manifolds
are installed. Vacuum conditioning of
the antennas will begin
later this week.
Work also continued on
the new glow discharge system. The glow
discharge
switch control chassis wiring was completed. The control chassis and HV
chassis were
bench tested. A second glow paddle was
completed and installed
on our vacuum test stand and the system pumped
down. The glow system was
installed on the vacuum test stand and both
probes were tested successfully,
one at a time, using the one available
control board. Glow discharge
currents
of over 4 amps, at 600 volts were produced and controlled.
This test
included operation of the PLC and Paragon control systems.
The second
control board is now complete and will be tested early this week.
Physics
-------
New
data from June 2000 runs was added to the H-mode Threshold database,
including
low density limit H-modes and hydrogen H-modes for isotope scaling.
An
analysis of the new data showed that the lowest density in which H-modes
are
achieved on C-Mod is slightly lower than before, now reaching as low
as 5 x
10^19 m^-3 whereas previous data found H-modes only as low as about
6 x 10^19
m^-3. Note that the
H-modes are much more difficult to achieve at these low
densities
requiring about 2.5 times the standard 1999 H-mode threshold scaling
power
(P (MW) = 0.054 nebar^0.49 BT^0.85 S^0.85, with nebar in 10^20 m^-3, BT
in
T, and S in m^2), which was a best fit to 10 tokamaks in the
International
H-mode Threshold Database for deuterium plasmas. Note that these are also very
marginal
H-modes which drop easily out of H-mode and never achieve good
confinement. An analysis of the H-mode run in hydrogen
plasmas showed that the
threshold was indeed higher in hydrogen than in
deuterium, as expected from the
1/M scaling found comparing H, D, and T
plasmas in JET. These Ohmic
H-modes
required a threshold power about 1.7 times higher than the 1999
threshold
scaling above. Note that
the ratio H/(H+D) ~ 0.6, so we did not achieve 100%
hydrogen plasmas,
which would be expected to require twice the deuterium
threshold power
based on the JET results. Within errors
of about 10 - 15%,
these results are in agreement with the JET scaling for
hydrogenic plasmas.
Travel and Visits
-----------------
David
Mikkelsen spent the week at C-Mod continuing his work on transport
modeling. Stewart Zweben was at C-Mod 8/8-10 working
on the external
connections and
hookup for the new edge turbulence imaging diagnostic.
Gerd Schilling spent
the week at PPPL on collaboration
administrative work
and discussions on diagnostic and rf topics.
All
of the collaborators on the heterodyne ECE diagnostic met at C-mod this
week
to discuss progress of and plans for the instrument. These included
Perry Philips, Romik Chattergee and Alan Lynn of U. Texas, Christopher
Watts
of Auburn, John Heard, Clarion Univ., Yongkyoon In of Univ. Idaho,
and
Amanda Hubbard of MIT. The
diagnostic is now operational, and several ideas
emerged to improve its
poloidal resolution, which is important for
fluctuation
measurements.