From: WOLFE@PSFC.MIT.EDU
Date:  4-JUN-2001  11:20:36
Subj: Alcator C-Mod Weekly Highlights                         

		Alcator C-Mod Weekly Highlights
			June 4, 2001
 
Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled, but only two were completed, due an unscheduled "clean" vent to
retrieve a detached viewing dump. A total of 14 plasma shots were obtained,
with a startup reliability of about 65%.
 
Progress was also made on the Divertor upgrade and Lower Hybrid projects.
 
Plasma operation is scheduled to continue this week. An extra run day has been
scheduled to make up for the time lost to the vent.
 
Operations
----------
 
ECDC and a low temperature (60C) bake were carried out over the long weekend
and during a maintenance day on Tuesday.
 
Wednesday's run was devoted to continuation of machine clean-up and
conditioning and RF conditioning of the D&E port dipole antennas. The density
was increased to ~1.6e20/m^3 with flattop currents of 800kA, at our standard
operating field of 5.4T. At least as long as divertor detachment did not
occur, the density was maintained by feedback on the gas puff, rather than
being dominated by wall fueling as had been the case in the previous week,
indicating the machine was cleaning up. The H/D ratio was between 0.3 and 0.6,
a considerable improvement over the previous week, when it was over
2. Between-shot vacuum conditioning of the J-port antenna was carried out in
preparation for operation into plasma. Development of a lower triangularity
shape suitable for a planned experiment matching ASDEX-Upgrade non-dimensional
parameters was begun.
 
Late in the run on Wednesday a viewing dump for the visible bremmstrahlung
array became detached from the outer wall and came to rest in the divertor
between B and C ports, where it was observed on one of the divertor camera
views. The run was halted, and on Thursday morning the machine was vented to
helium and the dump, a small piece of black-passivated stainless steel shim
stock, was removed using tooling manipulated from outside C-port. The system
was then pumped down again and, following overnight ECDC and a 60C bake,
operation resumed at noon on Friday, despite rather high H2O levels on the
RGA.  Not surprisingly, the vent resulted in significantly degraded machine
conditions. While the startup reliability was high (90%), the plasmas were
resistive, and short-lived. Discharges were limited to currents less than
about 500kA and pulse lengths of about .25 seconds. The machine conditioning
program has therefore been set back several days.
 
Additional baking and ECDC were carried out over the weekend, and operation
will be resumed on Monday.
 
Physics and Diagnostics
-----------------------
 
A comparison was made between the collisionality calculated at the q=1.5 and
q=2 surfaces in C-Mod with several scalings for Neoclassical Tearing Mode
(NTM) thresholds in DIII-D and ASDEX-Upgrade for a large number of high beta
shots (1.2<beta_N<1.7).  In a couple of these shots, large low frequency MHD
modes
were observed that increase in amplitude with increasing beta during the
discharge and then finally lock at an amplitude of about 50 G measured at the
wall and lead to a beta collapse.  Most of the C-Mod data lie at substantially
higher collisionality than the DIII-D and ASDEX-Upgrade NTM data, but the two
discharges with large MHD modes fell close to each of the
DIII-D and ASDEX-Upgrade scalings in terms of dimensionless parameters
betan/rhostari and nustari.  The C-Mod rhostari values for these discharges
fell in the range of 0.004 to 0.007 and the collisonalities ranged from 0.01
to about 2.0.   While the proximity of the threshold from these other
machines to these two shots with large MHD does not prove they are NTM's, it
is further evidence suggesting that these higher beta discharges at low
collisionality are at least approaching the NTM threshold.
 
Initial data was collected from the in-vessel Penning ionization gauges during
the May 25 startup discharges. These cold cathode ionization gauges,
mounted to the vacuum vessel wall at four locations, use the ambient magnetic
field inside the tokamak for operation. Three gauges are positioned in the
main chamber to record toroidal and poloidal variation in the neutral
pressure. One gauge is located in the lower divertor to record fast changes in
the divertor neutral pressure when the divertor bypass flaps are opened.  The
main-chamber gauges appear to be operational. No-plasma calibration shots
indicate that their signal is proportional to the torus neutral
pressure. Initial data during plasma operation indicate weak toroidal
variation in the neutral pressure (less than a factor of 2) and strong
poloidal variation (up to a factor of 5).  In order to avoid a saturated
signal, the divertor gauge must be operated in a reduced bias mode where a
non-linear pressure response might be expected. Nevertheless, this gauge
appears to provide information on the rate of change of pressures in the
divertor when the bypass flapper is opened.  Initial results suggest that the
transient neutral exhaust rate through the C-divertor flapper is small (~1
torr-liter/s). Further experiments are planned.
 
All channels of the FRC ECE system are working, and calibration is in
progress.  Some shots were lost because of CAMAC problems.  These
problems are being resolved.
 
 
ICRF System
------------
 
The J-port antenna was vacuum conditioned to ~32 kV and the first (of
three) tuning calibration factor, DC1, was obtained in preparation for
plasma operation. The remaining 2 calibration numbers require plasma
discharges to obtain.  We have continued to cross check power, voltage
and current measurements to properly monitor and characterize the
system performance.  D and E-port antennas have continued to operate up
to 0.5 MW each when plasma operation has permitted.
 
 
Diagnostic Neutral Beam Systems
-------------------------------
 
The DNB was baked for approximately 40 hours.  In the past, a bake
preceded the lowest water concentration, but several additional days
of beam shots were also necessary.
 
A small number of DNB shots are lost each day because of nuisance faults that
prevent a beam from being generated, but which are spurious.  One of these
incorrectly indicates an absence of the diverting magnet current.  The
diagnostic for the magnet current is slow and sometimes does not indicate that
current is present until the time is past for generation of a beam.  This
diagnostic is being replaced a faster system.  Design is complete and
fabrication is in progress.  A second nuisance fault is related to the "false
start fault" which when operating properly, indicates that the control system
has received multiple start pulses.  Diagnostics have been installed to search
for the source of this spurious fault condition.
 
Inner Divertor Project
----------------------
 
Ceramic-coated tungsten probes to be used in the new inner divertor probe box
were received and test-fit in an aluminum tile prototype. Existing molybdenum
tiles will be modified to accept the probes. A new stud face-off tool is
complete, and will be tested in the mock-up.
 
A detailed schedule for finishing the machining of the Rear Girdle plates has
been developed with the vendor. All of these plates are to be ready for silver
plating at the beginning of next month. Machining of the Upper Section of the
first C-plate is nearly complete.
 
Lower Hybrid MIE Project
------------------------
 
The TPS (Transmittter Protection System) Backplane board assembly is
complete. Parts to assemble TPS and CPS (Coupler Protection System) circuit
boards began to arrive. Work was begun on the overall Lower Hybrid wiring
interconnection schematic.
 
Work continued on the SFOL (Serial Fiber Optic Link) circuits which will be
used for communications in the LH control system. Progress was also made in
development of communications software for the active controller. The data
compression and decompression programs in Labview RT are now working. Tests
were performed in which analog outputs were wrapped back into the analog
inputs on the controller chassis to check for sampling problems; the data is
being evaluated.