Alcator C-Mod Weekly Highlights
			Mar 7, 2005


FY 2005 weeks of research operation planned: 17 weeks, completed: 1 week.

Plasma operations resumed at Alcator C-Mod last week. Four run days were
scheduled and completed, including three days of research operation and one
conditioning run. Also during last week, the lower hybrid system operated
into C-Mod plasmas for the first time.

Operations are planned to continue this week.

Operations
----------

Following the scheduled maintenance period, we resumed plasma operations on
Tuesday, March 01. The operating schedule remains on a split shift, with
runtime between noon and 19:00, in order to accommodate on-going work in the
cell and power room.  During four runs last week, we produced a total of 66
plasma discharges with a startup reliability of 72%. Experiments were carried
out in support of research in the MHD and Divertor/Edge Topical Science
areas, as well as one day dedicated to conditioning of the ICRF system.
During the run on Friday, the Lower Hybrid system was operated into plasma for
the first time.

Tuesday's run was dedicated to MP#413, "B-field scaling of error threshold at
constant q95 and normalized density", from the MHD Stability Group. For this
run the locked mode threshold was observed in a series of discharges with
3<B_T<7 tesla, keeping q95=3.4 and n/n_G=0.17 . The non-axisymmetric A-coils
were configured to produce a field that nearly cancels the 2/1 component of
the intrinsic error field, and ramped down during each shot until a locked
mode occurred. Assuming a power law scaling of the form B21/B ~ B^alpha_B, the
data are well-fit by an exponent alpha_B=-1.1, which is close to the value
previously inferred from experiments on JET under a somewhat different set of
constraints. Determining and understanding the scaling of the locked mode
threshold with field is crucial to extrapolation of present-day experiments to
a large burning plasma experiment such as ITER.

The run on Wednesday was primarily devoted to ICRF Conditioning
activities. The E-port dipole antenna (FMIT#2, 80MHz) was successfully
conditioned to 1.9 MW into L-mode target plasmas. At this power level, brief
H-modes were obtained with suitable target conditions, despite a rather high
H/D ratio of approximately 10%. The J-port four-strap antenna (FMIT#3 and 4)
was tested into plasma at 50MHz and operated successfully with both
transmitters, in contrast to last year's experience.  Last week's successful
operation was attributed to better suppression of a transmitter instability in
the FMIT#3 Final Power Amplifier stage. Operation of the D-port dipole antenna
was again restricted by an arc in the transmission line. This line has been
partially disassembled and is under inspection.

A planned piggyback experiment on Wednesday aimed at getting preliminary data
for MP#407, "Effects of 3D error fields on magnetic surfaces", proposed by
Lang Lao (GA) to the MHD Stability group, was not accomplished. Test shots
prior to plasma operation revealed a power supply instability and some arcing
on the coil case of one of the A-coils required for this experiment. The
experiment will be rescheduled when these issues are resolved.

Thursday's run was devoted to MP#323. The purpose of these experiments is to
assess the performance of a cryopump located in the upper divertor and to
provide guidance for optimizing the tile and baffle structures at the entrance
to the pumping chamber. In particular, we wished to make use of the new
'simulated divertor baffles' that were recently installed in the upper
divertor region. For this run, L-mode discharges were run with the magnetic
balance adjusted to SSEP=-3mm (i.e. slightly biased to the lower
x-point). This configuration is expected to be an important operational
scenario for the upper cryopump for high performance discharges, sending peak
heat fluxes to the lower divertor while providing high particle loading in the
upper divertor chamber.  Detailed pressure measurements from the upper and
lower divertor chambers were obtained at two densities, 1.3 and 1.6e20 m^{-3},
with gas puffing from different locations. From these data we expect to be
able to determine (1) the active gas inventory in the vessel (2) the upper and
lower divertor pressures, and sensitivity of cryopump baffle pressure to upper
secondary strikepoint location (3) gas conductance into various volumes:
cryobaffle, E-top 'slot' and under lower divertor (BC-bottom).

Friday's run was the first run of a multi-day series of experiments under
MP#409, "Phase space of the SOL Plasma", submitted by the divertor/edge
group. The overall goal of this MP is to map out plasma conditions in the edge
(profiles, turbulence, cross-field fluxes) over a wide range of currents,
field, and densities. Upper and lower null discharges were produced at 0.8MA,
5.4 T over a factor of six range in density. An extensive diagnostic set was used,
including the new Mach probe on the A-side scanning probe (ASP), Gas puff
imaging, fast diodes, the inner-wall probe, and vertical scanning probe (FSP),
boron Doppler spectroscopy at the inner wall, and Xybion camera looking at
D-alpha in the edge plasma. Clear differences were observed between upper and
lower null cases, beyond the different SOL flow behavior reported
previously. The upper null cases had lower temperature near the separatrix and
smaller gradients; MARFE behavior was also more pronounced in upper null cases
than for LSN discharges at the same density. Discharges with continually
increasing density were also produced with the ASP at a fixed location 6mm
outside the separatrix, taking continuous Isat and Vf data at 1MHz sampling
rate.  These data will be analyzed to determine if the character of the
turbulence changes as some fraction of the Greenwald density is approached.

Physics
--------

Analysis of the H/(H+D) ratio in C-Mod discharges since the beginning of this
campaign with molybdenum antenna protection tiles indicates that the cleanup
rate for the hydrogen is similar to that seen in previous campaigns with boron
nitride tiles.  It has typically taken approximately two hundred full length
discharges to reduce the H/(H+D) ratio to below 10%, at which point the vessel
has been declared ready for boronization.  This H/(H+D) level was reached last
week.  However, a survey of the H/(H+D) data since 1999 indicates that it
became more difficult to maintain low levels of hydrogen once the boron
nitride was installed in July of 2000.  The behaviour of the H/(H+D) ratio
following our first boronization this campaign will therefore be of great
interest and provide important information for our level 1 JOULE milestone.

ICRF System
-----------

The E-port antenna operated reliably, providing up to 1.9 MW into
plasma. Testing of the J-port four-strap antenna at 50 MHz into plasma was
carried out successfully. The J-port transmitters (FMIT#3 and 4) are being
re-tuned to 78 MHz.

DNB Systems
-----------

The installation of the new DNB is progressing well and is approximately 65%
complete after about six working days. All power and control cabinets have
been placed in the C-Mod cell and power input wiring is complete.

One of the 50,000 l/s helium cryo pumps suffered damage during shipment.
Repair parts were designed and machined. The dewar was welded and leak tested,
and the pump appears to have been sucessfully re-assembled, welded, and leak
checked.

The vacuum chamber was mounted on the support frame, and the bending magnet,
helium cryo pumps, beam dump, etc were mounted inside the vacuum chamber. The
two 50,000 l/s cryo pumps were mounted on top, and the chamber has been
evacuated and leak tested. Several small leaks were located and repaired, and
the chamber has passed a helium leak test showing no leaks greater than
approximately 5x10(-7)atm-cc/sec.

The hot cathode beam source has been assembled and has passed a leak test. The
source is ready for installation on the chamber.

The high voltage transformer tank has been assembled, evacuated and backfilled
with SF6 gas.

Lower Hybrid System
-------------------

Power testing using the Active Control System (ACS) to control the
transmitters was begun. On Friday, power from one klystron was coupled into
C-Mod, both into vacuum and into plasma for the first time. The coupled power
was 3-5 kW with a pulse width of 10 msec.

Travel, Visitors, and Remote Collaborations
-------------------------------------------

Lang Lao (GA) participated in part of Wednesday's run using the video
conferencing tools provided by the Access Grid as part of the Fusion
Collaboratory project; communications were carried out using a Unicast bridge
set up in the MIT Venue of the Fusion Collaboratory site. Dr Lao also used the
web-based electronic logbook interface and MdsPlus data system tools to
monitor the progress of the testing in preparation for his proposed piggyback
experiment.

 From Feb. 24 to mar. 4, I. Furno visited C-Mod from Los Alamos National
Laboratory (LANL) in the frame of collaboration with J. Terry involving
infrared (IR) thermography measurements of divertor tiles. The IR camera
(recently refurbished) has been installed on top of the Alcator igloo together
with the optical train (including two ZnSe lenses). The camera communication
link and the digital capture interface have been tested and work properly. The
system has been focused and the digital images of the divertor tiles
(including the newly installed Tungsten brushed tiles) seem to be particularly
clear. The last two days of the visit were devoted to testing the Mdsplus side
of the acquisition system for the IR camera. A migration from the old
VMS-based routines onto a Linux-based routines is necessary in order to write
the digital images in the Alcator C-Mod Mdsplus database; this work is in
progress.
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