Alcator C-Mod Weekly Highlights
			July 11, 2005

FY2005 weeks of research operations
Planned: 17 weeks
Completed: 12.5 weeks

Operations
----------

Research operations continued at Alcator C-Mod last week, with four run days
scheduled and completed. A total of 87 plasma discharges were produced with an
overall reliability of 74%. Experiments carried out last week supported
research in Transport Science and the Advanced Tokamak thrust, and contributed
to the C-Mod Level 1 (JOULE) target "Measure plasma behavior with high-Z
antenna guards and input power greater than 3.5 MW".  Boronizations were
carried out on Tuesday and Thursday nights, and during operation on Tuesday.

This week will be a scheduled maintenance week, with no plasma operations
planned.

Run Summaries
-------------

Tuesday's run was devoted to MP#417 and investigated the use of between-shot
boronization to improve wall-conditioning. This was the first time this
technique has been tried at C-Mod. Clear effects of as little as 10 minutes of
between-shot boronization were seen. Increasing the time to 20 and then 30
minutes showed monotonically increasing improvement (reduction of molybdenum
and radiated power, increase of peak and integral neutron rate, increase of
peak stored energy). Another interesting result is that the effects of these
short boronizations are almost completely gone after 1 shot. The 30 minute
boronization did have a small lingering effect on the second shot which
followed, with no additional boronization in between. That second shot was
slightly better than one with no boronization, but not as good as shots with
10 minutes of pre-boronization.  The RF ran very reliably at 3 MW throughout
the day, with very few trips; this greatly facilitated the shot by shot
comparisons of the effects of the boronizations.  For these experiments, the
range of the ecdc resonance was .50 < R < 1.03m (covering the divertor and
outer limiter and antenna hardware), with the exception of one case in which
the scan was changed to 0.44<R<0.80m (inner wall and divertor). The
boronization with this modified scan was clearly less effective.

Wednesday's run was devoted to MP#425 "Toroidal rotation in dimensionally
similar discharges with no momentum input", submitted through the Transport
Group by John deGrassie (GA). The goal of the experiment was to run C-Mod
discharges with dimensionless similarity to discharges previously run in
DIII-D and measure the toroidal rotation velocity (Mach number), thereby
establishing whether non-driven rotation can be described by the dimensionless
parameters of plasma physics.  DeGrassie participated in the experiment as
co-Session Leader using remote collaboration tools (Access Grid) from San
Diego. Because this experiment had not been envisioned when the DIII-D case
(an EC-heated H-mode) was run, it was necessary for C-Mod to operate in a
non-standard shape, specifically one with moderately high elongation and very
low triangularity, in order to match the DIII-D geometry. The C-Mod experiment
was carried out at 6T, corresponding to the Kadomtsev-scaled DIII-D field of
1.65T. We were successful in matching the target density range in H-mode as
well as q95, but the ICRF was unable to couple well to plasmas with the
desired shape, and the match in temperature, and therefore in beta and nu*,
was not achieved, although some shots were close enough for an approximate
comparison to be undertaken. These 'near DIII-D' cases will give a comparison
for a future try with a more standard C-Mod shape to obtain higher RF power
and a closer match to the physics parameters.

Thursday's run was dedicated to MP#422, "ITB formation - Ti critical gradient
dependence". This run, which continued the experiment begun the previous week,
is to test the hypotheis that the trigger condition for producing internal
transport barriers (ITB) in off-axis ICRF heated plasmas is explained in the
framework of suppressing ITG turbulence by increasing the ion temperature
scale-length relative to the critical scale-length. The experiment consists of
carefully documenting the scale lengths as the ICRF resonance is scanned
shot-to-shot by incremental changes in the toroidal field. ITB's were
established with the resonance on the low-field side (B(on-axis)~6.3T). As
observed in previous experiments the ITB threshold in field was found to be
quite sharp. Analysis of the variation of the temperature gradient as the
field was varied across the threshold value is underway.

Friday's run was devoted to MP#421 "Obtaining steady low-density H-modes",
submitted through the Advanced Tokamak thrust. The purpose was to test a
proposed technique to lower the density of steady H-modes at normal currents
by obtaining an L-H transition at low current and low target density and then
ramping the current to the higher value. Secondary goals were to obtain data
on the scaling of the low-density limit of the L-H transition and to explore
ELM behavior at low collisionality. We were able to make low density, low
current H-modes but were not successful in ramping these up to full current.
Contrary to expectation, the low density limit did not seem to be much lower
at low currents. Generally the transition did not occur until nebar was in the
range 0.9-1.1e20/m3. These low density, low current discharges had very strong
counter rotation - vphi was down below -50 km/sec! This may explain the
difficulty in making the transition, which occurs at about 0 km/s, as observed
in previous experiments. A steady EDA H-mode was obtained with nebar down to
1.e20 at 0.38 MA; for these conditions the confinement parameter H98y2 = 1.

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

Preparations continue for low power phase and amplitude calibration work to be
performed during the maintenance week.  A symmetric 3-window prototype was
brazed last Thursday.  A small leak detected in the corner of one waveguide is
undergoing careful analysis.  Preparations are being made to braze a flanged
3-window sample as soon as it returns from plating.

As part of the ongoing work to refurbish spare klystrons, cathode ceramic
metal parts have been redesigned and final drawing packages are ready to send
out to the machining vendor, the ceramic fabricator, and to the
ceramic-to-metal brazing vendor.

Diagnostics
------------

New data acquisition hardware (D-TACQ) has been installed and commissioned for
the Motional Stark Effect (MSE) diagnostic to acquire data at 1 MHz for the
full plasma duration.

Long Pulse Diagnostic Neutral Beam
-----------------------------------

We have had additional dialog with the Budker Institute on tuning the beam for
higher performance without faulting.  They have recommended a new protocol for
beam conditioning which starts at high voltage and increases the arc current
slowly as faults condition out. We have been following this conditioning
regimen and have seen a significant decrease in fault frequency as well as
reductions in third (repeller) grid current, indicating that the source
performance is improving.

Larry Grisham and Al Von Halle (both PPPL) visited MIT on Thursday and Friday
for discussions on approaches to conditioning the DNB.

Travel and Visitors
--------------------

Bob Granetz, Bruce Lipschultz, and Joe Snipes participated in the ITPA working
group meetings in Tarragona, Spain, last week, following the EPS
meeting. Miklos Porkolab was also in Tarragona, attending te IEA Burning
Plasma Physics and Simulation Workshop, W60.

Amanda Hubbard attended a meeting of the Nuclear Fusion Editorial Board in
Tarragona on Tuesday, July 5.

Larry Grisham and Al Von Halle (PPPL) visited MIT last week for DNB
discussions with Dexter Beals, Bob Granetz and Steve Scott.


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