Alcator C-Mod Weekly Highlights
			April 25, 2005

FY2005 weeks of research operations
Planned: 17 weeks
Completed: 6.25 weeks

Operations
----------

Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled and completed. A total of 78 plasma discharges were produced with a
startup reliability of 87%. All three days were devoted to the
post-boronization evaluation phase of MP#417, which directly supports our FY05
Level 1 (JOULE) target "Measure plasma behavior with high-Z antenna guards and
input power greater than 3.5 MW". Plasma operations are planned to continue
this week.

Plasma performance on Wednesday, the first operational day following the
boronization, was good, with low radiated power and high confinement; H89
factors transiently above 2 were obtained, with H89~1.7 sustained for several
hundred milliseconds.  Stored energies above 200kJ, with volume average
pressures up to 1.6 atmosphere and peak pressure ~4.6 atmosphere, were
obtained with up to 4MW of ICRF power injected on a number of discharges. A
density scan was carried out at a plasma current of 1MA, and the character of
the H-modes varied from "ELM-free" to "EDA" as the density was
raised. Interestingly, the H/(H+D) ratio was not lower than in discharges of
the week preceding the boronization; H/(H+D) was measured to be between 4 and
6% during ICRF. This effect may be due to the lack of deuterium discharge
cleaning following this boronization. The most beneficial effects of the
boronization were not fully maintained, with some performance degradation and
increase in Mo radiation already observed by the end of the first day of
operation.

Experiments continued on Thursday following overnight ECDC in D2.  Confinement
was reduced relative to the previous run, and the levels of Mo and Prad/P_in
were higher. The H/(H+D) was lower, around 3-4%, which is typical of
post-boronization operation. Optimizing the strike point location and puffing
D2 into the H-mode phase reduced Mo and core radiated power; effects on
confinement were small.  Comparing fiducial discharges from the beginning and
end of the day indicated that performance degraded over the course of the
experiments.

On Friday, we continued the post-boronization experiments, with particular
attention to reducing the Mo influx and the radiated power fraction. Puffing
of N2 to radiatively cool the divertor and SOL plasma was somewhat effective
in reducing Prad, but the pedestal temperature was also reduced. Steadier
H-modes were obtained with N2 puffing, both on its own and in combination with
additional D2 puffing into the H-mode. Energy confinement was not
significantly affected.  Comparison of similar shots from early
and late in the day indicated that there was no additional degradation in
confinement over the course of these experiments.

ICRF Systems
-------------

The ICRF antennas recovered quickly from the machine's first boronization of
the campaign.  Reliable powers up to 4 MW were injected.  We continued our
investigation into filament management for the final amplifier tubes.  Into
dummy load, one tube could have the filament voltage reduced while the other 3
did not have enough gain without full voltage.  Further investigation is
necessary to understand the differences among the tubes.  We also installed
ferrite suppressors on FMIT#1 and FMIT#2 in an attempt to reduce RF noise and
this change appears to have improved the transmitter stability.  Finally,
further tests confirmed that power from the J-port antenna is no longer
interfering with the GPC2 Electron Cyclotron Emission diagnostic.

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

Calibration of the CPS (Coupler Protection System) continued. Addition of
capacitors across the DC breaks in the CPS coupler was demonstrated to
eliminate interference from the ICRF system in those circuits.


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

Jean-Marie Noterdaeme (AUG/JET) visited last Friday to discuss antenna
operation in an all metal machine.

Marco Brambilla and Roberto Bilato (IPP Garching) started a one month visit to
the PSFC. They will be working with Paul Bonoli and John Wright on the TORIC
code, including problems related to the parallel implementation of the
full-wave lower hybrid solver.

John Rice participated in the 8th Meeting of the ITPA Transport Physics
Topical Group held in Kyoto, Japan, on April 18-21.
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