Alcator C-Mod Weekly Highlights
			July 2, 2001

Plasma operations continued at Alcator C-Mod last week. Four run days were
scheduled and completed. A total of 61 plasma shots were produced, with a
startup reliability of 85%. Three runs concentrated on conditioning and
performance characterization of the four-strap "J-port" ICRF antenna. The
fourth was an experiment to test the peeling-mode model of ELMs. 

Plasma operations are scheduled to continue this week. 

Physics and Operations: 
-----------------------

Following completion of repairs to the phase shifters, ICRF antenna
conditioning resumed on Tuesday and continued Wednesday. The J-port antenna
was tested up to 3.0 MW into plasma.

A fresh boronization was carried out over Wednesday night, with an average
deposited thickness of approximately 2000A. The J-port antenna conditioning
and testing were completed on Thursday.  The results of these experiments are
detailed in the ICRF section below. 

Friday's run was in support of MP#282A, to investigate the effect of edge
current density on ELM behavior in high beta_N (1.2<beta_N<1.6) C-Mod
plasmas. EDA H-mode plasmas were produced at 800kA and 1MA, with RF power
between 2.5 and 3.5MW. These discharges were in the regime where "grassy" ELM
activity begins to be seen on the H-alpha and other diagnostics, along with
the QC mode typical of the EDA. The edge current profile was transiently
altered by imposing current ramps, raising and lowering the surface loop
voltage by about 2V.  The goal was to test the theoretical prediction that the
consequent increase in edge current density would make the peeling modes more
unstable, leading to increased ELM activity, while decreasing Jedge with
reverse current ramps would stabilize the modes. This experiment was
technically challenging, as it was difficult to maintain good RF coupling
during these transients; there were also some perturbations to the equilibrium
shapes during the current ramps, which may complicate the analysis. However,
the preliminary result is that the observed changes in ELM behavior were 
opposite to the expectation: increasing the current seemed to suppress
pre-existing ELM activity, while reverse ramps resulted in short bursts of
ELMs in a previously quiescent discharge.  A number of shots have been
identified for detailed analysis.

Piggyback experiments this week continued to exploit the capabilities of the
new ultra-fast PSI camera. On different days, the camera was used to image
filamentary striations in the edge plasma and lithium pellet ablation
clouds. The latter technique can be used as a q-profile diagnostic. Using the
ablation light detected on a separate photodiode as a trigger for the camera,
it was possible to obtain 12 separate Li^+ ablation trails with time
resolution of 20 microseconds, corresponding to radial spatial
resolution of about 2 cm.

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

As mentioned above, the J-port antenna was tested to 3.0 MW into plasma.  It
heated well and reached ~26 kV briefly.  The antenna had numerous faults when
the voltage reached ~20 kV and degraded over a series of discharges.  The fast
data indicated that some arcs survived for 30-50 usec.  Up to 100 J could be
available to dissipate in the arc; therefore, we reduced the reflected-to-
forward power ratio necessary to generate a trip by 25% for Friday's run.
This successfully limited the arcs to ~15 usec or ~15 J per MW injected.  We
suspect arcing in the antenna strap is responsible for these arcs, because no
signal corresponding to the arcs were observed in the optical monitor signals,
and the antenna "paper clip" geometry could allow an arc on one antenna strap
half without resulting in a phase balance or reflected power fault in the
transmission line.  We often observe an interaction at the antenna midplane
near the septum and side limiters at about the location of the short.  This
interaction was more severe when running inner wall limited L-mode discharges,
and was weakly dependent on plasma current.  A phase scan showed that the
nominal [0,pi,0,pi] was the most effective heating phase and had little or no
negative edge interaction.  An outer gap scan was also completed and suggested
a gap of 1-1.5 cm was better than larger outer gaps.  Antenna performance was
insensitive to toroidal field from 5-5.6T.  Boronization recovery did require
a few discharges even though the antenna was voltage conditioned to 32 kV
before the run.  False phase balance faults are still occurring on retry when
other antennas are operating.  We will be examining a fix for this in the next
week.

The D-port antenna was out of service the previous week due to arcing in the
phase shifter.  There were arcs both across the teflon surface and across the
gap.  Additional arc tracks were found near the push/pull rod (but not on it),
suggesting that arcing was occurring across the conductor gap.  We had been
operating up to 40 kV with 20 psig of N2, which is close to the design
maximum.  The arc location was also an area with enhanced E-fields due to the
geometry.  Thus, the breakdown was probably a result of exceeding the phase
shifter operating limits in N2.  Future operation will be with 20 psig SF6,
which will raise the power handling by a factor >10 and result in a safety
factor of about 2 in voltage.  Once the phase shifter was repaired on Monday,
D-port operation resumed.  During operation early last week, the D-port
antenna experienced numerous false phase balance faults due to a bad
demodulator.  Another demodulator was installed for Friday's run and the
antenna fault protection behaved properly.


Diagnostic Neutral Beam System
-------------------------------

The beam was operated at full voltage and current.  There was an apparent
heating problem on the suppressor on Wednesday.  The last two days of the week
were devoted to parameter scans to improve the beam component mix and to
identify any remaining problems with the beam.  Based on these, we plan to
instrument the arc to assure that the arc current is shared among the
filaments and to assure that the arc SCRs are receiving appropriate trigger
signals.  This did not prevent us from continuing the development of CXRS,
MSE, and BES diagnostics.

Lower Hybrid MIE Project
------------------------

The construction of the Lower Hybrid launcher's test set up for the
reflected-power probe and for the "water dump" has been completed at PPPL.  It
is being assembled now and a probe is being fabricated; testing is expected to
begin next week.  The launcher design is proceeding well, with the FDR
tentatively scheduled for July 31 at PPPL.  The launcher will have radial
movement capability via vacuum bellows, and a vendor visit concerning these
bellows was very encouraging.  Plating of copper to provide a high electrical
conductivity skin on the titanium waveguide is proceeding at another vendor.
Ceramic windows with a larger radius of curvature at the corners will be
received this week for brazing tests.

Inner Divertor Fabrication
--------------------------

 All ribs (upper section and main body) on 12 Inconel Rear Girdle Plates were
completed on Wednesday, June 27th. There are two more operations: Milling the
bottom of the plates and wire EDM inside radius + remove corners.  Estimated
delivery date of all 12 Rear Girdle Plates is July 31st.

All 12 Inconel C-Plates were completed and 100% inspected at the manufacturer.
The C-Plates are expected to arrive in-house this week.

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

Gerrit Kramer (PPPL) was at MIT 6/26-28, working on MSE data.  Gerd Schilling
was at MIT 6/26-29, helping with the DNB operation and attempting to improve
the beam species mix.

Drs. H. Kishimoto, A. Kitsunezaki, and T. Ogawa, of JAERI, visited Alcator on
Friday. They toured the facility and met with Professors Porkolab, Parker, and
Hutchinson. 

Dr. Michael Holland from the Office of Management and Budget visited the PSFC
on Friday, June 30. He had discussions with scientists and students from the
center, and toured the C-Mod facility. He also presented a talk on the budget
scenario.