From: wolfe@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
July 1, 2002

Plasma operations continued on Alcator C-Mod last week. Four run days were
scheduled and completed. A total of 31 plasma discharges were produced with a
startup reliability of 45%. The purpose of this week's runs was continued
clean-up of the tokamak, diagnostic alignment and calibration, and discharge
development for the upcoming physics campaign. Progress also continued on
ICRF, DNB, and Lower Hybrid Systems. Plasma operation is scheduled to continue
next week.

Operations
----------

Three days of plasma operations in support of tokamak conditioning and startup
activities were carried out last week. In addition, one run day was devoted to
beam-into-gas (no plasma) shots with programmed toroidal and vertical fields
for calibration of the MSE diagnostic.

Cleanup and conditioning of the machine is proceeding well. By Friday, the
H/(H+D) ratio was down to less than 15%, and the startup reliability was above
60%. Disruptivity was also down, with 29 of 31 plasmas lasting into programmed
rampdown. A problem with the discharge initiation sparker encountered the
previous week has been remedied, and a new sparker system has been installed
as a backup.

Development of new equilibria for the physics campaign also made progress last
week. A series of double null discharges (abs(SSEP)<2mm) were produced with
both upper and lower triangularity>0.8 and elongations up to 1.85. These high
triangularity configurations are possible due to the new lower inner divertor
structure installed during the last year.

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

Diagnostics continue to be brought online and calibrated. The ECE system was
aligned on a series of constant-shape discharges. Both A and F-port scanning
probes were exercised during plasma shots, as were the divertor flappers.

The high-pressure in-vessel Penning gauge recently installed under the
B-C outer divertor module was connected to a new high voltage bias
circuit last thursday. The cathode/anode geometry of this gauge
was custom-built with dimensions approximately a factor of 10 times
smaller than the standard commercial units used at other locations
in C-Mod. The miniature Penning gauge was designed to take advantage
of the high magnetic field strengths in C-Mod, extending the
useful pressure measurement range to the 10s of mtorr level and
allowing a direct, fast time-response measurement of the high
pressures under an outer divertor module. During last friday's run,
the gauge was successfully operated, reporting divertor pressures of
up to 100 mtorr during local capillary gas injection tests. Pressure
readings taken during discharge 'fizzles' are found to generally agree
with readings from other gauges connected to the vessel, lending
confidence to the gauge's operation. The gauge appears to maintain
a discharge at moderate to high neutral pressures which is more stable than
the three other Penning gauges in the torus (commerical units).
Thus, plans are already developing to replace the 3 commerical gauges with
custom-built units. Experiments are planned to use the miniature gauge to
calibrate
divertor neutral leakage rates during the physics phase of the present run
campaign. New capillaries in the upper and lower regions of the chamber are
also operational, and should shed new light on plasma fueling and pumping.

The visible continuum array diagnostic became operational last week.

Additional new inner divertor diagnostics are also coming on-line. The halo
and eddy current rogowski coils (20 coils) are now all operational.

One of the two Gunn oscillators used for the upgraded reflectometer system was
found to have failed and will be sent to the commercial vendor for repairs.

TV plasma-imaging systems are being brought on-line, including two wide-angle
views of the discharge from A- and F-ports, which will provide a nearly
complete view of the plasma chamber; a divertor view; and a fast framing
camera view used to image turbulence in the SOL region near the
midplane. Problems with data acquisition for these cameras have apparently
been resolved, and the image data are now being automatically stored in the
MdsPlus tree.


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

The ICRF system is coming on-line. Following resolution of some issues in the
new control and fault protection systems, the J-port four-strap antenna was
vacuum
conditioned up to 35kV on Friday. Low-power (~30kW) conditioning pulses into
plasma were then obtained on the final few shots of the day. Plasma
conditioning of the J-antenna should proceed this week. Operation of D- and
E-port antennas is also imminent.

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

The beam was operated on three days last week, On Tuesday and Wednesday, it
was fired in synch with C-Mod pulses. On Thursday, a beam-into-gas experiment
was completed for MSE calibration. The TF, and EF3 and EF4 coils were
energized during these calibrations to simulate the fields due to plasma
current. The BES system was operated for measurement of the beam width. On
the last day of the week, there was a fault in the charging power supply
circuit which is being evaluated. Blown fuses were found but operation of the
beam will be continued only after consultation with the Budker team.


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

Miklos Porkolab visited the Ecole Polytechnique Federale De Lausanne/CRPP,
Lausanne, after the EPS meeting, June 24, and gave the Colloquium with the
title: The MIT Plasma Science and Fusion Center: A Paradigm of How to Operate
a major Fusion Laboratory in a University Environment. The colloquium was
well attended. Discussions followed by the Director, Prof. Tran and
Dr. Appert, Associate Director, about possible collaborations between the two
laboratories.

Gerrit Kramer (PPPL) was at MIT 6/25-26 to check out the two-channel
reflectometer upgrade and participate in MSE diagnostic startup with
the RFX beam. Steve Scott (PPPL) has joined the C-Mod team; this was his
first week onsite. Steve will be working on the MSE diagnostic.
Gerd Schilling was at MIT 6/25-28 to participate in the ICRF startup.







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