From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
June 30, 2003

Last week was a maintenance week at Alcator C-Mod. No runs were
scheduled. Plasma operations will resume this week.

The C-Mod Experimental Program Committee met at MIT on Thursday, with remote
participation from PPPL, U. Texas, and GA. Thirteen MiniProposals, most for
experiments to be carried out during the remainder of the present campaign,
were considered at this meeting. A tentative run schedule has been developed.

Operations
----------

No plasma operations were scheduled. The tokamak was maintained under vacuum
and 60C bake as the engineering staff performed preventive maintenance tasks
and worked on power system improvements.

The Netware server PC for the Paragon interfaces to the engineering and
diagnostic PLC's was replaced with a more reliable computer; the old server
had experienced several crashes in recent weeks, necessitating reboots of all
of the Paragon stations.

A second TMX (slave) supply to increase the voltage/current available to drive
the non-axisymmetric control coils was installed in the power room. This
includes AC input wiring and high current DC wiring and connection at
the slave supply, as well as control circuit wiring in both master and slave
supplies. The PLC programming work to operate the second supply has also been
completed. The original TMX master supply is still connected and ready for use
during this week's operation. Some additional wiring and testing of the
master/slave configuration into a dummy load will be required before the
combined power supply is ready for use.

Physics
--------

At the Monday group meeting, Robert Granetz showed results of observations of
beam heating by the diagnostic neutral beam in low density, low current ohmic
discharges. The beam power into the plasma is of order 100 kW, primarily in
the form of 50 keV hydrogen ions, injected perpendicularly to the magnetic
field. This reasonably mimics the ICRF H minority heating scheme. The
increase in plasma total stored energy due to the DNB was roughly 1 kJ, which
is just discernible on the EFIT calculation of stored energy. Plasmas with
locked modes generally show less efficient DNB heating, possibly indicating
enhanced loss of fast perpendicular ions. With the new long-pulse DNB,
expected in early 2004, it will be possible to modulate the beam throughout
the discharge, allowing energy confinement analysis of ohmic, L-mode, H-mode,
ITB's, etcetera to be systemically compared in a single discharge.

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

Initial current drive phasing tests for the J-port four strap antenna were
successfully completed. The phase control settings were determined from vacuum
measurements and agreed well with expectations. The J-port loops have been
reconfigured for heating phase for this week's runs.

Operation of the 500W Kalmus amplifier on transmitter #2 was verified to be
qualified for long pulse (5 sec) operations.

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

The location of the arcing in the circulator assembly has been determined by
extensive testing to be near the center of the circulator itself. The unit is
being crated and will be shipped back to the vendor for evaluation and repair.

Preparations for power testing of the lower hybrid launcher are proceeding.
All of the rigid elbows required for the test have been received. We also
received nine out of twelve waveguides with flanges (12' long) required for
the test. All of the brackets for holding the waveguides have been installed
in the cell.

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

The DNB turbopump which failed last week was temporarily replaced with a
spare, and the beam is operational for this week's runs. The failed pump will
be repaired and returned to service after the end of the present run
campaign.



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

Work continued on bringing up the NSTX imaging Xray Crystal Spectrometer. The
crystal was rotated slightly to improve the centering of the argon spectrum on
the detector. The crystal was also masked down to reduce the detector count
rates to acceptable levels. Calculations indicate that the etendu of this
spectrometer is much higher than that of the single chord C-Mod crystal
spectrometers, consistent with the observation of symptoms of high count rate
and detector/electronics saturation during C-Mod discharges. Measurements were
made to better understand the apparent high deadtime fraction of the
signal-processing electronics at high count rates, and to assess the
individual contributions of the TDC and the KBSI interface box to this
deadtime. Measurements on C-Mod plasmas with this instrument will continue
during the upcoming week's operation.

The 140 GHz channel of the reflectometer diagnostic system needed some small
retuning to compensate for temperature-induced frequency drifts. The 130 GHz
channel was also modified so that it can be tuned easily to allow for
frequency drifts due to temperature changes.

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

On Monday, 6/23, Larry Dudek, Doug Loesser, Rich Hawryluk, Joel Hosea,
Ned Sauthoff, Gerd Schilling, and Skip Schoen, all from PPPL, visited
MIT for a meeting on the Lower Hybrid Launcher. Scheduling of the
plating, brazing, and testing activities was discussed. Attending the
meeting from MIT were Ian Hutchinson, Earl Marmar, Ron Parker, Miklos
Porkolab, Jim Irby, Bill Beck, Bob Childs, and Rui Vieira.

Manfred Bitter and Ken Hill (PPPL) were at MIT this week making further
improvements to the NSTX imaging XCS. Gerrit Kramer was at C-Mod 6/23-25
working on the reflectometer diagnostic; he also installed his reflectometer
data analysis program on the Linux cluster to facilitate analysis of
fluctuation data.






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