From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
Oct 27, 2003

Plasma startup and conditioning operation continued at Alcator C-Mod last
week. Four run days were scheduled and completed. A total of 60 plasmas were
produced with a startup reliability of 80%. Diagnostic systems continue to be
brought online, and ICRF systems are being conditioned to high power. Progress
continued on the Lower Hybrid System.

No runs are scheduled this week, as most of the physics staff is participating
in the 45th annual meeting of the APS Division of Plasma Physics in
Albuquerque, New Mexico.

Operations
----------

The run on Tuesday had to be stopped early when a monitor in the alternator
indicated a low helium purity. Helium is used to reduce windage losses in the
alternator. On Wednesday morning, the problem was tracked down to an air leak
in the gas handling system. Once this was fixed, the helium purity stayed very
high the rest of the week (~ 99%).

The chopper power supply used for fast vertical position control tripped
during a shot near the end of the run on Thursday afternoon. The supply was
repaired, tested and returned to service Friday morning.

Machine condition continued to improve through the week. The H/D ratio is
presently at the 10-15% level.

Experiments to quantify the amount of gas pumped by the vessel wall under
different conditions are being carried out during this phase of C-Mod
operation. For these experiments, the gate valve to torus pumping system is
left closed during and following the discharge, and the pressure rise
monitored for up to ten minutes. The difference between the total fueling
during the shot and that released from the wall between shots
determines the increment to the wall inventory. These experiments are part of
a collaborative effort with Dennis Whyte (U. Wisc.).

Physics
--------

Robert Granetz presented results of orbit loss calculations of 50 keV
deuterons for the deuterium DNB run last year. The orbit code uses a
Runge-Kutta package to solve for the Lorentz motion of the beam ions in the
EFIT-derived poloidal and toroidal fields. The results show that there are no
first-orbit losses of beam particles, except for those born within ~1 cm of
the plasma edge. Pitch angle scattering can eventually cause the loss of some
of the beam particles, but on this timescale they lose enough energy to reduce
their fusion neutron cross-section significantly, so this doesn't affect the
D-D neutron rate significantly.

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

Conditioning of the antennas continued last week. We have achieved up to 5
MW total power from all antennas for pulses up to 100 msec. D and E-port
antennas have achieved 1.5 MW each simultaneously. J-port has achieved 2 MW
for 0.2 sec and is behind the other antennas in terms of conditioning. J-port
has had less operating time due to the change of frequency from 50 to 78 MHz
completed last week.

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

The DNB was operated in conjunction with C-Mod operation for most plasma
discharges.

The arc current slope-correction circuit has been bench tested and installed.
Final testing will be completed early next week.

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

Both rebuilt klystrons were successfully tested up to 46kV and 230kW for short
pulses. Further conditioning is required to bring up the pulse length.

A teleconference on the status of the Lower Hybrid launcher was held with
PPPL 10/22. Task updates were discussed and several issues were resolved.
All four couplers, which were undergoing a final cleanup procedure at
the vendor to prepare them for brazing, are now back in house at PPPL
for inspection.

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

Stewart Zweben (PPPL) worked with Princeton Scientific Instruments to prepare the
PSI-5 camera for delivery to C-Mod next week. This camera captures 300
frames at up to 250,000 frames/sec, and will be used for the Gas Puff
Imaging diagnostic as well as others.

Manfred Bitter and Ken Hill continued to carry out experiments on C-Mod using
the improved high resolution X-ray diagnostic from NSTX. Good spectra were
obtained during ohmic operation, but the signal/noise was degraded during high
power RF heating.

Marco Valisa from RFX completed his visit to work with the Diagnostic Neutral
Beam, which is on loan to C-Mod from the RFX Group in Padova. During his stay
he carried out several experiments to characterize the beam operation and
associated diagnostics, while becoming familiar with operation of the
beam.



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