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
Nov 10, 2003

Plasma operations resumed at Alcator C-Mod last week, after a one-week hiatus
for the APS Division of Plasma Physics Meeting. Four run days were scheduled
and three were completed. A total of 49 discharges were produced, with a
startup reliability of 56%. The first boronization of the campaign was carried
out on Thursday evening. Experiments were conducted in support of the RF and
Edge/Divertor topical science areas. The ICRF power into plasma was increased
to 5.2MW for 0.7 sec and 6MW for .45 sec. Work also continued on the lower
hybrid system development.

Plasma operations are scheduled to continue this week.

Operations
----------

Plasma runs were carried out Tuesday, Thursday and Friday; the scheduled run
on Wednesday was cancelled in order to investigate a ground fault indication
on the alternator rotor. The fault was cleared and, following consultation
with the manufacturer, operation resumed on Thursday. In addition to continued
machine conditioning, especially of the strike point surfaces for the upper
null configuration, two physics experiments were carried out. The RF group
completed experiments characterizing the effect of a pre-matching network on
the D-port antenna, as discussed below, and characterization of wall-pumping
and gas release continued.

Following Thursday's run, a fresh boronization was carried out, using 140 psi
of B2D6 resulting in an average deposition thickness of 2048A. The H/(H+D)
ratio on Friday was down to 3 or 4% and the carbon II light was down by a
factor of 3 or 4, indicative of a good boronization. The startup
reproducibility on Friday was also low (under 50%), which is also typical of
the first run following boronization.


Physics
--------

The amount of gas retained by the walls after each shot was monitored
throughout the week, with the goals of understanding what conditions affect
the wall pumping, assessing the wall H/D ratio and its relation to that in the
plasma, and determining the effect of boronization. The configuration was
varied from upper to lower single null, and RF power from 0 to 6MW. In
addition, a series of discharges on Thursday were run with helium majority
rather than deuterium; for these cases the hydrogenic content of the discharge
is determined only by the wall. As expected, helium operation also tends to
degas the walls. In a series of discharges where the RF was first on, then
off, and then back on, the H/D lowered during the RF off discharge and the
amount of gas required to maintain the discharge increased relative to the RF
case. This indicates that the RF is leading to more gas in the discharge,
possibly due to recycling effects or heating of surfaces.


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

Further power conditioning of the antennas has been accomplished. New high
power ICRF discharges were achieved: 5.2 MW for 0.7 sec without fault and 6 MW
with single trip for 0.46 sec.

Quarter wavelength pre-matching transformers have been added to the D-Port
antenna transmission line system. After proper consideration of the reactive
component of the plasma loading was included in the design, a large reduction
in the reflection coefficient as seen on the transmitter side of the
transformers was found (rho from 0.75 to 0.55). A 10 kV reduction in rf
voltages in other matching components was also seen.

These results suggest that pre-matching can be successfully used to limit the
voltage in the phase shifter-stub tuner matching elements. Furthermore, we
can limit the range of reflection coefficients that an active matching network
needs to cover.

DNB System
----------

Early in the week a 12 V supply in the DNB rack malfunctioned. A replacement
supply was installed after the end of Tuesday's run. Although C-Mod did not
run on Wednesday, the DNB was fired throughout the day for conditioning
purposes. A minor problem with an intermittent trigger cable was fixed.
During plasma operation on Thursday and Friday the beam was fired on nearly
all shots. Thursday's run provided useful BES data to compare helium and
deuterium plasmas. For the last shot on Thursday, the MSE polarizer was
installed and pitch angle data was obtained during a period of very long
sawteeth. The DNB continued to run well throughout Friday, with beam ion
current at 5 amps on many shots.

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

The front end couplers are presently being refurbished at PPPL. All bricks
and residual brazing alloy have been removed, and machining to reduce wall
thickness and lower brazing stresses is under way. Roughly half of the new
ceramic windows have been received following edge metallizing, the remaining
are due next week. The greater part of prototype braze testing has been
completed.

The forward waveguide part of the launcher has now been completely reassembled
at PPPL, the shroud has been welded, and vacuum leak checking is in progress.
Mechanical distortion due to the welding has been reduced from the first
assembly.

The Coupler Protection System (CPS) design is nearly finalized, and PC board
layouts for the I/O boards have been completed.

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

Joe Snipes attended the Active MHD Control Workshop in Austin, Texas from 3 -
5 November and presented results from Active MHD Experiments on C-Mod.

Perry Phillips of UT-FRC was on-site for several days to start repairs for the
FRC ECE radiometer. At present, it appears that the local oscillator for 16
of the 32 channels has failed. The LO has been sent to the manufacturer for
testing. The radiometer will remain in service with 16 core channels.

During the week prior to the APS-DPP meeting, Matt Sampsell prepared the BES
system for operation by calibrating the spectrometer, positioning the
fibers/views, and tuning the bandpass filters.

Manfred Bitter, Ken Hill and Sang Gon Lee (Korea Basic Science Institute)
were at MIT this week, installing a new detector for the X-ray crystal
spectrometer. Helium-like Argon spectra were measured during the runs, and
will continue in the coming week. Analysis of the data is ongoing.




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