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
Dec 30, 2002

The maintenance period continued at Alcator C-Mod last week.

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

FMIT#4 was retuned to 78 MHz from 70 MHz. Into dummy load, 1 sec
pulses up to 1.1 MW were completed. Further refinement of the tune is
required for higher power pulses.

Following removal of all the antennas, an initial inspection showed that the
modifications to the antennas prior to the 2002 campaign were successful.
Modifications to shield the BN-metal interfaces from the plasma were
successful on all antennas. No melt damage on plasma facing elements was
found except for a few washers on D and E-port antennas. The reason for the
minor melt damage to these few washers is unclear. The modifications to the
J-port current strap also appear to be successful. No arc damage was found at
the "bridge" location. The J-port maximum voltage
obtained in this campaign exceeded the previous limit by ~15%. The maximum
voltage limitation on all antennas appears to be related again to where E
parallel to B. For the D and E-port antenna, the voltage limit was ~10% lower
than in past years. The modifications made to the FS to shield the BN-metal
interface resulted in an increased electrical length of the antenna. This
changed the voltage pattern in the transmission line and placed a higher
voltage in a 20 ohm section of the vacuum transmission line. We are currently
considering modifying this section of line to 50 ohms. Initial analysis
suggests this would increase the voltage handling of this line section.
J-port also had similar damage at a low impedance section of the vacuum
transmission line. At the transition from coax to parallel plate transmission
line, a low impedance coaxial section had arc damage where E is parallel to B.
We are considering modifying this line section to ~50 ohms as well. The most
notable failure were the top and bottom end tiles. A total of four tiles
failed. The likely explaination is the disruption induced rocking of the
fastener assembly. As been reported earlier, a new tile design has been
completed and is undergoing testing.

DNB Systems
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During the week of 23 Dec, plans for redesigning and fabricating
replacements for two damaged MSE mirrors have been finalized. The new
design will use stainless steel clamps to hold the mirrors down over nearly
their entire perimeters (with a break to eliminate disruption eddy
currents). Teflon gaskets will be inserted (and trapped) on all edge
surfaces to pad the contact between the stainless steel and the glass.
This design requires shrinking the mirrors by about 0.5 cm on all edges.
To save time and money, we will use spare mirrors that already have
dielectric coatings, and cut off their edges, rather than buying and
coating new blanks. The new mirrors and mounts will be subjected to
appropriate shake tests.