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


Alcator C-Mod Weekly Highlights
August 16, 1999

Plasma operations continued at Alcator C-Mod last week. The principal focus
continued to be commissioning of the RF systems. Plasma runs were carried out
on Wednesday and Friday. Standard 1MA, 5.4T single null plasmas were run to provide
targets for the RF. The runs scheduled for Tuesday and Thursday were cancelled
to facilitate work on the RF system.

Physics & Analysis
------------------

An IDL routine has been written which automatically determines the confinement
mode (L-mode or H-mode) of the plasma by looking at the edge soft x-ray
brightness profile from the outboard midplane. The routine takes advantage of
the fact that the soft x-ray emissivity develops a pedestal shape in H-mode,
and that the emissivity at the top of this pedestal typically is 10-40 times
higher than the emissivity in L-mode. Initial testing indicates that the
routine is very reliable, but not fool-proof. The routine may become a
valuable tool for statistical analysis of large amounts of plasma data where
manual determination of the confinement mode is not feasible.

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

Last Monday, 8/9, we noted a large pressure excursion on the FMIT#3 2242 final
amplifier tube. Though the pressure came down, it never recovered to an
operational level. We continued to monitor the tube on Tuesday while work
continued on the RF power feedback systems mentioned in the last weekly
report. We operated C-Mod on Wednesday and successfully tested the feedback
system on the J-port antenna using FMIT#4. Having completed this test, we
stopped the run and did a high-pot of the 2242 tube. When the tube failed the
hi-pot test, we disassembled the cavity and removed the tube. We found that
one of the water cooled filament connections to the tube was badly discolored.
It is clear that this filament connection was not being cooled adequately, and
that the resulting overheating caused a leak. This incident is still being
investigated, and a report will be issued at a later date. The 8 to 9 year
old tube was installed at MIT in 1996. It operated successfully during our 40
MHz campaign in 1997, and during the many hours of operation needed to bring
up the new J-port antenna. All filament cooling lines on the other three
transmitters have been checked and found to be properly cooled.

We have now installed the 2274 2.5 MW tube from FMIT#2 into the FMIT#3 cavity.
The flow of cooling water to the tube has been verified and all associated
alarms and interlocks have been tested. FMIT#3, with the 2274 was tested at
low power during the run on Friday. This change will ultimately give us 4 MW
of RF power capability into the J-port antenna.

We are working with GA to obtain another 2274 tube for the FMIT#2
transmitter. We hope to have this 2274 in house by midweek, with installation
and testing typically taking about one full week.

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

Dr. Ricky Maqueda from LANL is on site for a week to investigate edge
fluctuations and "filaments" as seen by gated (~10 microsec), fast framing
(1000 fps) camera. He will also be running the IR camera system. This is part
of the C-Mod collaboration with LANL.