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


Alcator C-Mod Weekly Highlights
January 31, 2000


We continued to make good progress on invessel, RF, and DNB work last
week. Invessel work has begun to concentrate more or installation
and alignment instead of fabrication.


Engineering:

Invessel work last week at K-port included installation and alignment
of the bolo box, and changes to improve the view of the Z-meter array.
New diborane lines were also installed. Additional studs have been
added to better support these lines where they bend out radially.
The hole pattern in the lines has also been changed to improve the
distribution of diborane around the machine during boronization.
A new H-port mirror has been installed to allow the McPherson
VUV spectrometer view to be scanned across the divertor. Installation of
new flux probes on both limiters is proceeding. All tiles (fewer than 5)
that showed melting during the last run campaign have been replaced. All
antenna work is complete including installation of the new boron nitride
tiles. The new shutter for the MSE diagnostic has been installed and
made operational. During calibration of the MSE/BES system, it was discovered
that two of the mirrors in the in-vessel optics enclosure had come loose, and
the optical system had become misaligned. These components have gone back
to PPPL where the mount will be redesigned. The new CXRS poloidal view
telescope has been installed. Fabrications still in progress needed for
invessel work include rangefinder retro-reflector holders, and a new mirror
at A-port. This mirror will provide a new view of the J-port antenna. These
fabrications should not delay pumpdown. Finally, we have begun to throughly
clean the vessel to remove old boron deposits.

The input cavity and tube assembly for the FMIT#1 transmitter is complete
and installed. Work on the output cavity is nearly complete. We have also
begun work on modifications to the PPPL demodulators to improve their
response time.

The DNB was moved from its test area to the C-Mod cell last week. All major
in-cell components are in place and interconnection is in progress.
Air, water, and vent lines are also being installed.

Work continues on upgrades to the cryo system. We have sealed the
LN2 sump after making improvements to the gas, liquid, and transducer
feedthroughs. Wiring for the cryo control system has also been improved.
Work to improve the operation of the cryostat exhaust valve has been planned
and will begin shortly.


Physics:

Recent analysis of data from the omegatron ion mass spectrometer shows
that the magnitudes of the resonant ion current increases with applied
RF power up to a saturation value. A simple model of the collection
of resonant current reproduces the observations. The model assumes a
shifted half-Maxwellian distribution for each ion species, allows for
reflection of ions due to space charge in the resonant cavity, and
establishes a collection criterion based on resonant ion dwell time in
the cavity. Within the assumptions of the model it is possible to
extract from the experimental data the temperatures of individual ion
impurity species. It is found that the impurity temperatures are all
near the bulk ion temperature, approximately 3 eV, recorded with the
retarding field energy analyzer on the omegatron probe. From the
asymptotic value of the resonant current it is possible to estimate
the absolute flux of individual impurities. Fluxes of charged states
of boron dominate the impurities with levels near two percent the
deuterium flux. With further assumptions about the relationship
between the impurity velocity and impurity density at the sheath edge
it is possible to obtain estimates of the impurity density in the edge
plasma.


Travel and Visits:

David Winslow and Keith Carter (UT-FRC) spent 1/18 to 1/26 at C-Mod
installing the Texas turbulence probe on the DNB stand at F-port. The
vacuum system, electronics, and probe alignment work will begin during the
week of 2/14. It is expected to be ready for operation during the next
campaign.

Ron Bravenec (UT-FRC) was at C-Mod from 1/25 to 1/28 to assist Norton Bretz
(PPPL), Dan Simon (PPPL), and Howard Yuh (MIT) install the relay optics,
PEMs, and image dissector for the MSE/BES optical system.

Gerd Schilling (PPPL) spent most of last week at PPPL for discussions on the
C-Mod collaboration aspects of the PPPL Field Work Proposal.