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
July 17, 2000

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

On Sunday, July 9, the machine was backfilled with hydrogen for calibration of
the Thomson scattering diagnostic. The Raman scattering calibration was
carried out on Monday and Tuesday.

After completion of the Thomson scattering calibration, the vacuum vessel was
opened for manned access. The first survey indicated very low diborane
levels and no measurable radioactivity, allowing work inside the tokamak to
proceed without delay.

A general invessel survey was carried out on Wednesday, with video
documentation. The status of invessel components was generally good.
A few systems suffered light damage. Some ground glass has been found near
the MSE/BES optics, indicating damage to the lenses or mirrors. Some
internal optics such as the J antenna-view mirror, the DNB poloidal view,
and CXRS optical systems have been covered with boron, and plans are to clean
them in situ. Five molybdenum tiles (out of 8000) suffered damage from heat
flux and disruptions, and are being replaced. Additional work planned for
this opening includes some minor calibrations, alignment work for
diagnostics, and installation of the PPPL invessel telescope that will be
used for 2d imaging of edge fluctuations. The fiber optic bundle, bellows,
window, and miniconflat seals have been installed for this new diagnostic.
Imaging checks of the telescope system indicate that 1 mm spots will be
easily resolved.

This vacuum opening is scheduled to be completed within a week, followed by
leak checks, a 120C bake and ECDC in order to resume basic operations
at the end of August.

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

An inspection of the antennas was performed as soon as general invessel
access was possible. The J-port antenna was in very good condition compared
to the previous campaign. The protection tiles had little arc or melt damage,
indicating the new shorting strap between the tiles is functioning as
designed. The new insulating washer assemblies are in excellent condition,
as are stainless cups inserted to protect them. There are some vertical arc
tracks on the vacuum wall that suggest the antenna is at a different potential
with the toroidal field present. These arc tracks are consistent with the most
likely cause of our parasitic loading being the generation of plasma waves
between the back of the antenna and the vessel wall. We have already
completed grounding of the J-port conductor covers. Machining for the
remaining pieces has been completed and we have begun reassembling
the antenna. The remaining major item is the side plate installation, and
this should be finished by the end of the week.

An inspection of the D-port antenna revealed minor pitting and tracking on the
top left strap (looking out along the major radius). Video images during
plasma operation had already indicated a problem at this location that when
correlated with phase balance and reflected power measurements lead us to
suspect a transmission line or feedthrough. Inspection of the
feedthrough indicated tracking had indeed taken place at this location.
Clean up of of the feedthrough is progressing well, and reassembly will begin
this week.

The E-port Faraday screen shows indications of arcing near the BN tiles.
The mechanism for this problem is not as yet understood, but is certainly
under investigation.

The Mo side tiles have been removed from D and E-port antennas in preparation
for installation of the BN tiles. The BN tiles have been fitted and await
the fasteners for installation.

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

Stewart Zweben visited from PPPL all last week working with Jim Terry
to install the 2d telescope.