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
March 27, 2000


The C-Mod power supplies were tested into the machine last week. We
are in the final preparation stages for the start of the next run period.
Work continued on the Lower Hybrid test stand, ICRF transmitters, and the
DNB. Work is moving forward on the new divertor upgrade.


Engineering:

On Monday of last week C-Mod was pre-cooled to standard between run
temperatures. On Tuesday a full cooldown took place and power supply tests
into the magnets were begun. All supplies were tested during this first
day, as well as the pulsed gas system. On Tuesday tests of the
commutation banks were completed and a plasma attempt was made. H-alpha,
CII, visible bremsstrahlung, and magnetics diagnostics indicated an initial
breakdown with a good null. We then stopped operation for the rest of the week
so that work on the ICRF and DNB systems, which require power room access,
could continue. Operation will begin again this Tuesday with investigation
of reversed field startup optimization being the primary goal. The week will
also be used for diagnostic commissioning and checkout.

The DNB was operated into its beam dump at up to 47 kV. Repeatable 100ms
pulses were created at 45 kV with 5.2 A of beam current. The arc current
as a function of filament current was re-characterized with the longer cables
into the C-Mod cell. Operation of the beam in the C-Mod cell had
previously been limited to 20 kV by arcing between the accel and gradient
grids. This problem was traced to the initial voltage rise of the grids
which caused the two grids to be more than 16 kV apart (normal operation
has a 5 kV spacing at 50 kV). The problem was fixed by adding additional
capacitance to the gradient grid voltage divider and by adjusting the
mod-reg rise time. The accel and gradient grid voltage rises were
characterized as a function of mod-reg parameters.

We continued work on the Lower Hybrid test stand last week with the
installation and wiring of some of the crowbar components. Work on the
high voltage bus was also started. The low voltage power supply for the TPS
(Transmitter Protection System) chassis is operational, assembly of
the TPS cards is complete, and the cards have been powered up. Testing of
the TPS control functions has begun. The high voltage control wiring
installation is in progress, and work on the water cooling system continues.

As part of the divertor upgrade project, a detailed 3D computer model of the
C-Mod vacuum vessel is being produced with locations of studs, thermocouples,
and magnetics indicated. A design for the new divertor incorporating a
re-enforcing hoop that both supports the tiles and adds strength to the inner
wall is also being developed. A non-linear finite-element-analysis of the
new design is being carried out, and the response of the new hardware to
halo and eddy currents generated during disruptions is being quantified.
Installation of these new components is planned for our next up-to-air period.


Physics:

N. Bretz and R. Cutler, PPPL, worked at C-Mod from 3/20-3/23 this week with
H. Yuh of MIT installing the MSE and BES image dissector and the photoelastic
modulator, installing the shutter control, referruling the BES fibers, and
installing the MSE and BES fibers in the dissector at F-Port. The MSE
fibers were checked and connected to their respective photomultpliers in
the rough setup lab and a filter set was installed which will match operations
with a 45 keV hydrogen beam and a 5.3 Tesla on-axis toroidal field.
All of the electronics on the test cell floor and in the rough setup lab were
connected. The filter heater controls were tested though CAMAC and appear to
be working. H. Yuh is completing the I/O controls and MDSplus tree.
This work completes virtually all of the hardware installation for MSE and
integrates the BES fibers into the combined dissector.


Travel and Visits:

Dr. Nakanishi Hideya and Dr. Emoto Masahiko from the LHD experiment at NIFS
visited Tom Fredian and Josh Stillerman from 3/20-3/24 to discuss data
acquisition and analysis topics, including new data acquisition hardware,
MDSplus porting status, CCD camera applications and data handling, and data
visualization tools.

Bill Noonan and Ray Elton, from the U. of Maryland, were here last week to
prepare their spectrograph for operation during the next run campaign.