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
February 5, 2001



We continued work on the RF antenna redesign last week. Work also
continued on new diagnostics for the antenna and the new inner
divertor.


Physics
-------

Joe Snipes participated remotely in a continuation of the JET experiments
attempting to obtain Enhanced D alpha H-mode on JET on Friday, 2 February.
There were seven discharge with -pi/2 phasing of the ICRF antennas at 2.8 T,
and 2 MA, to drive current near the q=1 surface, in an attempt to destabilize
sawteeth and prevent long 'monster' sawteeth. The first discharges had less
than 2 percent hydrogen concentration and drove a substantial fast ion tail
that did stabilize the sawteeth such that large n=2 MHD modes were driven
after the first sawtooth collapse, which substantially degraded confinement.
Raising the hydrogen concentration helped to destabilize the sawteeth and
eliminate the large MHD modes. There were, however, problems with the ICRF
such that only 5 to 6 MW of RF power was available, which was insufficient to
get into H-mode. 2.5 MW of NBI was added on top of the available RF power
to stay in H-mode. Several long ELM-free H-modes were obtained as the target
density was raised in an attempt to find a perhaps narrow operating window in
which the EDA H-mode may exist on JET. However, there was insufficient time
to complete this density scan. There was no clear evidence of EDA H-mode in
the pulses obtained, but more analysis of the reflectometer and magnetic
fluctuations needs to be done to see if there were any modes present that
could be related to the Quasi-Coherent modes observed in C-Mod EDA H-modes.


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

The solid model design for the new antenna components has been completed, and
we have begun fabrication of prototype parts out of aluminum. The new design
incorporates better use of magnetic insulation and reduces the electric field
at critical locations. Combined with changes already being made to the front
of the antenna to reduce impurity injection at high power levels, we feel
these changes will result in better antenna performance. The prototype
components will be checked in our antenna test fixture for any design
errors after which the inconel stripline components will be made.

A schedule has been developed to implement the changes to the antenna.
We expect installation of the antenna with the new components to be
complete by 3/23. Following pumpdown, vacuum conditioning, and
plasma startup, plasma physics operation will begin in early May and continue
through early June. Please find our short term schedule at

http://www.psfc.mit.edu/cmod/operations/short_term.pdf

Maintenance continues on the ICRF transmitters. All cavities are being
inspected and refurbished where necessary. Mechanical and electrical systems
needed for tuning the transmitters are being reworked and checked. PLC
connections are being determined for the new automatic dummy load switching
system.


Lower Hybrid Project
--------------------

Work continued last week on development of the control hardware and
software for the lower hybrid project. Progress was also made on
the three klystron carts with installation of filament transformers,
chokes, and high voltage bus. Preparations for the Lower Hybrid
review to be held at MIT on 2/7 and 2/8 continued.

More information has been supplied by the vendors bidding for the
lower hybrid system circulators. The selection committee should be
able to reach a decision soon on what vendor to use.

A great deal of progress has been made on the Lower Hybrid Active
Controller system. The power supply for the I-Q detector and vector modulator
characterization tests was completed, and tests on the sample I/Q detector
were conducted. Initial results indicate some unexpected non-linearity.
Testing of the serial fiber optic link (SFOL) transmitter and receiver is
underway. A synchronization test for a multiple PCI chassis configuration
was done, indicating that only tens of nanoseconds of jitter is present.
This jitter is acceptable for our application.


Inner Divertor
--------------

Fabrication of the inner divertor components is proceeding.
Machining of the C-plates, which together with the rear plates, form
the major support structure of the divertor is now well underway.
The rear girdle plate inner and outer surfaces have been machined using
a wire EDM technique. Fixturing and tooling for cutting the holes is
now being set up. 350 of the 1300 tiles needed for the project have
been delivered. These tiles are now being delivered at a rate of about
200 per week. All bolts for attaching the C-plate to the rear girdle
plate have been received, age hardening, and will soon be silver plated.
All transition keys and tile screw keepers are complete and in house.


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

Ben Carreras from ORNL began a visit at MIT last week. He has been
working with Amanda Hubbard on interpretation of C-Mod H-mode threshold
data and discussing possible edge instabilities in the EDA regime.
He has also been working with Brian LaBombard the analysis of
fluctuation data.

A successful Preliminary Design Review of the Lower Hybrid launcher
was held at PPPL on 1/31; Ron Parker, Monty Grimes and Paul Woskov
attended from MIT.

Randy Wilson and Bob Ellis from PPPL visited last week to participate
in the ICRF antenna rework.

Martin Greenwald was at UCLA last week as part of the FESAC sub-panel
reviewing the US fusion science theory and modeling program.