From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
Sept 29, 2003

The maintenance period at Alcator C-Mod continued last week. Progress
continued on all systems in preparation for resumption of tokamak operations
in early October. There was also considerable progress on the Lower Hybrid
Project.

Operations
----------

The vessel was subjected to a 120 degree C bake beginning on Monday and
lasting through the weekend. Electron Cyclotron Discharge Cleaning (ECDC) in
deuterium was carried out simultaneously with the high temperature bake.

The alternator reassembly has been completed and the unit is expected to
return to service this week. The machine was on turning gear over the
weekend. Some instrumentation work remains to be completed prior to
operation.

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

Work began on tuning transmitter #4 (J-port) to 50 MHz in preparation for
plasma operation at that frequency in October.

DNB Systems
-----------

The diagnostic neutral beam system is being readied for resumed
operation. Instrumentation in the LHe dewar was repaired and re-installed. The
vacuum system is reassembled and ready for startup. A new turbopump has been
installed. A new connector flange that provides additional instrumentation
between the gate valve and the pump was installed.

The new 2d cxrs detector was installed and initial alignment completed by Bill
Rowan (UT-FRC). The detector will allow the number of spatial channels which
can be used simultaneously to increase to 40.


Lower Hybrid System
-------------------

The Lower Hybrid launcher work continued this week. Dielectric constant
measurements have been completed for four new ceramic window test samples at
the operating frequency of 4.6 GHz. The measurements result in a slight
reduction in window thickness, and the new dimension has been passed on to
the manufacturer for grinding.

Removal of the old windows from the first coupler through acid etching of
the braze material has progressed slowly. Seven bricks have been removed,
nine bricks are loose but not yet out, and the remaining eight bricks show
acid flow past the periphery. We are attempting to speed up removal by
purchasing baffling, acid pumps, and acid heaters/controllers in order to
direct acid flow better, increase the acid temperature to raise the etching
rate, and allow all remaining couplers to be etched in parallel. This is
expected to speed up this process considerably.

Our brazing vendor has performed eight additional braze coupon tests, six
with ceramic and two with bare metal. These tests concentrated on the two
alloys which had performed best in the initial investigation, Gapasil and
Silver ABA. Preliminary indications from a tapered gap test show good
wetting and filling. The Silver ABA seems to wet the titanium metal better,
but also wets the alumina better, which might lead to a greater cleanup
effort. A test summary report is expected from the vendor by 9/29.

Work is continuing with Bill Beck (MIT) on the concept of thinning the
coupler outer walls to reduce mechanical stresses on the ceramic. Two
thinned-wall concepts will be braze tested starting next week, with a total
of six to seven 3-channel samples. The braze alloy will be chosen after
receipt of the test summary report. Bill is also investigating measuring
post-brazed stresses.

The Forward Wave Guide that was successfully power-tested at MIT has been
returned to PPPL. Positioning holes are being reamed for final assembly, to
be followed by trial fit-up with flanges. A final assembly procedure is
being generated, and will be reviewed with MIT.

At MIT, work last week centered on testing of the circulators and the reworked
water-cooled dummy loads; improvements to these loads made by the vendor to
bring performance up to specification include silver plating and increased
water flow. The tests were carried out with the assistance of an engineer from
the vendor. After some on-site modifications, both loads successfully passed
full power, full pulse length tests. The next tests were with the circulator
terminated into a matched load. Full power and pulse width operation was
achieved. The output was then terminated with a shorting plate (to simulate
an arc) with the water load to Port 3 of the circulator. Full power and full
pulse width were again achieved.

During the last of the 5 second circulator tests, problems were encountered
with the klystron V3 of cart 2. The output power was found to be low early in
the pulse and then increasing in time. This behavior may be indicative of the
tube becoming "gassy". We are considering means to remedy this situation.
Meanwhile, the two rebuilt klystrons received earlier from CTL are being
perpared for installation and testing.

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

Garry McCracken visited the PSFC Sept 25 and discussed edge physics with the
edge group as well as the prospects for ITER.

On Wednesday of last week, Earl Marmar, Martin Greenwald, Bruce Lipschultz
and Miklos Porkolab travelled to Germantown to meet with OFES staff.
Discussions centered on recent research highlights and plans for C-Mod.
Attending from DoE were John Willis, Rostom Dagazian, Erol Oktay, Curt
Bolton, T.V. George, Steve Eckstrand and Arnold Kritz.

Catherine Fiore, Dimitri Mossessian, Amanda Hubbard, Miklos Porkolab, Darren
Ernst and Jerry Hughes (MIT), and Perry Phillips, Alan Lynn, and Matt Sampsell
(UT-FRC) attended the 9th Annual IAEA TM on H-Mode Physics and Transport
Barriers 9/24-9/26 in San Diego. Work was presented on control of ITBs in
Alcator C-Mod (Fiore), and the role of TEMs and neoclassical transport in
Alcator C-Mod ITBs (Ernst) . C-Mod/DIIID/ JET dimensionless scaling results
for the edge pedestal were reported by D. Mossessian and the role of plasma
and neutral transport in the edge pedestal in Alcator C-Mod was explored by
J. Hughes. A. Hubbard presented her work on local threshold conditions and
the fast transition dynamics of L-H mode transitions in Alcator
C-Mod. Phillips, Lynn and Sampsell presented posters on "Observations of Core
Modes during RF-Generated Internal Transport Barriers in Alcator C-Mod" and
"Measurements of the Quasi-Coherent Mode on Alcator C-Mod"



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