From: IRBY@PSFC.MIT.EDU
Date: Tue, 14 Jul 1998 09:30:24 EST5EDT4,M4.1.0,M10.5.0
Message-ID: <009C9292.7DE67C80.17@woods.uml.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 13, 1998



Engineering:

Work on the TF magnet is proceeding quickly. All upper and lower
horizontal arms have been electroplated with silver. The plating will
undergo tests this week to qualify it for operation. Electroforming of the
core finger has been delayed several days as we rework the fixturing. The
fixture must seal with absolute reliability against the flow of electrolyte
into turn-to-turn joints. A secondary seal is being added to insure that
this is the case. The TF core was prepared for plating over the weekend,
and plating of the lower core fingers will begin early this week. Though
somewhat dependent on the electroforming schedule, we hope to complete all
plating activities within the next 2 weeks.
Feltmetal tests also moved along quickly last week. Samples with
a 0.0005" layer of silver plating were run for 36,0000 cycles, at 800 psi,
-170 degrees centigrade, and a 2.4 mm stroke. The ability to run the
finger joints at 800 psi rather than the previously used 400 psi will,
according to our latest simulations, make the joints more reliable during
high field operation. The test was stopped not because of wear of the
feltmetal, but because of the need to proceed to other test conditions.
Examination of the feltmetal indicated it was still in good condition.
Inductive heating soldering tests were concluded very successfully
last week. We are now preparing to start production soldering of the lower
TF arms.
We have completed the initial conditioning of the DNB plasma source.
The source generates the plasma from which ions are extracted, accelerated,
and neutralized. Stable, predictable operation is critical for a diagnostic
beam. A plasma was generated in the source and its parameters
scanned across the typical operating range required for successful neutral
beam generation. This test was followed by a series of 50 shots to test
reproducibility near the expected initial operating point for the plasma
source. Parameter variation during this series was only a few percent
which is acceptable. Finally, the plasma source modulation was tested from
100 Hz to 1 kHz. Modulation will be used to improve diagnostic
sensitivity and will contribute to improving the usefulness of the beam
at high plasma densities. At present, modulation works adequately to 750
Hz. Improvements are expected when the new modulation capability of the
accelerator is implemented. With the successful completion of these tests,
the next major goal is operation of the accelerator and generation of a
neutral beam.


Physics:

At the weekly staff meeting, B. LaBombard discussed plans
for the continued operation of the A-port horizontal fast scanning probe.
The A-port probe was built and operated over the past three run campaigns
by Jim Reardon to study RF edge effects for his doctoral thesis. Now
that Jim has finished collecting data, the probe will be operated by
the edge group to collect data for scrape-off layer
transport studies. New Mach-Langmuir probe heads will be fabricated
for the probe drives to monitor parallel flows as well as density
and temperature profiles up to the separatrix. The new probe head
design will employ some boron nitride materials and should be more
resilient to plasma disruptions. Also, a slimmer, more rugged support stand
for the probe drive at A-port will be fabricated. The probe system
has a vacuum gate valve to allow damaged probe heads to be replaced.
For this reason, the system can also function as a sample manipulator (during
ECDC wall conditioning experiments, for example). Based on Jim Reardon's
experience, RF pickup and sheath rectification effects are expected to be
low at the A-port location (with RF injected at D- and E-port locations).
Planned experiments included: scrape-off layer profiles for transport
studies in RF heated H-modes, parallel flow profile measurements, and direct
comparison between Langmuir probe and reflectometry inferred density profiles.
The probe and the reflectometer are located on the same horizontal
port. Comparisons of fluctuation levels determined by the probe and
reflectometer should also be of great interest.
Professor C.S. Chang from the Courant Institute visited MIT and
the Alcator C-Mod Group on July 9-10, 1998. Dr. Chang first spent time
becoming familiar with the toroidal rotation measurements made by
John Rice on C-Mod. We also explained to him the various rf minority
absorption schemes and mode conversion electron heating experiments
that can be done on C-Mod. He also spent considerable time and effort
explaining his theory for the generation of plasma rotation by ICRH
to members of the C-Mod Group. Dr. Chang has been able to show
that ICRH can induce inward radial movement of the guiding center
orbits of fast minority ions without direct momentum transfer. This
non-ambipolar radial transport can generate a radial electric field
and subsequent plasma rotation. His theory agrees with many of the
qualitative features of John Rice's measurements. For example, the
predicted toroidal rotation for C-Mod is in the same direction as the
toroidal current (C0-), the direction of the rotation reverses with
a reversal in the direction of toroidal current (remaining C0-current),
and the rotation speeds decrease with increasing plasma current. Using
30-50~keV (H)-minority tail temperatures and the predicted minority
absorption power density from FPPRF and TORIC, Dr. Chang was able to
predict toroidal rotation speeds in the range of V_tor = (1-3) X 10^6
cm/sec and radial electric fields on the order of 100 V/cm for typical
L-mode discharge parameters. Plans for future modelling and analysis
of the C-Mod data over the next few months were also made.


Travel and Visits:

Ian Hutchinson was in Culham last week, Steve Wukitch was in
Garching, and Rejean Boivin was at Lausanne. Ian Hutchinson discussed
TF joints, spring plates, and feltmetal with the MAST group.
Ben Carreras from Oak Ridge is visiting the PSFC for two weeks. He is
working with the group on neutral particle modeling, L/H transitions,
turbulent transport, and H-mode regimes. He will also be collaborating with
the theory groups on various divertor problems.
Pablo Acedo and Ernesto Garcia from Carlos III University, Madrid,
are here for the remainder of the Summer collaborating on interferometry and
a laser rangefinder. They hope to develop instrumentation to measure the
edge density profile, and to monitor movement of the inner wall during
disruptions.