From: IRBY@PSFC.MIT.EDU
Message-ID: <009C4596.EC41AB80.21@woods.uml.edu>
Subject: Alcator C-Mod Weekly Highlights
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT Plasma Fusion Center
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
April 6, 1998


After an extensive set of measurements were made to document
finger joint locations, spring plate tension, and arm-to-core clearances,
the lower TF arms were removed late last week. The lower
arms and core fingers were found to be in very good condition.
We are now preparing to remove the TF core so that it can be carefully
inspected and cleaned up for re-installation. Analysis of
electromechanical and thermal effects near the finger joints continues.
In addition, the extensive set of thermocouple and joint resistance
measurements made during the last campaign and previous ones is being reviewed
and will be used as constraints for the modelling effort.
Progress on the DNB also continues. The high voltage cables for
the accelerator were terminated at the snubber in the test area. The snubber
was prepared for high voltage, and the accelerator current monitor was
reinstalled. A block diagram for the beam timing was completed. The
interface between the existing beam timing system and the CAMAC timing
modules required for synchronization with C-Mod shot cycle was designed, and
construction was begun. The filtering system for oil conditioning was
modified to begin conditioning the oil in the Mod/Reg isolation transformer.
Also for the Mod/Reg, the high voltage input cables and ground pads were
installed. Coatings required to control voltage gradients and prevent
breakdown in the high capacitance section of the high voltage transmission
system to the cell were completed. The CAMAC data acquisition, the entries
in the MDSPlus tree, and the CAMAC control for the beam profile monitor
were completed and tested.
Work continued on the tuneable RF systems. FMIT#3 was successfully
tuned and tested at 78 MHz (2 MW output into a matched dummy load).
Tuning and testing FMIT#4 has begun. The 9" coax plumbing has
progressed as well and a design up to the resonant loops has been
sketched. We can finalize the design when more information about
the resonant loop design becomes available from PPPL.
The D and E-port antenna protection tiles have been removed for
inspection. During the last campaign, the center upper protection
tiles on D-port were consistently bright (at visible wavelengths)
suggesting power deposition. Melting on the tiles above the current strap is
visible on the lower edge of the tile. These tiles can get close to the plasma
because the antenna upper protection limiters do not follow the plasma
toroidal curvature. We are examining the possibility of removing or
shaping the tiles to reduce this problem.
Analysis has begun of data from the inner-wall array of RF loop
probes. This array is directly opposite one of the fast-wave antennas, and
the loop probe signals often show a large transient when the RF is turned on.
A series of three consecutive shots has been identified in which the decay
time of the turn-on transient increases as the minority cyclotron resonance
is moved outward from the center to the edge of the plasma. This decay is
thought to be caused by heating of the bulk plasma and the generation of a
minority "tail". Comparison is underway between this observation and the
predictions of the Fokker-Planck code FPPRF.
Professor Peter Stangeby and his grad student, Steve Lisgo, from the
University of Toronto visited Alcator last Wednesday and Thursday. The
primary purpose of the visit was to discuss the current state of modelling
that the U. Toronto group is doing using DIVIMP and EIRENNE codes to simulate
C-Mod edge and divertor plasmas. Professor Stangeby and Steve Lisgo also gave
seminars on this work and some related work concerning perpendicular transport
in the SOL. Work is progressing well in using the combined DIVIMP-EIRENNE
model to understand the neutral dynamics and thus flows in the SOL. Steve
Lisgo has been concentrating on analyzing D_alpha brightness data from the
inner wall. He finds the best match to experiment when the emitting region near
the divertor entrance has an electron temperature near 1 eV. We discussed
other data that could be included in the modelling and thus improve the
constraints on the model.
Earl Marmar attended the US committee meeting on IAEA synopsis
submission in Washington D.C. last week. Seven C-Mod papers (one
together with authors from DIII-D) will be submitted for consideration by
the International Committee.