From: IRBY@CMOD.PFC.MIT.EDU
Date: Tue, 31 Mar 1998 11:02:25 -0500
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980331110225.22a2b3e7@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT Plasma Fusion Center
Newsgroups: sci.physics.plasma



Alcator C-Mod Weekly Highlights
March 30, 1998


All TF legs and upper arms were removed last week, and the machine
was moved to a support stand in the southeast corner of the cell. This
move was required before the lower arms could be removed. Inspections of the
upper arms indicate the need to replace feltmetal in many locations,
particularly on the inner most pads. We are currently reviewing the
early analysis of the finger joints in this location and also extending
this analysis with both analytic models and the better numeric tools now
available. Inspections of the lower arms that have been removed thus far
reveal very little feltmetal wear. Understanding this upper-lower asymmetry
has of course become a very important task for our analysis group.
We have continued preparations for the installation of the new RF
antenna. FMIT#3 was tested at 78.5 MHz with an output power of 1.5 MW into
a dummy load. This frequency will be used with the new PPPL antenna during
our next campaign. The coax transmission lines from the transmitters to the
cell are being modified to link up properly with the matching network to
be installed near J-port. Dr. Paul Bonoli and Ken Takase have been working on
porting ACCOME to the C-Mod ALPHA Workstations. This work was motivated
partially by planned changes at NERSC where they plan to remove from service a
compiler which was used by ACCOME. The equilibrium solver, lower hybrid and
neutral beam modules have been ported. Work remains on debugging the
lower hybrid module and writing an IDL post processor.
The antenna work at PPPL continues as well. The straps have
been plated and the Mo Faraday rods have been bent and stress
relieved. The rods are currently being brazed, and this process is
expected to take 2 weeks. The rods will then need to be coated with
TiC. Some of the back plane parts have been plated and are being
assembled for electrical testing and measurements of the completed
antenna. These tests and measurements will provide the remaining data
necessary to design the resonant loops.
Work continued on the diagnostic neutral beam. The ground pad for the
mod/reg was built and installed, nonconducting hardware was installed on
mod/reg HV cable docks, and high voltage cable was run from the mod/reg to
the cell wall. The fiber optic transmitter and receiver board testing was
completed. Construction and testing of the thermocouple system for
measurement of the unneutralized component of the beam was completed from
the signal conditioning chassis input to the CAMAC modules. Work continued
on fiber optic communication for the arc/fil/snubber voltage feedback,
monitoring and fault systems, the NWL controls, and the DNB timing system.
The HV ductwork termination box at the cell wall was installed for cut-to-fit
matching to the existing cell wall penetration.
Preliminary analysis of the C-Mod/Jet dimensionless identity
experiments shows rather good dimensionless agreement in the transport
parameters. However, it now appears that the shapes run on JET in the shots
most similar to C-Mod were somewhat different from those previously planned.
Therefore more analysis is needed to investigate the extent to which shape
differences have compromised the experiment or may explain the observed
differences in ELM behaviour.
During ELMy bursts, fast dips (20 ~ 30 microsec) have been observed
on our ECE grating polychromator (GPC) signals, which are correlated
with D_alpha and magnetic bursts. As the density increases, the dips have
been observed even on the inner-most GPC channels. One good candidate to
explain such dips is a density cutoff effect. When a narrow, sharply
localized edge density, high enough, but not thick enough to cutoff X-mode
second harmonic waves is assumed, evanescent wave-like features will occur.
Careful analysis of these wave effects has begun.
Ricky Maqueda from LANL visited C-Mod week-before-last to work on the
IR imaging periscope. It was found that the presumed boronization coating
on the sapphire window was in fact composed of small chips that had
broken off the edges of the ZnSe elements. Modifications to the periscope
are being planned to fix this problem for the next campaign.
We also plan to modify the view of the periscope to image the
divertor surface below the outer nose. Finally, the Kodak fast
framing camera hardware was removed from the C-Mod cell and shipped to Los
Alamos so that the intensified imager can be serviced.
Jim Terry attended the AIP Topical Conference on Atomic
Processes in Plasmas which took place at Auburn University from March 23-26.
He presented a very well received invited talk - "Study of Volume
Recombination and Radiation Opacity Effects in Alcator C-Mod Plasmas".
Paul Bonoli attended the Sherwood theory meeting last week.