From: IRBY@CMOD.PFC.MIT.EDU
Date: Wed, 27 May 1998 8:46:35 -0400
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980527084635.25a004f6@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT Plasma Fusion Center


Alcator C-Mod Weekly Highlights
May 25, 1998


Good progress on the TF refurbishment was made last week. All
feltmetal has been removed from both the upper and lower TF arms.
We are now removing the fiberglass layups from the outer end of
the arms in preparation for cleanup and replating of those arm surfaces
without feltmetal. The between plate surfaces on both the upper and
lower core fingers have been sealed. Sealing of the between turn surfaces
on the upper core with a silicon rubber compound is nearly complete.
As mentioned in previous reports, the core must be completely sealed before
we can begin cleaning, electroplating, and electroforming operations.
Work continues on inductive heating fixturing in preparation for soldering
tests to be conducted later this week. Progress on both small area and full
area feltmetal tests, and the diagnostics required for these tests has been
made. For the full scale test, a set of Rogowski coils is being designed to
measure both total current through the feltmetal pads and the current
distribution in the copper. In addition, thermocouple, voltage, and optical
measurements are being planned.
A great deal of progress was make last week in tracking down the
arc in FMIT#4. The problem has been isolated to the EIMAC 2274 vacuum
tube. We replaced the this tube with a backup EIMAC 2274 and
produced 1 MW of power at 78 MHz before we developed an "arc" (forward and
reflected power are equal) between the directional coupler looking into the
FPA input cavity and its output cavity. We do not think this arc is
related to the previous one, and this test therefore indicates that the
original fault was in the EIMAC tube. Dr. Gerd Schilling from PPPL visited
last week. We discussed the PPPL antenna and some data from the last campaign.
The antenna is in the assembly stage and electrical testing is not due to
begin for a couple of weeks. We are currently expecting the antenna to be
delivered the beginning of July. We intend to assemble the antenna while
the machine is still on its assembly stand. This plan will relieve some of
the pressure on invessel resources during machine reassembly.
We continue to proceed in the implementation of a DNB for C-Mod.
Conditioning of the oil in the tank shared by the arc/filament/snubber
supplies continued with steady improvement of the breakdown voltage to
28kV. Detailed design changes and associated documentation for the Master
Control Logic (MCL) chassis continued. The logic boards required in the
arc/fil/snub interface to the MCL were ordered. Compensation of the high
voltage dividers at the beam began. Modification to the MCL analog chassis
was begun. The kirk key relay interface chassis is now designed, built,
installed and wired to the transfer block switches and to the PLC
inputs and is awaiting testing. The first version of VAX software for
control and timing of a DNB conditioning shot via an IDL interface was
completed. Though it cannot as yet be used for hardware control,
the program can detect C-Mod state and implements a means for avoiding
collisions with C-Mod shots. Work continued on the MCL and PLC interface
circuits and PLC programming.
A review of progress on DNB diagnostics and design of F-Port was held
on May 20 at MIT. John Heard (Auburn) described the internal ECE optics. Ned
Eisner (U. Texas) discussed his design for the beam limiting aperture required
for MSE and BES. Norton Bretz and Bob Parsells (both of PPPL) traced recent
progress on design of MSE optics. Sanjay Gangadhara described the side
periscope. Bill Rowan (U. Texas) presented his design for the internal CXRS
optics and the group design of F-Port flange.
Further analysis has been done by Joe Snipes of fluctuations in H-mode
during the 980122 run in which plasma current and toroidal field were ramped
during steady Enhanced D-alpha H-mode (EDA). Although the fast (1 MHz and 2
MHz) sampled magnetic pick-up coil signals were limited to 131 msec and 65 msec
duration, respectively, a slowly sampled frequency comb filter array was used
to measure the fluctuations in frequency bands from 56 kHz to 640 kHz. The
fluctuations in the range from 56 to 320 kHz all increased when ramping the
current down from 1.2 MA to 0.6 MA. At higher current, the fluctuation levels
were considerably lower. One detailed case of a magnetic precursor to an L-H
transition was also analyzed in which a few oscillations were observed at
about 100 kHz just before the transition. It appeared to be high m and
n > 5 - 10 and was only visible on the outboard RF limiter coils, suggesting
a high n ballooning character. The mode rotated in the electron diamagnetic
drift direction. These characteristics are similar to Type III ELM precursors.
Edge profiles measured by the new A-port scanning Langmuir probe
during ohmic L-mode have been found to be similar to profiles simultaneously
measured by the F-port scanning probe. These profiles constitute our
best absolute density measurement in the edge and have been used by the
reflectometer group to provide a calibration.
Miklos Porkolab attended the NSTX Dedication Ceremony at PPPL, on
Monday, May 18th. He expressed the strong desire from MIT-PSFC to collaborate
on this project. Bruce Lipschultz, Jim Terry, Spencer Pitcher, and
Brian LaBombard attended the PSI conference in San Diego last week.