From: IRBY@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
October 18, 1999


The C-Mod maintenance period continued last week and preparations for machine
operation scheduled to begin this week were made. Cooldown of the machine
was begun on Wednesday, 10/13. By Friday, the TF magnet was cool enough for
some low current resistance tests to be made. These tests indicated that
warming up the machine had succeeded in reducing the TF resistances back to
nominal levels.

C-Mod operation resumed on Monday, 10/18, with successful tests of the TF magnet
followed by an RF conditioning run. Three out of our four transmitters ran
during the last half of the run. FMIT#2 was not operated because of a fault
indicator problem discussed in detail later in this report. Startup
reliability was 100%.

Physics
-------

The previously observed coherent high frequency magnetic fluctuations around
600 kHz in Enhanced D alpha H-mode (EDA) have now been observed again with
both D and E port antennas running, and the hydrogen concentration was
5% or less. There appears to be a threshold of between 1 and 1.5 MW of RF
power to drive these modes. They persist throughout the EDA phase and continue
after the RF is switched off for typically 10 - 15 ms. These modes have
also been observed on the Phase Contrast Imaging (PCI) diagnostic at about 400
kHz because of aliasing about the 1 MHz sampling frequency. The time history
and changes in frequency of the mode are clearly correlated with the fast
magnetic fluctuations. The 1 MHz magnetic fluctuations are aliased in the same
way, but the 2 MHz magnetic fluctuation data shows clearly the true frequency
of the mode. In some discharges the signal was strong enough on the PCI to
determine the major radial k number of the mode, and it was found to have
k_R = 2 cm^-1, which is near the limit of spatial resolution of the fast
magnetic pick-up coils. Because of the line integrated nature of the PCI
measurement, it is not possible to determine the radial location of the mode.

Engineering
-----------

FMIT#1 and #2 were each successfully tested into a dummy load at the 2.5 MW
level. The output power was limited by the screen current rollback and the
anode current overload on FMIT#1 and #2, respectively. The RF
instrumentation (Kalmus, IPA, and driver forward and reflected power) has
been upgraded, installed, and calibrated. Fault protection circuits were
tested in both system and component level tests. These tests
are now part of a routine maintenance program. A fault indicator in
FMIT#2 was found not to be resetting properly. The consequence was that the
system would turn off upon a fault, but the operator would not have a
record of which fault caused the RF to shut down. The problem was
identified as a low reset signal level caused by a overloaded line driver.
This circuit is now being repaired.

The phase feedback units have been installed in FMIT#3 and FMIT#4 and
reconfiguration of the low power RF chain in these units has been completed.
Testing of the phase feedback system during operation into vacuum has begun.
The phase feedback system will maintain a set phase shift between the two
j-port transmission lines.

We have completed a version of the resonant loop configuration
capable of [0,pi,0,pi] and [0,pi,pi,0] phasing of the j-port antenna using a
variable length stub. The measured resonant frequenies of the loops are 77.9
MHz and 78.11 MHz. The decoupling stub electrical length is 6.1 cm, and the
decoupler isolation is -50 dB. The variable length stub will eventually be
replaced with a fixed length stub once the proper stub length is determined.

Work progressed on the DNB last week. The design of the HV/Mod-Reg interface
has been completed and work has started on modifying and stuffing the
PC boards. Detailed wiring diagrams for the HV/Mod-Reg interface chassis has
been started. Installation of the low capacitance duct has continued, and
high voltage cables have been terminated at the diagnostic lab. The
Arc/fil/snub duct and cabling have been installed in the cell. Final
alignment of the duct and attachment to supports is underway. HV
cables in the cell transition box have been terminated.

C-Mod Quarterly Review
----------------------

On Wednesday, 10/13, a C-Mod DOE Quarterly Review was held via video
conference. Both MIT and U. Texas made presentations. MIT reported on C-Mod
engineering activities and new physics results, with emphasis on data from
the new high spatial resolution edge diagnostics. U. Texas reported on
data obtained during the current campaign from instrumentation that will
eventually be used in conjunction with the DNB operation. Reporting MIT work
were Rejean Boivin, Martin Greenwald, Bob Granetz, Ian Hutchinson,
Jim Irby, Yijun Lin, Bruce Lipschultz, and Steve Wolfe. Bill Rowan discussed
the U. Texas results. Rostom Dagazian, Ray Schwartz, Chuck Finfgeld,
John Willis, and Warren Martin represented OFES.