From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Approved: plasma@cs.uml.edu
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
July 6, 1999

The scheduled maintenance period continued at Alcator C-Mod last week. No
plasma operations were scheduled. This week is also a maintenance
week. Cryogenics and HEAT systems will be turned on, and the machine cooled
down in preparation for resumption of operations.


Physics and Analysis
--------------------

Further investigation of the rate of RF antenna phase balance faulting,
including all RF runs from 1998 and 1999, has yielded a good picture of where
the plasma should be for reliable RF operation. (Phase balance faults are
difficult to deal with because unlike arc detection faults and voltage limit
faults, there is nothing the RF system can do operationally to reduce the
faulting rate). Defining top (bottom) to be the closest distance from the last
closed flux surface (LCFS) to the top (bottom) of the RF limiter, and midplane
gap to be the distance from the middle (Z=0) of the RF limiter to the LCFS,
and asymmetry to be top minus bottom, it has been found that there are two
regions in midplane gap vs. asymmetry space where the RF runs with the lowest
probability of encountering phase balance faults. One region is centered
around asymmetry +0.33 cm, midplane gap 1.4 cm, (so that the plasma is
slightly down from center) and the other is centered around asymmetry -0.07
cm, midplane gap 0.91 cm (plasma is nearly up down symmetric but closer to the
outboard limiters). RF operation becomes very poor when asymmetry exceeds +/-
0.9 cm, or midplane gap exceeds 1.3 cm for negative asymmetry or 1.7 cm for
positive asymmetry.

A relative calibration was performed on the high-resolution (Auburn/U.Texas)
ECE system. A small ramp in the toroidal field was introduced near the end of
a discharge which resulted in adjacent ECE channels scanning the same plasma
volume. Under the assumption that the plasma does not change during this ramp,
a relative calibration of the 32 channels was derived. The resulting profile
was then absolutely calibrated by normalizing it to the other Te
diagnostics. A small anomaly was observed near the transition from the
gradient to the sawtoothing region. Other ramped discharges were investigated
to make certain this was not an instrumental problem. No problems were found
in these other discharges and the anomaly was attributed to the smoothing
required to remove the time dependance of the sawteeth.


Reviews
-------

A Cost and Schedule Review for the proposed C-Mod Lower Hybrid Current Drive
System was held at MIT on June 30-July 1. This project is planned as a
collaboration between MIT and PPPL. The panel (T. Bigelow (ORNL), F. Soldner
(JET), R. Pinsker (GA), T. Intrator (LANL), and W. Ferguson (LLNL)) heard
technical presentations by Miklos Porkolab, Ron Parker, Ian Hutchinson, Paul
Bonoli, and Joel Hosea (PPPL).

A quarterly review of the Alcator C-Mod programme was held at MIT on July 2.
Rostom Dagazian and Mona Bradford from DoE attended in person. For off-site
participants, the viewgraphs were posted to the web using our show-station,
and telephone audio links were provided. David Mikkelson, from PPPL, gave his
presentation remotely, also using the show-station technology. The agenda from
the review follows.

A Engineering and Operations
1) Jim Irby -- Status of the ICRF systems and the diagnostic
neutral beam

B Recent Results - Experiment and Theory
1) John Rice -- Toroidal rotation and impurity transport in ohmic
H-Modes
2) Earl Marmar -- Evidence of neoclassical impurity transport
effects in the
H-mode pedestal
3) Amanda Hubbard -- Experimental tests of temperature profile
stiffness
4) David Mikkelsen -- Testing theoretical predictions for the
critical ion
temperature gradient length in C-Mod
5) Paul Bonoli -- Advanced tokamak modeling

ICRF Systems
-------------

FMIT #3 and #4 were each operated up to 150kW into the J-port antenna last
week. A transmission line arc, which had limited the J-port conditioning
program, was diagnosed and repaired. Dry N2 gas in the transmission line was
replaced by SF6, which allowed higher voltages (up to 28kV) and powers to be
obtained. In addition, FMIT#4 also required some retuning to eliminate
self-oscillation. The remaining test for FMIT#3 and #4 is to operate
simultaneously into vacuum with 250 kW total through the antenna.

FMIT#2 and #1 were successfully high potted to 35 kV with the transmitters
fully assembled. Initial tuning and testing of FMIT#2 into the dummy load was
begun. After reducing the high voltage to 24 kV (due to a suspicion that the
cavity was breaking down due to the humidity), 1.5 MW was obtained. A couple
of problems were also identified. The RF was gating off without cause. There
was also breaker trip that is either a result of a faulty overload signal or a
malfunction in the breaker. These are being investigated.

The arc detection and voltage limit protection circuits were characterized and
upgraded. The MIT arc detection modules respond within 5.4 usec to an excess
in the ratio of reflected to forward power. The PPPL demodulator response
time was improved from 80 usec to 4 usec. The time required to shut the RF
off once an arc or voltage signal is received is now less than 1 usec. The
improved response time will reduce any damage associated with an arc.



Travel and Visitors
--------------------

Joe Snipes presented a talk at the CRPP in Lausanne on "Fast Particle Driven
Modes in Alcator C-Mod".

Randy Wilson visited last week to help with J-port antenna
conditioning following the lower hybrid cost and scheduling review.