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


Alcator C-Mod Weekly Highlights
October 16, 2000

Plasma operations continued on Alcator C-Mod last week. Three run days were
scheduled and completed. A total of 52 plasma shots were produced, with a
startup reliability of 95%. Experiments included ICRF development, an
investigation of SOL transport in (ohmic) L-mode plasmas, and high-performance
H-modes.

A fresh boronization has been carried out, and plasma runs are scheduled to
continue this week.

Physics
---------

An experiment to explore the dependence of SOL fluctuation
characteristics and attendant cross-field particle transport on the
collisionality of the SOL was carried out last week. A fine density scan was
carried out with both scanning probes and the Lyman-alpha array
operating. Analysis of data from this set of discharges should allow us to
compute Deff profiles across the SOL for a variety of collisionalities and to
relate changes in particle transport to fluctuation characteristics. In
addition, we also obtained CII and CIII plume images from the 'burping' FSP
probe. From these, we hope to infer the magnitude of Er and perhaps its
variation across the SOL under a variety of collisionalities.

On Friday, we attempted to extend our H-mode database using the increased RF
power available from the four-strap J-port antenna, together with the two
dipole antennas at D- and E-port. Total RF power employed was over 4MW, but at
these power levels we experienced a number of trips and impurity
injections. Some of this behavior may have been due to the relatively high
hydrogen fraction; H/D in these discharges rose during the RF pulse, typically
exceeding 0.1 after 1 second. Nevertheless, a number of 1MA shots with
Wmhd>200kJ, and neutron rates over 1e14/sec were obtained. At moderate target
densities (nebar>1.1e20) the H-modes had an EDA character, with good
confinement (H>1.7) and low radiated power for many energy confinement times.
At lower target density, we encountered ELM-free H-modes which were apparently
terminated by development of MHD activity which grew to large amplitude and then
locked. These discharges, which are phenomenologically different from plasmas
obtained previously at lower power, are undergoing detailed analysis.


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

We successfully coupled ~4.5 MW to H-mode plasmas. We also made a comparison
of D, E and J-port separately in H-mode. The fault protection circuitry was
properly faulting both J-port transmitters when one had an arc indication. We
were limited in power because the FMIT#4 gain appeared to decrease compared to
previous operation (only 1.4 MW into the dummy load). This problem was traced
to a lower stage tuning problem in the transmitter, which has been corrected.
FMIT#4 has been tripping frequently at times which often correspond to
sawtooth crashes. We will analyze this behavior further to determine if there
is an arcing or plasma coupling problem. At high plasma currents, the
sawteeth seriously impact the antenna loading on all antennas.

DNB Systems
------------

A modification to the DNB duct was completed for review. It includes
instrumentation of the duct for pressure and light emission. Fabrication has
begun and will be completed during the current week after including the
results of the design review. Possibilities for including differential
pumping of the duct for the remainder of the campaign were examined. This
appears unlikely, but can be incorporated for the next campaign.

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

Ian Hutchinson, Earl Marmar, John Rice, Miklos Porkolab, Jim Hastie, and Bob
Granetz and Joe Snipes were at the 18th IAEA Fusion Energy Conference in
Sorrento, Italy.

Following the IAEA meeting, Joe Snipes went to the International Confinement
and Modeling Database meeting in Frascati, Italy from 11 - 13 October and
presented the latest C-Mod results on the H-mode threshold experiments at low
density and in hydrogen to check the isotope scaling of the threshold.

After the IAEA, Bob Granetz spent 3 days at Garching attending Combined
Workshop of the Expert Groups on Pedestal Physics, Divertor, and
MHD/Disruption/Plasma Control, orgainzed under the IEA ASDEX-U/US Implementing
Agreement.

Stewart Zweben came to MIT 10/10-12 to participate in the C-Mod experiments
and to present a dry run of his APS talk to the C-Mod physicists. Norton
Bretz came and presented his APS dry run 10/12. Gerd Schilling also presented
his APS dry run 10/12.

Chris Brunkhorst (PPPL) was at MIT on Monday, 10/16, to help with the re-tuning
of the FMIT#4 transmitter.