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


Alcator C-Mod Weekly Highlights
June 21, 1999


Last week continued the scheduled maintenance interval on Alcator C-Mod. No
plasma operation was scheduled. This week is also a maintenance week.

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

Statistical analysis of fast magnetic fluctuations arising from edge
turbulence shows that they often propagate at a well-defined speed that
can be extracted automatically. This speed is now incorporated into the
routine data-analysis as a monitor of a critical edge-barrier parameter.
H-modes have perpendicular edge propagation speeds up to 40 km/s, with
enhanced-D-alpha modes perhaps slightly faster than ELM-free. In L-modes
the velocity is roughly four times smaller, but still in the electron
diamagnetic direction.

Temporal cross correlation analysis between the magnitude of high frequency
electron density fluctuations at the plasma edge (measured by reflectometry)
and the x-ray emissivity pedestal width show that there is a strong cross
correlation between bursts of reflectometer fluctuations and temporary
increases in the x-ray pedestal width. There is no measurable timing
difference between the broadening of the x-ray pedestal and the onset of the
reflectometer fluctuations. Any timing difference must be smaller than 100
microseconds. These bursts show up in the H alpha signals as well but the
cross correlation analysis does not yield a clear result.

The collapse of the x-ray emissivity pedestal has been studied during an H-L
transition. Simultaneous measurements at the outboard midplane and the top
show that the collapse starts at the pedestal location in both views and then
propagates inward at a velocity of approximately 150 m/s. The collapse happens
at the same time at the two locations (to within 24 microseconds).

The x-ray pedestal width at the outboard midplane is a strong inverse function
of plasma current in EDA H-mode, and is significantly wider in EDA H-Mode than
in ELMfree H-mode. However, the top x-ray width does not seem to vary at all,
but remains between 1.5 and 3 mm regardless of plasma current or type of
H-mode. In some low current EDA H-mode cases the pedestal is more than twice
as wide at the outboard midplane. This could indicate that the instability
causing the increased particle transport in EDA is mostly localized to the
outboard midplane.

ICRF System
------------

We continued testing FMIT#3 and #4 into dummy load. FMIT#3 was operated up to
1.5 MW reliably into dummy load. The limit is set by the FPA anode current.
FMIT#4 was operated to 1.8 MW reliably into dummy load. The power limit is
set by the available DC power, which has been reduced from the nominal value
to gain operating experience under conservative conditions. Compared with #4,
#3 has higher FPA anode current. The difference stems largely from the
different tubes in each transmitter, 2242 and 2274 in #3 and #4 respectively.
To obtain these results, each transmitter underwent tuning adjustments and
testing of the RF control system. Vacuum conditioning of the J-port antenna
using FMIT#3 and #4 will begin this week.

The screen crowbar and bias regulator units have been installed in FMIT# 1 and
#2. This installation completes all new hardware installations in the #1 and
#2 transmitters, and brings them into compliance with the standards set for
transmitters #3 and #4. Testing of the #1 crowbar unit and testing of the RF
control system for units #1 and #2 has also begun.

Diagnostic Neutral Beam
------------------------

The digital control board for the high voltage cage has been completed, the
board design sent out for production, and procurement of parts begun. Layout
of the screen control board is currently being completed.


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

Several C-Mod physicists attended the EPS Conference on Controlled Fusion and
Plasma Physics, in Maastricht, the Netherlands last week. Amanda Hubbard
presented a talk on "Pedestals and Confinement in Alcator C-mod H-modes."
Poster presentations were given by Joe Snipes, Rejean Boivin and Spencer
Pitcher, and on behalf of Jim Terry (who didn't attend).

Miklos Porkolab also attended the EPS Conference in Maastricht. In his
capacity as APS DPP Chair, he also attended their Board meeting and Program
Committee Meeting for the 2000 EPS meeting, to be held in Budapest.