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
August 21, 2000

Operations
----------

Plasma operation resumed on Alcator C-Mod last week, following the
manned-access vent in July. Operations were aimed at re-establishing routine
operation, machine conditioning prior to re-boronization, and diagnostic
alignment and calibration. Two plasma run days were scheduled and completed,
with a total of 23 plasmas produced and a startup reliability of over 50%.
The resumption of operations was carried out about one week earlier than
originally scheduled.

Power systems tests were completed on Wednesday, and an initial plasma attempt
resulted in a breakdown but no current rise (a fizzle). Several full length
plasmas were obtained on Thursday, and the machine showed signs of cleaning
up, although carbon levels and H/D remained high. Friday's run was marked by a
substantial improvement in startup reliability and reduced carbon
levels. Standard 800kA, 5.4T diverted discharges were run for machine
conditioning. The observed H/D ratios continued high, with values around 0.7;
this hydrogen concentration is similar to that observed at this stage of the
conditioning process in previous campaigns, implying that the use of heavy
water in the initial backfill at the start of the vent did not effectively
saturate the walls with deuterium, as had been hoped.

In addition to the standard conditioning discharges, we also ran plasmas with
ramping toroidal field to vary the edge q, in support of the turbulence
imaging experiment. We also produced cases with dynamic scans of the outer gap,
both for the benefit of the turbulence imaging and to evaluate equilibrium
reconstructions using an expanded set of magnetic diagnostics.

Plasma operations are scheduled to continue this week, with three run days
planned.


Physics
-------

The first data were taken with a camera system designed to visualize edge
turbulence. The camera has a tangential view (parallel to the field lines) of
a 6x6 cm region with resolution better than 1 mm. The camera was gated for 2
microseconds with images taken every 16.67 msec. The region of interest is
illuminated by light from neutral helium which is puffed from a capillary tube
at the outer limiter radius. Very clear images of turbulent structures were
obtained. This system was designed and operated by Stewart Zweben as part of
the PPPL collaboration.

A set of eight poloidal field loops located on the outboard limiter were
incorporated into the equilibrium reconstruction (EFIT) for the first
time. These loops, which were formerly dedicated to study of magnetic
fluctuations, are 10 to 15 cm closer to the plasma than the original loops
mounted on the outboard wall of the vessel, and are expected to improve the
accuracy of the reconstruction of the outer separatrix location. A series
of shots in which the plasma was moved into contact with the outboard limiter
was carried out to help test the new reconstructions. Detailed analysis of
these data is in progress.

RF Systems
------------

We began vacuum conditioning D and E-port antennas and completed the J-port
transmission lines preparations. We also repaired the screen crowbar unit for
the FMIT#2 transmitter, and also repaired the one of our spare crowbar units.

The Lower Hybrid launcher Conceptual Design Review was held on 8/17 at MIT.
The PPPL participtants were Joel Hosea, Stefano Bernabei, Gerd Schilling, Doug
Loesser and Joe Rushinsky. From MIT, Miklos Porkolab, Ian Hutchinson, Ron
Parker, Jim Irby, Monty Grimes, Dave Gwinn, Dave Terry, Bill Beck, Bob Childs,
Rui Viera, and Steve Wukitch were in attendance.

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

Tests of the upgraded glow/boronization system on our vacuum test stand were
completed last week, and the system was reinstalled in the cell. A full
checkout of the boronization system using helium rather than diborane will
occur early this week followed by installation of the diborane bottle in the
cell. Tests of the glow discharge system are also planned.

The installation of new instrumentation for long pulse operation is
proceeding. We will complete installation of the new conduit this week and
begin running the cabling. A new CAMAC crate for the new instrumentation is
also being prepared.

The DNB mod/reg stopped producing pulses during our last week of plasma
operation in June. This problem has been tracked down and fixed. The DNB
beamline is now being leakchecked and prepared for operation.

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

Stuart Zweben was at C-Mod from 8/15-18, working on the completion of the new
edge turbulence imaging diagnostic. First images of edge turbulence structure
were observed on 8/17, and are now being optimized. Norton Bretz was at C-Mod
8/16-18, continuing the Motional Stark Effect diagnostic calibration and
resolving possible interference issues with the Beam Emission Spectroscopy
diagnostic. MSE is again ready for the Diagnostic Neutral Beam to resume
operation.