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
Oct 11, 1999

The maintenance period continued at Alcator C-Mod last week. No plasma runs
were scheduled.

The machine temperature was brought up to 40C last week, and the cryostat and
magnets have been purged with dry nitrogen for several days to dry all
surfaces. The seal at the top of the cryostat has been replaced. A humidity
monitor was installed. We expect to begin cooling the magnet back down this
week.

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

High resolution measurements of the Balmer-alpha spectrum have been routinely
used for monitoring the minority hydrogen concentration in majority deuterium
plasmas since the beginning of the present run campaign. Examination of the
data has revealed small but systematic shifts in the line emission, which is
due to motion of the neutrals. With the present view, which is in the radial
direction, 9 cm below the midplane, the measurement is sensitive both to
radial and poloidal convection. If the shifts are interpreted as being due
entirely to poloidal motion, typical speeds are in the range of 1 to 20
km/second, in the electron diamagnetic direction. During high power ICRF
heated H-Mode plasmas, the speed increases, and this increase in speed is
smaller with increasing plasma current. This scaling is qualitatively similar
to that seen for the core toroidal rotation measured from doppler shifts in
highly charged argon ions. The data are being compared with edge rotation
measurements from ion species such as boron, as well as with the rotation
inferred from analysis of edge magnetics data.

An asymmetry in density pedestal parameters measured at the midplane and at
the top of the plasma has been observed. The midplane width as measured by the
visible bremsstrahlung array is always at least twice as large as the density
pedestal width near the top of the plasma measured by edge Thomson
scattering. There are cases when the midplane width is much wider than the
top, the difference in width sometimes reaching an order of magnitude.
Although positions of the pedestals are usually well correlated, the pedestal
at the midplane is normally located about 10 mm inside the separatrix, while
the top pedestal is measured on the separatrix within the uncertainties of the
position measurements.


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

Modifications for the master control system for the diagnostic neutral beam
are in the final design stage. These modifications are required to
interface the beam controls with the C-MOD main control system. Progress is
consistent with the schedule which was presented at a review of the status
and schedule for the DNB in September.

ICRF System
-------------
Modification of FMIT#1's cavity was successful. Into dummy load,
2.1-2.4 MW of RF was produced. The output power is now limited by the
power supply instead of the effective tube plate impedance. This was
done by adding parallel inductance in the lower 1/3 of the cavity.
Changes to FMIT#2 are in progress.

In disassembling FMIT#2, evidence of arcing and damaged finger stock was
found. This was not unexpected since there was one instance where light,
presumably from an arc, was observed coming from the cavity. At the time of
the observation, we made several tests without being able to repeat the
observation. Since the tube was properly protected by the protection
circuits, action was not taken. Upon closer investigation, we found that the
poor contact caused by about 1/16" offset of the cavity center conductor. We
have corrected the alignment problem and have begun to reassemble. In
addition, the filter capacitors were high potted to ~30 kV. The cavity has
been high potted to 35 kV for 5 minutes with only 10 uA of leakage current.
We have also changed the teflon sleeve that is used to extend the potential
arc path around the blocking capacitor. This path has been increased by 1.5"
(from 0.5"). Arc tracks were also found on the tuning element, so-called
donut and on the outlet water pipe. The PVC pipe was replaced and the donut
was carefully cleaned.

For J-port, we have removed the 78 MHz resonant loops in preparation for a
loop configuration capable of (0,pi,0,pi) phasing. Initial tests of the
resonant loops will begin shortly. Work on the decoupling loop is also
required.

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

Miklos Porkolab attended the workshop "Frontier Science at the National
Ignition Facility", held at the Hilton Hotel in Pleasanton, Ca., Oct. 4-6.
The workshop was sponsored by the MIT PSFC, and it brought together scientists
from Universities and the National Labs, as well as from abroad, to discuss
the potential use of NIF for science oriented experiments. It is anticipated
that approximately 15% of the shots may be available for basic science
oriented experiments, and that outside users will be encouraged to form teams
to propose such experiments. The meeting was highly successful, with over 170
registered participants, including more than 50 university scientists and many
foreign scientists attending. It was proposed that an Advisory Committee be
elected to promote the idea of groundbreaking science on NIF, interact with
the NIF Council, and propose future workshops.

Stewart Zweben of PPPL visited C-Mod Oct. 7-8 as part of his collaboration on
edge turbulence imaging. He gave an informal conceptual-level design review
on the installation of a re-entrant coherent fiber bundle, which would be used
for imaging a local hydrogen or helium gas puff along the direction of the
magnetic field. This diagnostic is designed to provide 2-D images of the edge
turbulence with millimeter spatial resolution and microsecond time resolution,
which can be used to check theories of the L-H transition and SOL turbulence.
Preliminary data obtained by Jim Terry from a tangential view of the puffer of
Spencer Pitcher showed strong turbulent fluctuations in the range up to about
100 kHz, which suggests that good 2-D images of this turbulence should be
observable.

Professor Cy MacLatchy from Acadia University, Nova Scotia
visited the PSFC on thursday and friday to view results obtained
from recent impurity injection "plume" experiments. A technique of
constructing an approximate 3-D representation of the plume from a
series of 2-D "slices" was also discussed.