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
Sept. 11, 2000

Plasma operations continued on Alcator C-Mod last week, with three run days
scheduled and completed. A total of 55 plasma shots were produced with a
startup reliability of greater than 88%. Experiments were carried out in
support of ICRF development, H-mode and pedestal characterization, and SOL
transport.

Plasma operations are scheduled to continue this week.

Physics
-------

Documentation of pedestal parameters (MP#271) continued with the run on
Thursday. A scan of toroidal field was carried out with EDA H-modes at a
current of 800kA, with two RF power levels, 2.5 and 2.0MW. Under these
conditions, none of the pedestal parameters show a B-dependence in the range
4.5T<Bt<6.0T.

One run last week was dedicated to completion of part 1 of MP 276, "Transport
Experiments in the Scrape-Off Layer during Ohmic L- and H-mode using
Gas-Injection Plumes." Plumes generated via gas-injection of deuterated
ethylene (C2D4) through the F-bottom scanning probe are imaged from two
near-perpendicular locations (F-side, F-top) with coherent fiber bundles
optically coupled to gated, intensified CCD cameras through beam-splitter
optics and bandpass interference filters. The optical system allows for
imaging of C^+1 and C^+2 emission simultaneously at each view location. On
Friday, an Ohmic L-mode density scan was carried out at fixed current (I_p =
0.8 MA) and fixed field (B_T = 5.3 T), during which the density was varied
from NL04 = 0.5e20 to NL04 = 1.2e20/m^2 and plumes were generated at various
depths in the scrape-off layer, ranging from right at the separatrix to 2 cm
outside. For plumes generated in the far SOL, cross-field dispersion
indicative of ExB flows was observed in both top and side views for all
densities. The dispersion was relatively uniform along the plume length,
consistent with having small electric fields and long ion lifetimes in this
region. Strong flows to the divertor were also observed in these cases. For
plumes located closer to the separatrix, the nature of the cross-field
dispersion changed to a ``boomerang''-type character, with dispersion
varying along the plume length. At the highest densities, plumes generated at
the separatrix appeared as though they ``hit a wall'', the cross-field
dispersion being highly asymmetric with most emission located in the direction
away from the separatrix. Parallel flow reversal was also observed for plumes
near the separatrix. Analysis of this data is underway.


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

We continued to investigate the operational characteristics of the J-port
antenna. With short pulses, we had several comparison discharges at various
power levels up to 1.5 MW. In terms of the impurity injections, the
performance improved compared to last Friday. We obtained ~0.25 sec pulses at
1.5 MW before injections from the antenna corners occurred. These injections
are largely N and Ti (spectroscopy). The N presumably comes from the boron
nitride tiles on the antenna front edges, while the Ti comes from the TiC
coating on the Faraday screens. These injections occur only in the presence
of plasma and when the J-Port antenna is excited to high power. The power
improvement may be a result of conditioning; however, the rate of improvement
is quite slow now. Unfortunately, the heating efficiency of the J-Port
antenna is still lower than the D- and E-Port antennas. From a power balance
analysis, J-port heating efficiency was ~50%, versus ~75% for D- and E-ports.
This heating efficiency is little better than that measured in June, before
capacitors were added to bypass rf currents on the antenna backplane to the
vessel wall; this modification was expected to reduce end leakage and power
coupling to plasma surface waves.

Because J-port operating as a four-strap antenna has a lower heating
efficiency than the D- and E-port dipoles, and also has impurity generation
problems, we are changing its configuration to a two-strap antenna. In
previous tests, we obtained good heating efficiency with the two middle straps
driven as a dipole, and the outer two straps shorted to ground. Experiments in
this configuration should begin this week.

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

The DNB operated into the plasma on Friday, after 2 days of repairing minor
problems with the control system. The beam ran reliably at 46 kV, >4.7 A in
conditioning mode. Beam-induced signals were seen on 8 of the 10 radial MSE
channels, the largest on the outside edge and diminishing toward the interior.
The signal-to-noise is poor even on the outside edge for the raw (unmodulated)
signal. However, neither the beam nor the spectral location of the channels
has been optimized. The purpose of this first look at signal levels has been
to determine if the filters are tuned to the correct part of the Stark
multiplet (pi+). The plan is to scan the filters and look for the spectral
features in question.

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

Work continues on the design of the cryostat upgrade. We are modifying the
cryostat so that removal of horizontal flanges can be done without removal of
the cryostat or igloo. A CAD solid model of the cryostat has been completed,
and a mockup of the new boots that will seal the cryostat to the vacuum vessel
is being made. Design of new penetrations in the cryostat for instrumentation
wiring is also well underway. We are also considering the addition of 6"
diameter horizontal ports between our large racetrack ports. These new ports,
if implemented, will allow for new RF feedthroughs and new diagnostic
installations. A CAD model of the the vessel has also been completed and will
be used for FEA analysis of the stresses produced by the new ports.

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

Martin Greenwald was in Varenna Italy attending the EU-US Transport task force
meeting.

Norton Bretz came to MIT 9/6-8 to continue checkout of the MSE diagnostic, as
described above.