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
June 1, 1999

Operations:
-------------

Plasma operations continued on Alcator C-Mod last week. Three run days were
scheduled and completed. This week is a scheduled maintenance week.

A run was devoted to exploring the effect of varying the outer gap on the
midplane neutral pressure. The motivation for these experiments comes from
recent experimental results on C-Mod which indicate that the midplane pressure
is insensitive to gas escaping from the divertor; this has led to the
hypothesis that the midplane neutrals are determined by main chamber
recycling, independent of the divertor. In this case, the plasma-wall
separation might influence the observed pressure. Difficulties with vertical
stability were experienced with outer gaps much beyond 2.5 cm. It does appear
that the right gap influences the midplane pressure (0 cm to 2.5 cm), and to a
lesser extent the divertor pressure. The best way of quantifying the effect
is in the compression ratio, which varied from CR = 165 with 0 cm right gap to
values as high as CR = 270 with right gap 2.5 cm. Helium probe, FSP and ASP
profiles were also obtained during the day. These profiles, not yet analyzed,
should shed light on the interaction of the SOL plasma with the limiters at
the outside midplane. We believe this is the factor which primarily
determines the outside midplane pressure. We also found an anti-intuitive
result: with small right gap (e.g. 5 mm) the opening of the flapper has a
larger perturbation on the midplane pressure, i.e. a 50% effect compared with
15% effect at right gap = 2.0 cm.

Thursday's run was in support of MP#213A,"Temperature Fluctuation profile
measurements". The purpose was to take initial measurements of T~/T via ECE
correlation radiometry using F-port high resolution ECE system. This was the
first dedicated run for this instrument. Reproducible plasmas were
established for a fiducial-like discharge. Initial correlation measurements
showed very small low frequency ( f < 50 kHz) fluctuations in only one
correlation pair. Moderate signal contamination (pi phasing in the
correlation) was seen in two of the channel pairs. One section of the
correlation system had very low signals, and a rebuild of the section during a
cell access did not improve the signals remarkably. Improvement of the
grounding and testing of the correlation system is continuing. In addition to
the correlation system, we collected high-frequency data on both grating
polychromator instruments; these data will be used for an initial search for
evidence of SOC-like fluctuations in Te.

The purpose of Friday's run was to investigate volume recombination in the
divertor under detached ohmic conditions. The high-density operation required
for these experiments was unfortunately subject to high disruptivity, which
seriously limited the productivity of the run. It was noted that the ohmic
detachment threshold was higher than in previous experiments, by at least 20%;
no reason for this increase was immediately apparent. Extremely high densities
were measured on the outer divertor probes (up to 4.0e21 m-3 in the "death
ray") with the divertor still attached. These may be the highest measured on
C-Mod to date. This represents an extension of the high-recycling regime. The
state of the bypass flaps did affect the detachment threshold; with the flaps
open, the detachment threshold was at lower density. Good Balmer spectra and
Lyman radiative recombination spectra (for T_e) were obtained.

The C-Mod weekly staff meeting was available for remote access
starting last week. Access to the speaker's slides was provided over
the web using the new digital overhead projector. Audio was made
available using the ES-Net audio bridge. This new capability will allow
C-Mod collaborators to more easily participate in meetings at the PSFC.

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

Repair and upgrading of the transmitters was again carried out in the
evenings. The high voltage crowbar circuit, which diverts fault currents in
case of an output or driver tube arc, was found to have excessive time delay
and trigger level problems; these issues have been addressed. In addition, a
re-work of the water-cooling system for the final output tubes was begun; this
work is now complete on the #3 and 4 transmitters.

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

Charles Skinner (PPPL) continued to work on edge ion temperature and flow
measurements with the Fabry-Perot diagnostic. Randy Wilson helped with ICRF
transmitter operation and diagnostic issues.

Tom Fredian attended a NTCC (National Transport Code Collaboratory) working group
meeting at Lehigh University on Tuesday. Tom is providing assistance in
interfacing
the physics codes to experimental data via an MDSplus data access gateway.

Phyllis Rhoney, Lew Randerson, and Bill Davis visited Wednesday and Thursday from
PPPL to speak with MDSplus users and developers to gain some insights that may
help
them with the use of MDSplus on the NSTX experiment. Tom Fredian and Josh
Stillerman
discussed some implementation details of MDSplus. Steve Wolfe, Martin Greenwald,
Bob
Granetz, Howard Yuh, Rejean Boivin and other C-Mod scientists discussed their use
of
MDSplus for plasma shape control, diagnostic setup, data acquisition and
integration
of large analysis codes into the MDSplus data handling system.

Bruce Lipschultz went to JET for several days for discussions about
divertor/boundary physics and EDA attempts on JET. One subject of discussion
was the wall-core recycling analysis done at C-Mod. JET also sees similar
shaped edge profiles leading to a diffusion coefficient increasing across the
SOL, although their error bars for the diffusivity are large. Results
regarding flow profiles in the SOL were also compared. There are reversed
flows in the JET SOL extending farther out into the SOL than at C-Mod with
higher Mach numbers (0.3-0.4). The JET Team have started analysis of this
data and believe there are Pfirsch-Schluter flows that account for most of
this effect. They do see a flow reversal when the magnetic field is
reversed. Considerable time was spent reviewing the recent JET attempts to
make EDA plasmas. Following the C-Mod practice, they aimed for high q, high
triangularity plasmas with ICRF only. There were no obvious signatures to
indicate they had obtained EDA H-modes in these experiments. They did obtain
good confinement with little or no ELMs. It was noted that the confinement
improved as the triangularity was increased prior to turning on the RF
heating. The JET group are hoping to continue this work in future runs.