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
May
22, 2000
Plasma operations continued at Alcator C-Mod last week. A
fresh boronization
was carried out on Monday night; an average deposition
layer of 2048
Angstroms was laid
down, followed by 7 hours of ECDC in D2.
Following the
boronization, three run days were completed. A total of 56
plasmas were
produced with a startup reliability of 75%, including the
post-boronization
recovery and conditioning day. Physics experiments were
devoted to
systematic studies of SOL fluctuations in L-mode, and an
investigation of
the quasi-coherent mode in ohmic EDA H-mode plasmas.
Plasma Physics
operation is scheduled to continue this week.
Physics
--------
A
run was dedicated to an exploration of the dependence of fluctuations in
the
SOL and attendant cross-field particle transport on collisionality in
the
SOL (MP#267). Key diagnostics employed in this study included two fast
scanning
langmuir probes, operated in fluctuation mode; the divertor
probes, also in
fluctuation mode; fast visible diodes viewing the helium
puff from the
outboard midplane limiter-mounted capillary; tangential
Lyman-alpha diode
array; limiter-mounted particle flux probes; and edge
Thomson scattering. A
density scan was carried out at fixed current and
toroidal field. The
fluctuations recorded by the scanning probes clearly
showed differences as the
density was lowered. At low density, the
fluctuation level at the separatrix
appeared to be much lower, increasing
with distance from the separatrix. As
has been seen previously at low
density, both mach probes record a strong
'reverse' flow, favoring higher
Isat on the 'West' probe relative to the 'East'
probe. It was noted that when this strong flow
occurs, the down-stream probes
of the Mach probes (East) see much more
Isat fluctuation than the upstream
probes (West). This is perhaps evidence
that the 'wake' of the probe has a
higher turbulence level.
A
run-day was devoted to study (MP#253) of the fluctuations characteristic
of
the so-called "EDA H-mode", which exhibits good energy
confinement,
moderate particle confinement and lack of large ELMs, and is
therefore
considered to represent a highly favorable regime for a tokamak
reactor. The
key to understanding EDA seems to be in the quasi-coherent
fluctuations which
have been observered by the reflectometer, Phase
Contrast Interferometry
(PCI), and probes. These fluctuations seem always to accompany EDA and have
been
shown to drive significant particle flux.
The purpose of this experiment
was to localize the mode relative to
the density and temperature pedestals and
to correlate both mode amplitude
and convected flux with the local
gradients. Because the range of the
scanning probe is limited by incident heat
flux, this run was carried out
in ohmic EDA discharges at the lowest possible
input power. This was done
by reducing the plasma current and toroidal field,
while keeping a fixed
time-evolution of q. By fine tuning the
Bt and Ip
ramps, we were able to get shots with an L/H transition followed
by an EDA
transition. On several
shots, we had probe plunges well inside the LCFS while
in the EDA
phase. With PCI and the reflectometer,
the fluctuations were
similar to those seen before. PCI saw k_R ~ 5-6 cm^-1. The A-side scanning
probe saw
considerably lower k_theta ~ 2-3 cm^-1; this result differs from
previous
experiments using the vertically scanning FSP, which observed
k-values in
the same range as the PCI. The quasi-coherent fluctuations were
seen over
a distance of ~7.5mm, but we were unable to access the inner edge of
the
mode with the probe.
A new analysis provides qualitative
spectroscopic evidence that the separatrix
location at the outer midplane
differs from that reported by the EFIT
code. Previously, the SOL profiles
as measured by the "midplane" diagnostics,
specifically the
radially scanning probe (ASP) and HeI line-ratio measurement
have
indicated that the outer separatrix location is ~5mm inside of its
EFIT-determined
location. This is the case even in
plasmas with an L-mode
edge. The spectroscopic evidence supporting these
measurements comes from the
observation that the atomic He puffed from the
outboard gas jet should be
almost fully ionized after passing through a
line density of plasma of n_e x
l=2x10^17 m^-2 [Schweer, et al., J. Nucl.
Mater. 196-198 (1992),p174].
Assuming the Thomson edge density profile is
mapped correctly by EFIT from the
top of the plasma to the midplane, then
too much atomic He is observed at
radii inside of the n_e x l value quoted
above. However the observation is
consistent with this n_e x L value if
the actual separatrix on the outboard
midplane is ~5 mm INSIDE of the EFIT-determined
one.
ICRF Systems
------------
Further ICRF antenna
plasma conditioning and antenna comparison experiments
were performed this
past week. Although the impurity
production associated
with the J-port antenna is greatly reduced compared
to the last campaign, its
heating efficiency appears to be less than that
of the D-port antenna. A MW
from J-port heats as effectively as 0.3-0.5 MW
from D-port; a fresh
boronization of the machine improved the comparison
but the relative
efficiency remains low.
The Mo source rate from J-port is about 2-3 times
that of D-Port at
the same power, but the plasma core radiation increased by a
much smaller
factor. Antenna loading into plasma
appears to be insensitive to
antenna-plasma gap variations, suggesting parasitic
loading is a major
component of the antenna loading. A possible source of parasitic loading
may
be the antenna back feed straps. We are considering replumbing the
antenna to
reduce the parasitic load by isolating those elements with the
suspected
largest parasitic load.
If the parasitic load is the cause of the poor
heating efficiency
and our analysis is correct, the heating efficiency should
improve. In addition, we are planning for antenna
modifications that should
allow us to improve the overall antenna
performance.
The E-Port antenna was inoperable last week due
to arcing inside the
transmission line, near the machine. The transmission
line has been repaired,
the antenna vacuum conditioned, and E-port is now
ready for resumed operation.
DNB Systems
------------
The
DNB was operated into plasmas on all three plasma-operation run days. The
filament and snubber 480V inputs
were switched to a new circuit. This
switch
provided a substantial improvement to the filament current during
C-Mod
pulses, essentially eliminating the portion of the arc current droop
due to
the filaments. Additional
perveance scans were done at 34 kV and 37 kV.
The
gradient grid voltage was varied as a fraction of the
accelerator grid voltage
to determine its effect on accelerator current
and perveance. The suppressor
grid
voltage was also varied to determine its effect for each of the gradient
grid
settings. The scans were done at 39
kV.
Travel and Visitors
-------------------
Y.
Nakamura has arrived from Japan for a two week visit to continue the
JAERI/MIT
collaboration on disruption research.
Our neutral point
experiment is scheduled to be run on C-Mod later
this week.
Chris Watts of Univ. Auburn visited for two days. He helped with work on
the e-mode
heterodyne radiometer.
Gary Taylor was at C-Mod for three days,
installing faster digitizers
and improving shielding on GPC2, which is
now no longer bothered by
RF pickup from the J-Port antenna. David Mikkelsen spent the week
here
for discussions and work on transport modeling.