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 30, 1999

Plasma operations continued at Alcator C-Mod last week. Three runs were
carried out, although the run on Tuesday was terminated after a single shot,
so that further work could be carried out on the ICRF system. A total of 37
plasmas were produced, with typical currents of 1MA. Start-up reliability was
95%.

The run on Wednesday was dedicated to MP#208, which is designed to study
transport effects in the SOL using the fast-scanning gas-injection probe. The
probe was used to inject trace amounts of ethylene (C2H4) for plume imaging
and impurity screening studies. A video system was used to capture images of
CII and CIII "plumes" to provide information on parallel and cross-field
transport in the SOL in a variety of ohmic plasma conditions. The objectives
of this experiment were:
(1) Investigate the dependence of cross-field spreading of C+1, C+2
(i.e., cross-field transport) at fixed locations in the SOL as
a function of plasma density (or collisionality)
(2) Investigate dependence of cross-field impurity transport on
SOL location (rho) in fixed discharge conditions
(3) Investigate parallel flow of background plasma (via parallel C+1,
C+2 spreading)as a function of SOL location and plasma
collisionality.
At a core density of 1.4e20, with deep insertion of the probe (close to the
LCFS), reversed flows were seen and plumes exhibited a corresponding parallel
shift away from the divertor. Good CII and CIII plumes were seen on most of
the shots. From analysis of the difference in CII and CIII emission patterns,
we hope to learn something about the cross-field and parallel transport of
impurity ions in the SOL. These data are expected to form part of a doctoral
thesis.

Thursday's run was devoted to continuation of the ICRF commissioning
activities. RF operation was carried out with the E-port dipole antenna and
with the J-port (PPPL) four-strap antenna. Peak combined power delivered to
the plasma was 3MW. Up to 1.5MW was coupled to the plasma from the E-port
antenna, driven by FMIT#2. However, later in the day this transmitter
exhibited an oscillation which led to persistent fault conditions. This
behavior is being investigated. Over 1.5MW was coupled through the J-port
antenna as well. Heating efficiency appeared to be lower with the J-port
antenna than with E-port, and impurity production was higher. Spectroscopic
diagnostics indicated influxes of iron, nickel, and titanium associated with
high-power operation of the J-port antenna. Further conditioning of this
antenna, both in vacuum and into plasma, is planned.


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

Perturbations of floating potentials in the far scrape-off layer plasma have
recently been observed during ICRF antenna conditioning discharges in which
the J-port antenna has been operated. These perturbations appear to be caused
by RF-sheath effects localized to antenna surfaces. During unfavorable phasing
of the J-port antenna with 1.3 MW of input power, the F-port scanning probe
records floating potentials as high as +200 V on portions of the profile which
connect along field lines to J-port antenna protection tiles. These
discharges correspondingly show large core impurity increases in response to
the RF power pulse. In contrast, during favorable phasing at the same power
level, floating potentials exhibits a much smaller increase (+50 V), the
impurity influx is less severe, and these discharges are seen to transition
into H-mode. Further evidence for RF-sheath effects localized to antenna
surfaces is supplied in data from the A-port scanning probe. Unlike the F-port
scanning probe, at no point in its trajectory does the A-port probe connect
along magnetic field lines to J-port antenna surfaces. Consequently, only a
small decrease (-30 volts) in the far scrape-off layer floating potential is
seen during J-port antenna operation. This relatively small change is
independent of antenna phasing, impurity influx, and attainment of H-mode.
These observations suggest that ICRF antenna operation can be greatly
improved by replacing the molybdenum antenna protection tiles with electrically
insulating boron nitride tiles. This idea is presently being considered for
the next in-vessel maintenance period.

Recent experimeents have improved our understanding of the source of
molybdenum impurities in ICRF-heated discharges. In the past, the inner wall
has been identified spectroscopically as the source of the molybdenum found in
the plasma core during current ramp up, when the plasma is limited by the
inner wall. However, until the beginning of the current experimental campaign,
there had not been a consistent correlation between the spectroscopically
measured molybdenum sources and molybdenum core densities during the steady
state portion of the plasma discharges and especially during RF heating. This
was the case despite systematically monitoring a number of surfaces including
the outer divertor (biggest source observed), the inner divertor, the inner
wall, the antenna protection limiters, and the ICRF antennas at the
midplane. During the vacuum vent, significant erosion had been found along the
top protection tiles of both (D & E) antennas, which prompted us to redirect
two spectroscopic views to look at the top of the two antennas instead at the
midplane. Our measurements since then have shown that indeed the top
protection tiles seem to be the main source of molybdenum during RF
heating. In L-mode, we have found clear correlations between the antenna Mo
sources and the core Mo levels. The antenna source rates (and the core Mo
density) correlate with the RF power levels. The correlations can be mainly
attributed to a local effect (e.g. local sheath acceleration of ions) rather
than to a global effect through the heating of the plasma, although the latter
has been also observed. Other sources have been excluded from being the main
contributors to the observed Mo core levels because of lack of correlation
between Mo sources and Mo core density. Specifically, it has been observed
that the outer divertor and inner wall source levels decrease significantly
during elm-free H-modes in contrast to the unaffected antenna Mo source and to
the increasing (due to transport) molybdenum core density.

Recent runs have provided additional examples of the "Enhanced Neutron" mode,
characterized by neutron rates that increase by up to a factor of 3 just after
the H-L transition. We have previously attributed this effect to spontaneous
formation of an internal transport barrier (ITB) triggered by the profile
changes following the H-L transition. Analysis of results from runs on 990818
and 990819, which had mostly ohmic H-modes with some ICRF at the end of the
H-mode, show that the increased neutron rate can be well fitted using the
measured central electron temperature from the ECE GPC system and the central
density calculated from the visible bremsstrahlung array. Most of the
increase in neutron rate comes from the temperature increase in these
discharges; the central density is in fact falling at the time of the increase
in neutron rate. This result appears to be in contrast to the the older ICRF
dominated H-modes where the central density remains high after the H-L
transition for a brief period when the neutron rate increases. Large low
frequency (2 - 10 kHz) MHD modes appeared just before the peak in the neutron
rate on most of these discharges. They appear to be m=1, n=1 in the core and
m=2, n=1 on the edge magnetic pick-up coils. These modes become so large as
to lock to the wall, but so far, they have not led to disruptions. The large
MHD can be present even when there is not a large increase in neutron rate.
While there are also cases in which there is no clear MHD mode on the
magnetics and there is a large increase in the neutron rate, the central soft
x rays do see an m=1 mode. Calculations of the profile of the density from
the visible bremsstrahlung indicate that the large m=1 mode begins when a
steep gradient in the density and pressure arrives at the sawtooth inversion
radius.

Work continued on development of the FRC ECE radiometer. Perry Phillips
(UT-FRC) was on site and continued work on the ECE profile calibration. In
addition, the two channel ECE fluctuation system was re-installed with
additional gain and will be used in the next week.

ICRF System
------------

FMIT#2, connected to the E-port antenna, had a high power 2274 tube on loan
from General Atomics installed and tested up to 2 MW into dummy load. After
initial plasma operation with power up to 1.5MW, an excessive self-oscillation
was observed in the FPA stage. We have begun adjusting the screen, bias, and
filament voltage to eliminate this problem.

Further progress on J-port plasma operation was initially hampered by an
intermittant gate, which prevented FMIT#3 from pulsing reliably. A problem
with the gate reset was identified and corrected. The ICRF antenna power
waveforms now show good tracking of the desired waveform, even in the face of
antenna load changes resulting from plasma L-H transitions. This indicates
that good progress has been made in the rebuild of the transmitter feedback
circuits. The FMIT#3 and #4 transmitters now appear to be operating reliably
at moderate power levels. Conditioning of the J-port antenna will continue
this week.

A rebuilt 2274 tube on loan from PPPL arrived late last week and the
installation process began immediately. The tube was inspected and
successfully high potted. Initial testing into dummy load should happen by
the end of this week. Once this tube is installed in FMIT#1 (connected to the
dipole antenna at D-port), all four transmitters will have high-power 2274 FPA
tubes, which will bring the rated total source power up to 8MW.


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

Joel Hosea and Randy Wilson visited C-Mod last week to participate in
the new antenna startup experiments.

Dr. Sakae Besshou of Kyoto University and the Heliotron E project
has been visting C-Mod for the last two weeks. Dr. Besshou is in charge of
designing the magnetic and bolometric diagnostics for the new heliotron
at the institute affiliated with Kyoto University. He is visiting in
order to consult with C-Mod scientists and observe C-Mod operations.

This is the last week for three summer students who have worked
productively at C-Mod since the end of May. Two, Dominique Huebner
and Stefan Krotz, are from the the University of Wurzburg, Germany.
They have worked on spectroscopic diagnostics and RF systems respectively.
The third student, David Smith, is from Northwestern University. David has
improved and updated the MIST impurity transport code and worked on other
spectroscopic analyses.