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
24, 1999
Alcator C-Mod resumed plasma operations last week.
Three experiments were
conducted. One run-day and part of a second were
lost due to a malfunction on
Tuesday of the solid-state interrupter
circuit on one of the OH coils. This
was repaired on Wednesday, and
operations continued through the remainder of
the week.
A
series of discharges were produced to determine if the regime of strong
main
chamber-wall recycling persists in very low core plasma density
discharges.
Specific measurements in these low (<6e19/m^3) plasmas included
determining
whether the flapper influences midplane neutral pressure and the
measurement
of parallel flows to the divertor with scanning Mach probes. A
comparison
to the ionization rate in the main chamber from H-alpha/Ly-alpha
measurements
was also made. Results indicate that the flapper does not
influence the
midplane pressure at all at lower densities. It does influence
the
divertor neutral pressure, although to a lesser degree (~25% drop in
divertor
pressure when flaps open compared to 50% drop at higher
density). Data
from the Mach probe and Lyman-alpha profile measurements are
being
analysed.
Thursday's run was devoted to an examination of cold-pulse
propagation and
non-local transport effects. Ken Gentle (U.Tx.) proposed
these experiments,
and served as Session Leader. The objective was to
inject sufficient
quantities of carbon by laser ablation to drive
significant, fast reductions
in edge temperatues and cause prompt core
temperature rises. The effect has
been
widely observed at low densities; the operating space in CMOD was to be
explored. Low power, low density target plasmas were
chosen to begin. Both
diverted and
limited configurations were run, with no significant differences.
Weak,
but clear evidence of prompt interior heating was obtained. The effect
disappears at higher density
and is strongest at low currents (low power.)
Better, more conclusive
experiments will require larger carbon injections:
prompt peak radiated
power on the 2pi bolometer greater than the 300 kW
maximum in this
series.
Friday's run was dedicated to ohmic H-mode studies,
specifically to
determination of impurity transport in these discharges in
support of
MP#249. There were many extended periods of Ohmic ELM-free and
EDA H-mode in
several of the discharges. There was good correlation
between the rotation
velocities determined by Doppler shifts and from the
fast magnetics. Many of
the plasmas had large stored energy increases;
shot 28 had a stored energy of
160 kJ, comparable to the highest stored
energies in RF-heated H-modes during
the current campaign. EDA H-modes
were obtained at plasma currents as high as
1.44 MA. Edge Te and x-ray
pedestals were measured. The impurity confinement
during ELM-free H-modes
were much longer than the discharge length, which led
to the termination
of the H-mode. For EDA Ohmic discharges, the confinement
was shorter, but
the injection often caused the plasma to return to ELM-free
behavior. The
main result is that the impurity confinement in Ohmic H-mode
plasmas is
very similar to ICRF H-mode discharges.
Physics and Analysis
---------------------
High
resolution measurements of radial profiles of deuterium Lyman alpha
emission
have been performed near the outer midplane in L and H mode
discharges.
With knowledge of edge electron density and temperature we
can determine the
ionization profile (and also the neutral density
profile). While the
ionization profile is relatively broad across the
separatrix in L mode,
it appears to shift progressively inside the separatrix
as the discharge
makes a transition to H mode. This shift may contribute
to the usual
density rise observed during H mode, although further
analysis is required
in order to fully quantify this change.
Ion impurities in the upper
divertor scrape off layer plasma have been
observed using the Omegatron
probe. The Omegatron probe combines
a
gridded energy analyzer and an ExB ion mass spectrometer. Ion species
with mass to charge ratio
M/Z=2, 4, 6, 10, and 11 have been observed,
with the M/Z=2 intensity
always dominant in deuterium plasmas.
The
intensities of the M/Z=6, 10, and 11 species are larger after
a
limited plasma discharge than they are after a series of diverted
plasma
discharges. When present, the
intensities of the M/Z=10 and 11
species appear in proportions that match
the natural isotopic
abundances of boron-10 and boron-11. An increase in the intensity of
the
M/Z=4 species is observed after puffing helium into a tokamak
discharge,
either through an inner wall capillary or through the
helium probe at the
midplane of an outer limiter. During a
plasma
discharge the evolution of the M/Z=4 intensity follows the
inventory
of helium injected; this is expected since helium is a
recyling
impurity. Impurity
resonances can be identified unambiguously at
intensities down to a
hundred nanoamperes, which corresponds to
approximately 0.5% of the total
current in the collection cavity.
Efforts to improve resolution further
are underway.
Travel and Visitors
--------------------
Miklos
Porkolab attended the National Academy of Sciences Review in
San Diego on
May 16-18 and made a presentation,"Wave Propagation and
Interaction
with Particles in Magnetized Plasmas:RF Heating and Current
Drive". He also attended the FESAC meeting at PPPL
on May 20,21, where the
PPPL Program review was given as well as a review
of the three Proof of
Principle experiments.
Earl Marmar was
also at the NRC-NAS review in La Jolla; he gave a presentation
titled
"Diagnostics and Facilities Needs".
Martin Greenwald
travelled to PPPL to deliver a seminar to their
theory group on C-Mod transport
experiments. He took the
opportunity
to consult with the local staff on various physics issues,
software
and data sharing, and "Snowmass" planning.
As
noted above, Prof. Ken Gentle of the University of Texas visited C-Mod
this
week to direct the cold-pulse experiments.