Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Feb 20 12:29:11 1996
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Feb. 20, 1996

Plasma operations continued on Alcator C-MOD last week. Three runs were
completed, out of four originally scheduled. Successful runs included
continuations of the programs in H-mode studies at 5.3 and 7.9T, and modulated
direct electron heating at fields around 6.5T. The latter run was in direct
support of Ph.D. thesis research. Friday's run was abandoned due to a
malfunction of a motor in the 13.8KV AC breaker feeding several of the PF
supplies. A replacement motor has now been procured and installed.

A fresh boronization was carried out on Monday night. Tuesday's run continued
the program on D(He3) heating at 7.9T (MP#117). In order to speed the
conditioning process which has been found necessary after boronization, a series
of 1MA D(H) H-mode shots at lower field (5.3 and 6.2T) were carried out; data
from these shots is suitable for the boronized h-mode study (MP#132). Shots at
6.2T had the minority resonance outside the sawtooth inversion radius.
Although the central temperature increase was lower, as expected, the stored
energy increase was comparable to on-axis resonance heating. This behavior is
very different from the pre-boronization result. After seven of these high power
"conditioning" shots, we proceeded to the high-field portion of the run. Good
H-modes were obtained at 1.2MA and 2.5MW of launched RF power. Stored energies
for the best shots were in the range of 200kJ. Previous difficulties using the
instantaneous change in slope of the diamagnetic signal as a diagnostic for
absorbed power were identified as being due to saturation; this was corrected
and preliminary analysis of the present data indicates about 60% absorption,
which would imply H-factors (relative to ITER89-P) well above 2.

H-mode studies at 5.3T, using D(H) heating, continued on Wednesday in support
of MP#132. The program included use of laser blow-off impurity injection to
measure impurity confinement in ELM-free h-modes and a triangularity scan at
1MA (MP#145). The upper triangularity was varied from 0.25 to >0.7, with the
lower triangularity (active x-point) remaining at about 0.55. The highest
triangularity cases were close to being double null. Good H-modes were
obtained for all cases, with gross confinement and stored energy not
noticeably affected by shaping over this range. However, the higher
triangularity cases seemed to obtain ELM-free H-modes more easily. The good
H-modes for this run were ELM-free and were characterized by fast initial
rises in density, stored energy, and radiated power. At some point the
radiated power becomes too high, and the stored energy degrades, sometimes
leading to an H-L transition. The good phases of the H-modes had H-factors
above 2, Tau_E_global of 70-80ms, and 1.4 times ITER elm-free H-mode scaling.

Laser blow-off experiments were done throughout the run. Scandium was injected
into the H-mode discharges, and its transport was tracked by observing lines
from both Li-like and He-like Sc. Two regimes of H-mode impurity particle
confinement were observed, one with essentially infinite confinement time
(during the phase with good H factors) which degraded (typically at the same
time that the stored energy stopped rising) to a regime with tau_imp
approximately equal to 100 ms. This is still significantly larger than the
L-mode tau_imp, which is typically 20 ms. Both regimes of H-mode impurity
confinement with boronized walls have longer confinement times than the H-mode
confinement previously measured with unboronized walls. For a set of constant
shots, spatial scans of the Li- and He-like charge states were done in order to
measure the local impurity transport coefficients. Profile peaking was
observed.

Thursday's run was devoted to mode-conversion electron heating in H(He3)
plasmas, using modulation of the RF power to analyze power deposition and
transport. This was the final run in a series being carried out by one of our
graduate students in support of his thesis research (MP#90A). Central and
off-axis heating were obtained at two currents, 600kA and 1.2MA. The residual
deuterium concentration was again found to have important effects on the
location of the mode conversion layer. The University of Maryland
spectrograph was successfully used to view Helium levels in the edge. At both
currents, some very interesting interaction of the RF and sawtooth behaviors
were seen. During off axis heating, sawteeth were at times greatly diminished.
Very promising data was obtained for transport studies; more detailed analysis
will follow.

Kevin Fournier, affiliated with The Johns Hopkins University and LLNL and one
of our collaborators in theoretical atomic physics, visited on Feb 14-17. He
was observing experiments to measure the relative intensities of lines from
various Mg-like ions (Sc, V, Cr, Ni, Zr, and Nb) and to show us the recently
calculated, divertor-relevant, cooling rate curve for Kr, showing a very
strong low temperature peak.

Operations will continue next week, with four run days scheduled. Two weeks
remain in the current campaign.