Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue May 2 10:49:38 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
May 1, 1995

Plasma operations on Alcator C-MOD resumed last week after a one-week
scheduled maintenance period. Following power system checkout, several plasma
shots were run on Monday to verify that all systems were operational and to
evaluate machine cleanliness after the vent the previous week. Four physics
run days were scheduled and completed.

A detailed study of x-point characteristics (MP#109) was carried out on
Tuesday and Wednesday. For these experiments the x-point was positioned
somewhat higher than the standard location to provide good viewing by the
various spectroscopic diagnostics. The x-point was observed in three regimes
on each shot: low-density sheath-limited divertor, high-density detached
divertor, and with RF to re-attach the divertor at high density. Goals of the
experiments included determination of what impurities are responsible for the
radiation coming from the X-pt region; determination of the reason for the
location of the peak in emissivity near the X-pt; obtaining profiles of
electron density from below the strike points to above the X-pt using
stark-broadened Balmer lines and the views of the K and A bottom arrays;
documentation of the presence or absence of molecular hydrogen lines in the
visible and/or in the VUV; measurement of the optical thickness of the n=3 to
n=1 line (Lyman_beta) in H for views through or below the X-pt. The Chromex
spectrograph covered the spectrum from 400 nm to 1300 nm for 14 different
spatial locations (5 looking at the outer divertor, 8 looking at the inner
divertor and 1 from K-top viewing the whole floor of the machine). The
McPherson view was scanned across the x-point for 2 different wavelength
settings (C-V & C-VI in one case, C-III & LyBeta & O-VI for the other case).
After completing these scans, the x-point radial location was scanned from 54
to 57 cm with diagnostic settings held fixed to see how the x-point location
affects the radiation pattern. Radiation stays at the x-point through
detachment for the case of the x-point far from the inner divertor; with the
x-point near the inner divertor the radiation peak is at smaller major radius
than the x-point before detachment and shifts inside the x-point after
detachment. Deuterium Paschen series lines were investigated. These lines are
Stark-broadened (width approximately 30 A) and should allow density
measurement. C-VI and C-V emission indicated heating at the x-point during
RF. C-III, Ly-Beta and O-VI emissions at the x-point are strongly affected by
the H-mode. This could be due to profile or density effects. The Mo-I emission
is primarily found at the outer divertor (not the inner divertor) and then
only at low densities. The CD band emission in the divertor was measured and
spatial profiles were obtained.

PEP mode studies were carried out on Thursday, using the Li pellet injector.
Attempts to optimize pellet/RF timing indicate that the RF should be turned on
just before the time of maximum xray peakedness. A scan of target density
indicated that a value of about nebar=1.6e20/m3 gave optimum results. At lower
currents (600kA) it was found that the PEP mode duration and the post pellet
particle confinement time doubled. However, the peak neutron rate was less
than at the higher currents. Ramping the current down from 800kA to 600kA
immediately following pellet injection resulted in a PEP mode with a neutron
rate characteristic of the 800kA shots with the longer duration typical of the
600kA case. Images of the pellet ablation pattern ("cigar" pictures) were
obtained for analysis of the poloidal field angle. Double pellet injections
were used on several shots to determine whether the current profile differs
significantly between PEP and normal discharges. The EFIT code has been
modified to accept the cigar angle data as inputs in determining equilibrium
reconstructions.

An arc in the E-port transmission line limited the power levels available
during the latter part of the PEP mode run on Thursday. This line was
disassembled Thursday night and the arc site determined to be at a Teflon
insulator. Repairs and clean-up were carried out over the weekend, and the
system is expected to be back online for this week's runs.

On Friday an experiment (MP#111) was carried out to explore a spectroscopic
edge/divertor electron density diagnostic for C-Mod and future tokamaks such
as ITER. The Ni- and Co-like charge states of Pd, Mo, and Nb exist near the
edge of the plasma. The ratio of electric quadrupole transitions (E2)to the
electric dipole transitions (E1) in these ions have a strong density
dependence in the n_e ~ 1E20 - 1E21 m-3 range. The laser blow-off impurity
injector was used to introduce these elements into a series of reproducible
shots which had a density ramp going from nebar ~0.9e20 to 2.3E20/m3. The line
ratios were measured successfully using the Johns Hopkins Molybdenum Monitor
diagnostic. The amount of the injected impurity was monitored using the 2pi
bolometer and the McPherson spectrograph. HIREX observed lines from Mg-, Na-,
Ne-, and F-like Nb, as well as lines from Ne-like Pd. The McPherson measured
Mg- and Na-like lines from each of the 3 elements. Looking at the X-pt region
the McPherson observed the Ni-like Nb spectrum. Preliminary results indicate
that the line ratios in Mo agree with the theoretical predictions at lower
densities. The preliminary result for Nb is that the ratios are in qualitative
agreement with theory. Mark May from Johns Hopkins was Session Leader for
these experiments; Kevin Fournier from JHU was also at C-MOD participating in
this run.

Rob Pinsker from DIII-D is beginning a one-week visit, during which he will be
collaborating with the C-MOD ICRF group. Ben Welch is visiting from the
University of Maryland. Dr. Mike Finkenthal is visiting from Johns Hopkins
this week for discussions concerning future collaborations on divertor
diagnostics.

Chris Brunkhorst has arrived from PPPL for an extended visit under the
auspices of the C-MOD/PPPL RF collaboration. He will be helping with the
engineering on the ICRF system.

Martin Greenwald attended the ESNET Steering Committee meeting in Austin. The
meeting's stress was on ESnet future services and international connectivity.