From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
Sept 25, 2000

Plasma operations continued last week. Four run days were scheduled, but only
two were accomplished, due to a loss of vacuum accident late Wednesday
afternoon. The experiments carried out included an investigation of the
dependence of pedestal widths and the EDA/ELM-free boundary on current ramp
rate; and a study of recombination and ionization in the divertor region.

Late in the run on Wednesday, C-Mod experienced a sudden loss of vaccuum which
was quickly traced to a hole in a bellows in the beamline of the Diagnostic
Neutral Beam. INdications are that the damage was a result of a portion of the
beam impinging on the bellows. The port was blanked off and the machine pumped
back down successfully, and the vessel was baked to 100C for two days,
followed by discharge cleaning in deuterium over the weekend.

Plasma operation will resume this week; five run days have been scheduled, to
make up for the time lost.

Physics
--------

An experiment was carried out to quantify the dependence of H-mode pedestal
parameters on dI/dt, which is presumed to influence primarily the edge current
density. Previous experiments had indicated that the width of the soft xray
pedestal was influenced by current ramps. A second objective of these
experiments was to determine if the transition from ELM-free H-modes to the
EDA regime was sensitive to the current ramp rate or edge currents.
Experiments were carried out at 0.8 and 1MA, with positive and negative ramp
rates up to 1.25MA/s. In these experiments, no clear dependence on dI/dt was
observed in the edge Thomson scattering profiles; a barely discernible effect
was noted in the xray pedestal widths in the 1MA case, but it was smaller than
expected and nearly masked by the well-established dependence on total
current. However, the visible continuum profile (representative of ne) showed
a strong variation with dIp/dt. No consistent dependence of the EDA/ELM-free
boundary was observed.

The goals of Wednesday's experiment, which will contribute to a doctoral
thesis, were to characterize the magnitude, location, and evolution of the
volume recombination and ionization in ohmic detached plasmas and EDA H-mode
plasmas; and to obtain spatial temperature and density profiles using D_gamma
to continuum profile ratios. Spectrally filtered TV images of the divertor
region were obtained in D-alpha, D-delta, D-gamma, and continuum emission. The
initial observations from this run include:

1) Detachment on the outer leg was observed by the cameras as a
region of emission that extends up the outer leg.
2) There were no obvious differences in the locations of the
D_alpha, D_gamma, and "continuum" emission regions during any
conditions. (2-D inversions are being generated)
3) During EDA H-modes the emission (D_alpha, D_gamma, and
"continuum") changes from being located at the outer
strike-point and weakly on the inner nose to no observed
emission on the outer leg or strike-point and strong emission
on the inner nose.
4) Absolutely calibrated 2-D profiles are being generated for all
shots in this run.

ICRF Systems
------------

Further analysis of the J-port antenna results by the ICRF and Engineering
Groups has led to new insights into the performance of this system. A subtle
error in the antenna transmission line model had led to a phase error of 180
degrees. Based on an assessment of the antenna waveforms of the previous week,
and the subsequent model revisions, a half-wavelength segment of transmission
line was removed from one of the two active current straps. ICRF heating was
attempted during Tuesday's run with the antenna in the two strap
configuration, and heating efficiency equal to that from the D- and E-port
dipoles was observed. Overlap of all three antennas brought the total power up
to 4MW.

The antenna was then reconfigured to a four-strap configuration, and heating
experiments were continued in piggyback mode on Wednesday. The two antenna
halves were initially brought up to the 0.5MW level individually, for
conditioning and phase checkout. These experiments were intended to test the
modified model and preliminary analysis indicates good agreement between the
model and experiment. Additional experiments will be performed this week in an
effort to develop a proper transmission line configuration for heating
experiments.


Diagnostic Neutral Beam Systems
-------------------------------

The BES diagnostic is being used for measurements of the beam width by
grouping its channels poloidally. The results of the analysis show a clear
reduction in beam width from ~13 cm to ~ 9 cm as the beam current is increased
from about 3.6 A to 5.1 A at a fixed accelerator voltage of ~44 kV. This is a
very positive result which indicates that the beam is focussing properly and
will assist with development of the diagnostics. In particular, it will be
used to benchmark the beam's internal thermocouple measurements. This work
needs to be repeated at intervals to assure reproducibility and to assist in
diagnostic development. The spectra that will be used by MSE and BES were
observed with good spectral resolution. The predicted spectrum agrees with
that measured. The MSE polarizer was positioned to transmit both pi and sigma
components, but because of the low TF (3.5 T), the sigma and pi components
were merged. With the experience gained during this work and with the
predictions developed in the course of it, we will be able to easily repeat
the measurements with the polarizer set to transmit only pi and then only
sigma components. Continued analysis of existing data at higher TF may
provide additional information as well. Both the perveance measurements and
these spectral measurements benefited from firing the beam into neutral gas.

As mentioned, The bellows in the DNB duct developed a leak and is in the
process of being replaced. The replacement bellows will be provided with
protection from internal beam interaction and from external abuse as well.

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

Norton Bretz and Garrit Kramer came to MIT 9/18-22 for further work
on the MSE diagnostic.