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
June 18, 2001

Plasma operations continued at Alcator C-Mod last week, as four run days were
scheduled and completed. A total of 59 plasma shots were produced with a
startup reliability over 90%. Physics experiments carried out included studies
of the EDA H-mode, investigation of the role of molecular recombination in the
observed continuum emission in the C-Mod divertor, and continuation of earlier
work on disruption mitigation.

Plasma operations are scheduled to continue this week.

Physics and Operations
----------------------

The first boronization of the campaign was carried out Monday night-Tuesday
morning, June 11-12, with an average deposition 2050 A. Approximately 3.5
hours of ECDC in helium at 5.0e-5 Torr followed boronization. Tuesday's run
was devoted to boronization recovery and conditioning of the J-port four-strap
antenna into plasma. The machine ran well, with no startup or other
difficulties which have been associated with some previous post-boronization
runs. The H/D ratio was in the 5% range, which is satisfactory for the
hydrogen minority ICRF heating scenario. Antenna conditioning was making good
progress, until the J-port system encountered a problem with an external
phase-shifter, as described below.

Wednesday's run was devoted to MP#294, Divertor Continuum Emission Studies;
this experiment constitutes part of the doctoral dissertation research of one
of our graduate students. The purpose of these experiments was to obtain high
resolution images of D_gamma and continuum emission in the divertor at various
densities, determine the spatial relation of the D_gamma and continuum
emissivities, and to test the hypothesis of molecular recombination as the
cause of the "anomalous" continuum emission in the divertor. Plasmas were run
with the same density programming in deuterium, and with strong helium
puffing, to vary the possible contribution of molecular deuterium to the
observed continuum emission between 420nm and 430nm. The continuum signal was
observed to decrease by about 25% during the mostly helium shots, while the
deuterium lines dropped by approximately a factor of 4 to 10. This modest
decrease in the continuum when compared to the significant decrease the
deuterium leads us to believe that the continuum is not dominated by molecular
recombination.

The second part of the run consisted of a series of shots in which the
separation between the separatrix associated with the upper x-point, normally
the secondary separatrix, and that associated with the lower x-point, which
normally defines the plasma boundary, was varied dymamically during the
discharge. In some cases the ramp was extended until the plasma went from the
usual lower single null to an upper single null. This was done to determine
whether or not the emission band seen near the inner wall is associated with
the inboard-most (secondary) separatrix. All results seem to indicate that
the emission follows the inboard-most separatrix, suggesting different radial
transport characteristics for the two regions separated by the inboard-most
separatrix.

The run on Thursday concentrated on the quasi-coherent mode and the transport
which results from it. Related Miniproposals were #253 and #292. Ohmic EDA
H-mode plasmas, which are more suitable for investigation with the fast
scanning probes, were produced in the usual manner, by ramping the toroidal
field down to induce an L-H transition and then bringing the field back up.
Specifically, the goals were: to look for q-resonance effects; to compare
the particle (and energy) flux convected by the fluctuation with local
particle and power balance; and to verify the extent and radial structure of
the quasi-coherent mode.

The motivation for the q-resonance study stems from the tentative
identification of the QC mode with the family of resistive ballooning modes,
which would be expected to be localized to rational q-surfaces. The
experiment varied the programming of the toroidal field ramps to produce a
slow scan of the edge q, looking for reproducible changes in the QC mode
amplitude, frequency, or wave number. Phase Contrast Interferometry (PCI) was
the principal diagnostic for this experiment, while the scanning probe was
used to collect detailed fluctuation data. No clear q resonance effect was
seen. Changes in mode amplitude and frequency were observed, but the
strongest correlation was with sawteeth. There are other excursions which are
not correlated with sawteeth and these need to be carefully compared to the q
time histories. Data were obtained from the probes, edge Thomson scattering,
and from the Lyman alpha array for the transport studies. On one shot, the
scanning probes apparently traversed the entire radial extent of the mode,
which measured 1-2 mm, as had been found in earlier experiments. Ten shots
were selected for further analysis.

Friday's run was an attempt to extend the encouraging results of last year's
neutral point experiments (MP 264) by programming the hybrid plasma control
system (PCS) to jump to specialized feedback programming at the thermal
quench, with the goal of extending the period of post-quench vertical
stability. Thermal quench disruptions were induced by firing silver-doped
lithium pellets into established discharges. We started off by reconfirming
last year's neutral point result that post-thermal-quench plasmas show
enhanced vertical stability when run about 3 cm above the midplane. Then we
proceeded to try out two different PCS programming setups, which we hoped
would promote vertical stability for even longer periods after the thermal
quench. The alternate programming showed some promise, but overall was not as
successful as had been hoped for.

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

E-port antenna conditioned very quickly and J-port was making good progress in
reconditioning following boronization. (FMIT#1 water system was repaired this
week and unavailable for plasma operation till 6/13.) Both antennas had clear
indications of heating. However, it became clear that J-port needed to be
voltage conditioned again; therefore, we decided to switch to vacuum
conditioning. During conditioning, the FMIT#4 phase shifter began breaking
down after it reached 25 kV. During the inspection, we found that the
push/pull rod and a teflon insulator had become tracked. The push/pull rod
material was replaced with G7, which has a lower index of refraction and
dissipation factor. We have had very good success with replacement G7 rods in
our other stub tuners and phase shifters. The tracked teflon insulator was of
more concern because it is the second time an insulator at this location has
shown this behavior. We have discussed this with the vendor, but have not
identified a strong candidate explanation. The focus has been upon the center
conductor and the teflon insulator itself. The center conductor was carefully
inspected to ensure proper contact was made when fully assembled. Sharp edges
were rounded and mating surfaces were blended (these could have enhanced the
local electric field). Additional vent holes were added to ensure proper
evacuation and purging of assembly. New teflon insulators were installed and
the assembly was cleaned. All ICRF systems should be ready for operation this
week.

Diagnostic Neutral Beam System
-------------------------------

The arc supply for the beam was tested in open loop. It is normally
run as a regulated supply. We wanted to determine whether we could
make an arc current with a flatter profile avoiding the initial
overshoot and the slight increase in current toward the end of the
pulse. There was not significant improvement and the arc supplies
were returned to regulated mode. The beam appears to be fully
recovered from the bake and is now operating reliably. For the BES
diagnostic, the filters and the filter rotators were calibrated by
illuminating them with a continuum source and observing the
transmisson with a spectrometer. Some beam profile measurement were
completed and BES will now return to fluctuation measurments. The
CXRS diagnostics continued to make observations of helium spectra and
analysis is underway.

Inner Divertor Project
----------------------

Work on the new probe box assembly is proceeding, and is now approximately 70%
complete. Mock-up building is moving forward, and a new Stainless Steel Wall
was installed.

Good thermal contact between the inner divertor and the inner wall is required
for proper operation during high heat load conditions. The dimensional specs
for the inner wall components are therefore very important. An inconel
flame-spraying technique for correcting tolerance errors in the radius of the
upper portion of some C-plates was tested on a sample piece. A sample of
Inconel 625 plate was sprayed with a layer of Inconel 625, approximately
0.020" thick. Then part of the plate's sprayed surface was machined, leaving
a layer 0.015" thick. The porosity was checked, and the plate was baked to
confirm proper adhesion of the coating. The process produces a satisfactory
result.


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

David Mikkelsen (PPPL) visited MIT last week to work on EFIT calculations of
Alcator C-Mod plasmas and to discuss possible tests of profile stiffness. Gerd
Schilling was at MIT 6/12-15, working mostly with the Diagnostic Neutral Beam
group on aspects of beam operation.

John Cary from the University of Colorado visited MIT on Thursday June 14,
2001. He had discussions with Paul Bonoli and Martin Greenwald concerning
modelling needs at the Alcator C-Mod facility. Information from these
discussions is being incorporated in the revised project plan of the National
Transport Code Collaboration (the NTCC). John was told that the two most
needed capabilities for C-Mod in an integrated transport modelling code would
be an ICRF Fokker Planck package and a lower hybrid current drive module. John
also discussed data acquisition and access issues related to MdsPlus with Josh
Stillerman and Tom Fredian. Howard Yuh also gave John Cary and in-depth tour
of the Beowulf cluster under development at C-Mod.