From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Alcator C-Mod Weekly Highlights
July 14, 2003

Plasma operations continued at Alcator C-Mod last week. Principal experiments
included MiniProposals submitted in the RF Physics and Advanced Tokamak areas,
along with a piggyback experiment from the Transport Physics topic. Good
progress was also made in the Lower Hybrid project.

The week of July 14 is scheduled to be the last week of plasma operations of
the summer experimental campaign at Alcator C-Mod.

Operations
-----------

A fresh boronization was successfully performed on Monday night, prior to the
week's operation. Four run days were scheduled last week, including two
extended runs of 10 hours each. Several difficulties with power and the ICRF
systems resulted in an abbreviated run on Tuesday. The machine ran well for
the rest of the week, and a total of 95 plasma shots were produced with a
startup reliability of over 80%.

There was a fault in the TORVAC PLC after the last shot on Wednesday; the
system was restarted and vacuum cleanliness was not compromised.

Experiments on Thursday and Friday involved mode conversion studies in
D-H-He3 plasmas. In order to increase the hydrogen fraction, overnight
ECDC in H2 was carried out over Thursday night. These experiments were
scheduled near the end of the campaign to avoid compromising the low H/D
ratio required for H minority heating which is our standard heating
scenario. ECDC in D2 is being carried out over the weekend, and we
will monitor the resulting H/D ratio in this week's experiments, which
do not depend critically on maintaining H/D at very low levels.

Physics
--------

Tuesday's run in support of MP#349 was designed to evaluate the performance of
the four-strap (J-port) antenna in current-drive phasing; this evaluation was
completed on Thursday. The symmetric and -/+90deg phase appeared to heat as
well as the dipole antennas at D and E within experimental error. With -90deg
the sawtooth period in L-mode was 5 msec and was 16 msec with +90deg. In
H-mode, there was little affect on the sawtooth period.

Mode conversion current drive experiments (MP#345) were begun on Thursday,
following completion of the antenna evaluation in standard D+ majority
plasmas. However, the attempt to switch over a small number of
discharges from low to high H/D ratio, as required for the planned
MCCD experiments, was not successful. The MCCD experiment has been
re-scheduled for this week.

The principal experiment on Wednesday was in support of MP#351 on q-profile
control with ICRF heating during the current rise. The main emphasis of this
experiment was to obtain MSE data to document the evolution of the current
profile with early ICRF heating. The target plasmas were similar to those in
experiments carried out last October, although in this run only on-axis
heating was employed. Up to 4.4MW of on-axis ICRF power was injected during
the current rise. As in the previous experiments, both ELMy and ELM-free
H-modes were obtained during the current ramp phase. A number of machine
pulses with and without plasma were devoted to MSE calibration shots to
facilitate interpretation of the profile data.

The secondary {"piggyback") experiment on Wednesday was in support of
MP#353, on the study of striations in Li pellet clouds. Images of
eleven pellet injections were taken using the PSI ultra-high speed CCD
camera. Six sets of images were taken during ohmic operation, and five
sets of images were taken during rf operation, three of these
occurring while the plasma was in H-mode. In all, a good radial scan
of the plasma was made during both modes of operation yielding high
quality images of the ablation striations. The observed motion of
these striations is believed to be indicative of the local structure of
the Er field, and we have speculated that these observations may
related to the presence of zonal flows. Comparison of the L- and
H-mode data with code simulations will contribute to the understanding
of this phenomenon.

Friday's run was devoted to initial studies of mode conversion flow drive
(MP#343A). After overnight ECDC in H2, about half the run was dedicated to
obtaining a satisfactory species mixture to get mode conversion at the He3-H
hybrid layer for investigation of flow drive associated with mode
conversion. The ICRF power was modulated to facilitate observation of local
heating associated with the mode conversion layer using the ECE
diagnostics. In several subsequent shots, possible indications of flow
correlated with the RF modulation were observed on the high resolution soft
x-ray spectrometer channel which was viewing the surface corresponding to the
approximate location of the MC layer. However, the observations were not
conclusive, and further attempts at higher power and at different fields were
not successful. Analysis of these data is in progress.


Lower Hybrid System
--------------------

High power tests of prototype lower hybrid alumina vacuum windows were
conducted this week at MIT. Over 50 kW of source power was
successfully coupled through these windows with a 1 second pulse
length (system losses to window of 0.5 db). This power level is
approximately a factor of two above what will be needed during lower
hybrid experiments on C-Mod. Power levels of over 140 kW were
delivered with 40 microsecond pulses without electrical breakdown or
increased reflected power. A VSWR of less than 1.11 was measured
during these tests.

All 50 plates of the forward waveguide assembly have completed a 200 C vacuum
bake at MIT following replating, and followup inspection revealed no plating
faults,other than some localized oxidation that should have a negligible
effect on the losses of the stacked waveguide, and a very smooth surface. The
plates have been returned to PPPL for initial assembly and electrical loss
measurements. If the losses are sufficiently low, < 0.4 dB per channel, then
no further polishing will be needed.

Electropolishing of the waveguide vacuum shroud has been completed at MIT, and
the shroud has been returned to PPPL for reassembly. Additional design,
specification, procurement, and fabrication tasks are in progress at PPPL.
Ceramic window rebrazing tests using a coupler prototype are in progress at a
commercial vendor.

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


Randy Wilson (PPPL) was at MIT 7/8-11 to participate in ICRF experiments and
for technical discussion on the Lower Hybrid tasks. Gerd Schilling was at MIT
for the week to participate in the evaluation of phased operation of the
4-strap antenna and the ICRF current drive and flow drive experiments.

Ken Gentle of UT-FRC visited last week to discuss QC mode with Matt Sampsell,
to continue working on the analysis of FRCECE data, and to look for
opportunities for experiments for the next campaign. He has been pursuing his
long-standing interest in electron thermal confinement phenomena at DIII-D and
is exploring ways to expand this on C-Mod.

Joe Snipes, Dmitri Mossessian and Ian Hutchinson attended and presented papers
at the 30th European Physical Society Meeting on Controlled Fusion in St
Petersburg, Russia. Martha Redi (PPPL) also authored a paper, based partially
on C-Mod results, which was presented at the meeting.










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