From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: 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 21, 2003

Research operations continued at Alcator C-Mod last week. Two and a
half run days were scheduled and completed, bringing the 2003
Spring-Summer experimental campaign to a close. C-Mod is now entering
a maintenance period.

During the two experimental campaigns in fiscal year 2003, C-Mod operated for
a total of 54 Research Days, exceeding our Level 1 Milestone target of 52 days
(13 weeks times 4 operating days per week). A total of 1443 plasma discharges
were produced with an overall startup reliability of 80.5%.

Operations
----------

A total of 48 plasma discharges were produced with an overall startup
reliability of 89% in support of three MiniProposals on Tuesday and
Wednesday of last week. The final half-day of operation on Thursday
involved power system only (no plasma) shots in support of an
experiment to quantify non-axisymmetric error fields of the poloidal
field coils.

The new boron injector was operated for the first time during
Tuesday's run. We hope the injector will reduce somewhat the need for
boronizations and help maintain more constant wall conditions during
run campaigns. This device is designed to inject boron at rates of up
to 2 mg/pulse at pulse rates of up to 30 Hz. The initial tests were
run at deposition rates too low to actually affect the plasma because
of a still undiagnosed problem believed to have been caused while
refilling the injector with boron before the run. However, the laser
scattering system used to measure the boron deposition rate, the BIII
detector used to monitor the boron light from the plasma, the injector
control system, and the magnetic shielding around the injector, were
all tested successfully during this first day of operation.

Wednesday's run was devoted to MP#345, Mode Conversion Current Drive (MCCD)
experiments, employing D-He3 plasmas at 8 tesla toroidal
fields. This experiment marked the first 8T operation of this year's
campaign. All systems performed nominally; startup reliability was
100% and all plasmas ran to full programmed length. Post-shot recool
time was increased to 20 minutes, in accordance with previous
practice.

Following completion of operations on Thursday, the tokamak is being
warmed up to room temperature. Minor maintenance, including a clean
vent to repair leaks and replace some vacuum components, will be
carried out over the coming weeks. The major activity during this
period will be the scheduled inspection of the alternator which
provides 13.8 kV power to the C-Mod power systems. Additional
maintenance and upgrades to the power and RF systems will also be
undertaken.

Physics
--------

On-going analysis of experiments conducted earlier this month
indicates that in Internal Transport Barrier discharges with off-axis
(HFS) ICRF heating, the barrier is maintained as the field is ramped
up such that the resonance moves closer to the axis by 1-2cm. In these
cases, the central density rise associated with ITB formation was
arrested, but the barrier was not destroyed.

DNB Systems
-----------

The DNB was switched to deuterium for the first half of the week in order to
accomplish MP 354, which was a study of fast ion confinement. This was the
first time that deuterium had been used in the beam, and the changeover was
uneventful. The beam was used to generate a perpendicular deuteron tail
population in the plasma, and their confinement time can be estimated from the
resulting increase in the 2.45 MeV neutron rate, although a full TRANSP
analysis is required to get quantitative results. Measurements of the
beam-target neutrons were made under a variety of discharge conditions with
excellent signal to noise. The new A-coils were used to compare plasmas with
and without locked modes at a number of densities, currents, and fields.
Preliminary results suggest that locked modes do not enhance fast particle
losses. This bears directly on the issue of ICRF heating efficiency, since
that also involves a perpendicular ion tail.

The DNB was switched back to normal hydrogen for the latter part of the
week. The changeovers from H->D and D->H took only a couple of shots to
reduce the previous component levels to the few percent level.

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

The initial mode conversion current drive experiment at 8T was
successfully carried out on Wednesday. The mode conversion power
deposition was clearly observed and localized near the plasma core and
the location was controlled by both magnetic field and He3
concentration. Up to 2 MW were coupled from the J-port antenna in
heating, co- and counter-current drive phasing.

Initial tests of the two quarter wave transformers to mitigate high
voltage in the phase shifter/stub tuner sections of the D-port
transmission line suggested the position of the transformers was not
optimal. Further measurements are required to optimize their
location.

Transmitter #2 developed a driver bias problem which precluded operation of
the E-port antenna last week. Since RF operations were focussed on
current-drive phasing experiments using the J-port antenna, research
operations were not significantly impacted. The problem is under
investigation.

During operation in May and June of this year we completed the 3rd quarter
target of the Level 1 C-Mod Milestone associated with the ICRF system
performance: "Begin high power ICRF heating experiments with modified antenna
systems and identify candidate target plasma conditions likely to lead to high
temperature and low collisionality." Also in June, we successfully
demonstrated 5MW operation for 0.5sec, which satisfies the first part of the
target for the 4th quarter. Assessment of the stability and confinement
properties of the resulting plasma is expected to be complete by the end of
the fiscal year.

Lower Hybrid Project
---------------------

We continued to make progress on the tasks associated with the Lower
Hybrid launcher replating this week. The forward waveguide plates
have now been cleaned, stripped, replated, vacuum baked, inspected,
and partially assembled at PPPL into a stack of 5 plates for RF loss
measurements. Measured losses of 10 waveguide channels were in the
range of 0.3 dB each, well within acceptable levels. A trial stackup
(white glove handling, extreme cleanliness) of 25 of the plates for
one complete half of the front waveguide will be performed next, and
the losses will be remeasured. If successful, this will allow us to
save schedule time and effort by eliminating the surface polishing
steps.

Chemical stripping of the copper from the front end coupler had also removed
part of the braze material fastening and sealing the ceramic microwave
windows, resulting in gaps and vacuum leaks. Rebrazing tests using
different alloys are under way at our industrial vendor; prototypes will be
available for inspection next week. We plan to procure a full set of backup
ceramic bricks and titanium frame material in case a refabrication is
necessary.

The weekly MIT/PPPL videoconference to discuss technical progress and track
the launcher schedule was held on 7/17.

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

Doug LaBrie (PPPL) visited C-Mod on Friday to retrieve the detector and data
acquisition components of the NSTX high-resolution xray diagnostic, which had
been being tested at C-Mod during operations earlier this month. An upgraded
version of this diagnostic will be installed at C-Mod during the next
experimental campaign.



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