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
July 16, 2001

Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled and completed. A total of 27 plasmas were produced, with a startup
reliability of about 85%. The primary focus of the week's operation was
recovery from the previous week's vent, and reconditioning of the ICRF
antennas.

Physics operation is scheduled to continue this week.

Operations
----------

Following the up-to-helium "clean vent" on Friday, July 6, the vessel was
pumped down and a 120C bake was begun. On Sunday morning a low current, low
voltage, glow discharge was started in helium at approximately 25 mTorr. On
Monday the glow current was increased to 3000 mA at about 300 volts at 10
mTorr. The bake was dropped to 60C on Monday afternoon. These conditions
were continued until Tuesday morning when the glow was stopped for several
hours for antenna conditioning. The glow was resumed overnight at 3500 mA,
270 V, and about 12 mTorr. The glow was stopped early Wednesday morning and
the vessel temp was dropped back to 35C and cooldown of the magnets to
operating temp was begun. ECDC in D2 at 2e-4 Torr was run for about 1 hour
before beginning plasma operation on Wednesday.

The glow cleaning regimen described above differs from our usual post-vent
cleanup procedure, which has emphasized baking and ECDC in deuterium. The
high quality plasmas produced during recovery shots on Wednesday and Thursday
indicate that the helium glow, combined with a sustained high temperture bake,
is a highly effective technique. Good plasmas were readily obtained, and the
initial H/D ratios were only about 0.3, compared to H/D>1 observed following
our previous vent. Plasma operations on Wednesday and Thursday were limited as
the engineering staff investigated and remedied a noise problem which caused
TF crowbars following several successful shots. However, after only a
relatively few conditioning discharges the H/D ratio had dropped to the 10-20%
range, adequate to carry out a boronization on Thursday night.

The vessel was boronized with about 2000 Angstroms of boron, followed by 2
hours of ECDC in helium at 5e-5 Torr, and 1 hour of ECDC in D2 at 2e-4 Torr.
Machine operation continued on Friday, with H/D ratios at the beginning of the
day below the 5% level. The main emphasis of this run was reconditioning of
the ICRF antennas. Both E-port (80MHz) and J-port (70MHz) antennas were
brought up to the 1MW level; this is actually the first time that the J-port
antenna has been operated into plasma at 70MHz in its four-strap
configuration. The D-port antenna experienced repetitive fault indications
that may have been due to a faulty demodulator circuit in the
instrumentation. ICRF conditioning should be completed early this week,
followed by physics operation.

Physics
-------

This week we obtained first "movie" images of edge turbulence with the Gas
Puff Imaging diagnostic using the Princeton Scientific Instruments PSI-3
camera viewing parallel to B. This camera can capture 12 frames at up to 1
million frames per second. Nice images of the motion of edge turbulence in
the radial vs poloidal plane were taken at 100,000 frames per second and
above.


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

The beam was operated at full voltage and current through the week with no
significant problems. The sharing of arc current among the four filaments
(which form one electrode for the plasma arc) was checked. Arc current is
equally shared to within trimming adjustments that can be made via filament
current (same as filament temperature) adjustment. We further conclude that
it would be useful to measure this sharing on every shot. We experimented
with an operating mode in which the arc is run for longer times for
conditioning while we are also generating beams. This resulted in the beam
not firing into C-Mod on a couple of shots and will therefore not be used
again. Arc conditioning will continue before and after regular operation.
This week we expect to increase the length of time that the arc can be turned
on during arc conditoning beyond the present 300 ms. An experimental plan for
MSE calibration was developed and we await the opportunity to do the
calibration which will also further our understanding of reionization. CXRS
measurements continue to progress with observation of small signals. BES is
able to observe fluctuations in ambient D-alpha emission. Data was taken with
beam and will be analyzed and tested for the effects of parasitic
fluctuations.


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

We completed the reconfiguration of the coax transmission line and tuning the
FMIT#3 and #4 to 70 MHz. Before tuning FMIT#3, we had to repair a broken
mechanical switchgear. The current transmitter tune is not ideal. The match
between driver and final is not fully optimized, but should be adequate at the
power levels required for near-term experiments. The arc detection system
testing was completed and initial antenna conditioning was completed for the
J-port antenna at 70 MHz. The decoupling is not as complete as it was for the
78 MHz configuration; however, it did not limit the operation of the antenna
into plasma. Antenna matching calibration factors were obtained in the
initial operation and about 1 MW was injected into L-mode plasmas
successfully.

The D and E-port antennas were operated successfully after the up-to-helium.
D-port was fairly unreliable and a malfunctioning fault detector was found and
thought to be the cause. The module has now been replaced in preparation for
this week's runs.

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

Work continued on the ladder-logic PLC program. A new design has been
developed for the coolant pump controller interface. This design has
significant improvements over the initial preliminary design which eliminate a
PLC, I/O rack, and dedicated optical interface circuit. These changes reduce
the project costs by over $10,000. Circuit design and PC board layout for the
Lower Hybrid Timing was completed. This board will provide properly
synchronized 6 and 48 kHz clocks for the LH control and data acquisition
system.


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

An independent CMM check on one of the C-Plates confirmed the vendor's data
that all 89 critical dimensions were within the required dimensional
tolerance. Work on the probe box parts is 90% complete. A design modification
is being carried out to add new probes for the bottom portion of the C-Plate
and modify 2 probes at the top in order to remove less material from the
C-Plates. Mock-up building is moving forward. Material (0.020" thick SS
strips) for "lining" under the BP coils has arrived, and we will complete the
strips and install them on the mockup wall this week. Manufacture of the Rear
Girdle Plates has resumed, following a vendor vacation. There are two more
operations: Milling the bottom of the plates and wire EDM of the inside radius
removal of the corners. Completion of these milling operations is promised
for this week.

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

Thawatchai Onjun, a graduate student from Lehigh University, is visiting the
C-Mod group for two weeks. He has been looking at experimental edge pedestal
data, and is comparing this with his model.

Gerrit Kramer was at MIT 7/9-11, working on the MSE diagnostic. Stewart
Zweben brought up the fast camera 7/11 and stayed through 7/13, installing the
camera and obtaining the new images mentioned above. Gerd Schilling was at
MIT for the week, mostly helping the with diagnostic neutral beam
conditioning.

Earl Marmar was at U. of Wisconsin, Madison, on Tuesday and Wednesday last
week for a meeting of the Next Step Options Program Advisory Committee.