From: IRBY@CMOD.PFC.MIT.EDU
Date: Tue, 19 May 1998 9:57:43 -0400
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980519095743.25a0031a@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT Plasma Fusion Center


Alcator C-Mod Weekly Highlights
May 18, 1998



We have begun removal of feltmetal from the upper TF arms as the
first step in preping them for new feltmetal installation. We have also
completed cleanup of the core insulation and begun sealing it to protect it
during the electroforming and plating activities. Two new inductive heating
coils have been fabricated and sent out to Pillar, the company supplying our
new heating unit. They will test the coils with a new transformer designed to
improve matching between the heating unit and the coil. Fabrication of the
fixture needed to hold the feltmetal during soldering is nearing completion.
This fixture will also be tested at Pillar before the end of the month. High
current density tests of feltmetal samples with 2 mm of sliding travel will
begin in the next few days. Full scale tests of a sliding joint at full
current are expected to begin by the end of month.
Further analysis has been performed on the total radiated power
emissivity at the edge of the plasma. A photodiode array with
2 mm radial resolution was used for these measurements. The emissivity
scale-length was found to decrease from 2-3 cm to 3-4 mm after
a transition from L to H-mode. Experiments where plasma current was
modified during the H-mode, showed a correlation between the scale-length
and the plasma current. When the current was increased the scale-length
decreased, and vice-versa. However, the biggest change was observed to be
outside the separatrix. In addition, a toroidal field scan was also performed,
but no change in scale-length was observed.
Results from ECDC experiments performed last fall are to be presented
at the upcoming Plasma Surface Interactions conference in San Diego.
Measurements were performed to determine ECR plasma conditions. Toroidal
asymmetries in ion saturation current density were observed, indicating
local toroidal plasma flow. The ECR plasma was used to remove a
diamond-like carbon coating from a stainless-steel sample. Removal rates
peaked at 4.2 +/- 0.4 nm/hour with the sample a few centimeters outside the
resonance location. The plasma did not remove the carbon from the sample
uniformly, possibly due to plasma flow. Yields were calculated (Y ~ 1e-3) to
be lower than other published results for chemical sputtering of deuterium
ions on carbon, possibly due to toroidally asymmetric plasma conditions.
Development of the DNB for C-Mod continues. The Kirk key safety
interlock mechanism for the Mod/Reg has been finished. Circuit board layout
was completed for the logic boards required in the arc/filament/snubber
interface to the Master Control Logic (MCL). HV cabling was completed between
the snubber in the beamline area and the accelerator and grid voltage dividers.
Insulating supports for the cable run to the beamline were fabricated.
Calibration and testing of the communication links to the high voltage divider
system at the beamline continues. A new current limiting resistor mount for
the suppressor was fabricated and installed. The embrittled filaments in the
beamline plasma source were replaced with newly fabricated filaments. Work
continued on the MCL and PLC interface circuits, PLC programming, and VAX
software development.
We are currently disassembling FMIT#4. Last week we found an
arc track on the tube contact ring. We surmised that an arc was
occurring between the control grid and the contact ring. Shorting the
grid would cause the tube to produce its maximum current which would
result in a crowbar. After removing the arc track and inspecting the
gap, we assembled the transmitter and tested it. It still crowbarred
when the power was in the range of 250 - 500 kW. We intend to perform some
other tests and insert another tube in FMIT#4 to test whether the current
tube is causing the problem.
The PPPL antenna work is progressing. We currently expect the
antenna to be delivered in the beginning of July. The pacing item is to
complete the antenna electrical testing. A thorough testing of the antenna
on the test stand is needed to prove that the antenna will work in an
engineering sense when installed in the machine. Depending upon the
TZM Faraday rod testing, the Faraday screen may be delayed.
Martin Greenwald travelled to Berkeley, California where he
attended a DOE sponsored review of the Energy Sciences Network.
At the review he made a presentation entitled "Users Perspective:
Fusion Energy Sciences". Following the review, he attended a meeting
of the ESnet Steering Committee. Bob Granetz attended the ITER Disruptions
and MHD Expert Group meeting in San Diego last week.