Date: Tue, 14 Apr 1998 10:14:23 -0400
From: IRBY@CMOD.PFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Message-ID: <980414101423.22a670da@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT

Alcator C-Mod Weekly Highlights
April 13, 1998



On Friday of last week the TF core was removed from the vessel,
completing disassembly of the machine. The TF core can now be carefully
inspected and cleaned. Inspection and documentation of the TF arms and legs
was also completed last week. Fabrication of the new TF leg continues
as final machining is completed of the copper components, feltmetal is
processed and tested, and soldering and fabrication tooling is brought
back into operation. A review of the TF fault is planned for May 6th at MIT.
Conditioning of the oil in the DNB Mod/Reg isolation transformer
began. Its input and output voltage divider buffer circuits were redesigned
to reduce the noise in the circuits. The schematic and layout for the fast
optoisolator boards to be used in the timing system were completed along
with patch cables and I/O backplane wiring. Eight channels of fault
circuitry for the arc/fil/snubber voltage feedback and monitoring systems
were completed. The basic plan for the new timing system was completed so
that software can be started this week. Both required and preventive
maintenance were performed on the cryo pumps by a factory technician.
Work continued on the tuneable RF systems. We are currently focusing
on tuning FMIT#4 to 78 MHz. However, recent tests indicated a
crowbar problem above 500 kW. We are debugging this problem. The 9" coax
plumbing for the new antenna has been installed from the power
room to the cell. The DC breaks will be installed following RF leakage
tests. The PPPL 4-strap antenna assembly has had a problem with the TZM
Faraday shield rods. Several of the braze joints and threaded rod sections
have cracked. This problem is being evaluated.
Analysis of the global H-mode threshold scaling for some of the
recent high field (6 - 8 T) data suggests that the toroidal field scaling may
be less than linear. Additional data at 8 T fell on top of or slightly lower
than previous 8 T data adding more statistical significance to a less than
linear toroidal field dependence of the H-mode threshold. The data still
fall between P/(nBS) of 0.015 and 0.03, when the absorbed power fraction is
included based on a break-in-slope analysis of the plasma stored energy.
However, a square root toroidal field dependence also falls within the error
bars, suggesting that the H-mode threshold may not be as high as previously
thought at high toroidal field. Nonetheless, it is clearly more difficult to
get into H-mode at high field, so there is a positive toroidal field scaling.
Further analysis of the toroidal field dependence of the H-mode threshold
is required for scaling to ITER or for scaling to a future high field
ignition machine.
Analysis of data from our new phase contrast interferometer (PCI)
shows a rise in 75-150 kHz turbulence during H-modes. The turbulence frequency
changes as plasma density, temperature and other parameters change.
Frequency vs. wave number plots allow a dispersion relation can be measured
for this kind of turbulence. Also, a very strong PCI signal is obtained when
a pellet is injected into the plasma. Future plans include upgrades
for a wider beam, a new detector, and heterodyne measurements of RF waves.
Analysis from the 88 GHz reflectometry channel modified as part
of the PPPL collaboration for high resolution fluctuation measurements
has begun. Fluctuations in the 100 kHz range have been measured during
H-mode operation. The narrowband components have frequency shifts that
can be correlated with sawteeth and H-alpha activity. A novel aspect of
this new fluctuation diagnostic is that both the upper and lower
sidebands of the AM modulated signal are monitored. These sidebands are
separated by only 265 MHz and should therefore usually be well correlated.
However, we have found that during H-mode formation, soon after application
of RF power, the correlation can be poor. We are investigating both
instrumental and plasma physics related explanations for this effect.