Date: Mon, 12 Jan 2004 17:38:16 -0500
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


Alcator C-Mod Weekly Highlights
Jan 12, 2004

Plasma operations resumed on Alcator C-Mod last week. Four run days were
scheduled and three completed; Friday's run was cancelled due to a fault in
one of the ICRF Transmitters. Progress continued on the Lower Hybrid project.

Operations
----------

Plasma runs were carried out Tuesday-Thursday last week in support of
experiments in the Transport and RF Physics programs. A total of 71 discharges
were produced with a startup reliability of ~85%. A fresh boronization was
carried out over Tuesday night.

Tuesday's run completed MP#363 on LH Threshold physics in Single- and
Double-null and Limited equilibria. The main thrust of this day's experiments
was to determine threshold powers and corresponding plasma parameters in the
Inner Wall limited configuration. This configuration proved to have an
extremely high L-H power threshold. ICRF power up to 5.8MW was applied in a
series of limited discharges. Of these, only one achieved H-mode; assuming 80
percent ICRF absorption and adding ohmic power, Ptot was 5.0 MW, as compared
to 1.3 MW at threshold in LSN (ie 3.8 times higher); this is more than in
either DN or USN configurations. In contrast, equilibria limited near the
bottom of the inner wall, at the "nose" plate, transitioned to H-mode at
essentially the same power as the lower null discharges. These results
contribute to the level 1 JOULE milestone SC6-1b, "Compare energy confinement,
H-mode thresholds, and divertor particle dynamics in Alcator C-Mod
discharges".


Prior to boronization on Tuesday night, a series of low density plasma
discharges were run to "unload" the walls, to evaluate the effect on the
boronization process. A total of 120psi of diborane was used (a "medium"
level), followed by several hours of ECDC in helium. Subsequent recovery shots
and RF antenna re-conditioning proceeded rapidly on Wednesday. The remainder
of the run on Wednesday and the run on Thursday were devoted to continuation
of MP#376 on ICRF Heating efficiency and performance. Variations in density,
and midplane neutral pressure were performed on Wednesday, with higher plasma
performance observed at the lower range of neutral pressure. A scan of the
outer gap on Thursday indicated the dependence of heating on this parameter
was insignificant, as had been previously observed. PEP mode shots were
obtained using Li pellet injection, with similar stored energy to the
gas-fueled shots. Finally, strongly radiating (near 100%) discharges were
produced by puffing of Argon and Neon, to check efficiency of RF heating and
calibration of spectroscopic and bolometric measurements.

Physics
--------

During recent LSN, DN, and USN discharges, chirping modes with a frequency
between 200 and 400 kHz and k=1-2 cm^-1 have been observed by the Phase
Contrast Imaging (PCI) diagnostic. The modes are most intense following a
sawtooth crash and multiple modes appear during the sawtooth evolution. These
modes appear to be suppressed by the sawtooth precursor. During monster
sawteeth, multiple chirping modes are observed between 200-300 kHz. The
identification of these modes is still under analysis.


ICRF System
-----------

On Wednesday, the FMIT#1 transmitter (D-port) experienced a fault of the final
power amplifier (FPA) tube. This tube had ~2500 filament hours, which is
beyond its estimated mean lifetime. A spare tube is being readied and will be
installed this week.

On Friday morning, a control system fault of the 13.8kV breaker for FMIT#4 led
to damage of some components in the soft start and crowbar cabinets. Required
repairs and impact on operations are being evaluated.


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

Installation of the circulators has begun. Three of the twelve units have now
been installed.

Following measurements of the fitup between the forward and rear waveguide
assemblies, the forward waveguide was shipped to a PPPL vendor to complete
machining to assure proper gasket compression. Final assembly and tests are
expected to begin at MIT by the end of the month.

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

Ron Bravenec (UT-FRC) spent the week at MIT to setup the BES diagnostic for
beam width measurements and for poloidal wave number and phase velocity
measurements of the QC mode. This involved moving fibers and acquiring new
calibration data.

Doug Loesser (PPPL) was at MIT 1/8 to review the Lower Hybrid Launcher
assembly schedule and for final fitup measurements of the waveguide
assemblies. Joe Frangipani and Dave Miller were also at MIT 1/8-9 to assist
with the waveguide fit-up, measurements and transport back to the PPPL
vendor.

Dr. Maxim Umansky (LLNL) visited the PSFC for January 8 and 9. Dr. Umansky
presented a seminar on the BOUT edge turbulence simulation code and had
detailed discussions with C-Mod scientists, Miklos Porkolab, Peter Catto, Jim
Terry, Brian LaBombard, Olaf Grulke, Martin Greenwald, and Bruce Lipschultz
regarding comparison of future BOUT simulations of C-Mod with our extensive
edge turbulence data. We are also planning C-Mod experiments that would
benchmark certain crucial aspects of BOUT.




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