Alcator C-Mod Weekly Highlights
			October 14, 1997
 

The maintenance period on Alcator C-Mod continued last week.  Re-assembly
of the alternator has begun.  Work on the DNB and RF systems continues.
Analysis of data from the last run period is proceeding.

The rotor is now back in the alternator housing and gas baffles and
bearing assembles are being installed.  Installation of new instrumentation
for monitoring the exciter voltages and currents, exciter varistor currents,
and alternator ground current is also proceeding.  A review of the alternator
instrumentation and protection circuits will be held on October 20th at
MIT.

Wiring and programming of the DNB vacuum control PLC has been
completed. The system has been tested, minor problems
identified, and the required changes are being made.  Leak testing of the
beamline was completed and repairs implemented.  Now that the beamline is leak
tight testing of the cryo pumps can began.  Supports for the
the high voltage, low capacitance transmission line leading into
the test lab were installed.  A proposal for installation of Langmuir probe
and ECE diagnostics on the DNB flange was distributed for comment.

Work on RF systems is proceeding. The tuned arc detectors have been
adjusted and incorporated into the PPPL demodulator unit.  The detector
has been tested successfully on the bench and is awaiting in-situ
testing. The FMIT#1 and #2's power feedback circuit has been tested to
determine if the RF power can be modulated at rates of up to 1 kHz.  Under
conditions of high reflected power it was found that one of the amplifiers
did not meet the 1 kHz spec.  Added isolation of the feedback circuit should
fix this problem.

Experimental investigations seeking to illuminate the role of D2 molecules
in volume recombination in the C-Mod detached divertor plasmas are of
interest because Molecular Activated Recombination (MAR) has been predicted to
be a major, if not dominant, pathway for volume recombination [1,2].  The
presence of molecules in the C-Mod plasma has now been detected by observing
emission lines from the Werner bands of D2. The Werner bands are produced by
electron impact excitation of D2 and have wavelengths in the 90-130 nm
spectral region. The thresholds for excitation are in the 10 eV range and
the bands are therefore observable only in regions where Te is greater 
than about 3 eV.  These ``hotter'' regions will contain significantly fewer
D2 molecules than colder regions near the divertor plates. Thus the lines
are extremely weak.  Nonetheless, by comparing D2 molecular spectra measured
in C-Mod's ECDC plasma, a few of the Werner band lines were
identified in spectra from C-Mod tokamak discharges. The brightnesses of
these lines were compared with the brightness of Lyman alpha (2p-1s) which
is emitted from atomic deuterium as is seen very strongly in the same spectra. 
After removing the effects of the different excitation efficiencies, the
average ratio of atomic to  molecular  density was estimated to be ~50
in the 2-15 eV region viewed by the spectrometer. This ratio is within a
factor of 2-3 of what is predicted by the simple 1-D modeling of Knoll,
Krashenninikov, and Pigarov, and is interpreted as confirmation of the
presence of D2 in the divertor plasma.  

The ECDC cleanup of the machine will begin again in a couple of days.
An invessel view of ECDC will be available again on the web at:

	http://www.pfc.mit.edu/cmod/inside.html

He Huang, UT-FRC, arrived for an extended stay as a visiting engineer.
He will commission the  DNB ACCEL supply as his first task.

Gary Taylor and Bob Cutler from PPPL visited last week to continue
commissioning the new GPC-2 grating polychromator.  The detector was cooled
and they verified that all 19 channels are working.

Randy Wilson visited from PPPL for two days last week.  The coax design
for the new PPPL antenna, experimental plans for the upcoming campaign,
and long range plans for the PPPL/MIT lower hybrid current drive
collaboration were discussed.

Jim Irby attended the SOFE conference in San Diego last week and presented
a paper on Alcator C-Mod and some recent results.

1) Post, D.E., J. Nucl. Mater. 220-222 (1195)

2) Krashenninikov, S.I., Pigarov, A.Yu., and Sigmar, D.J., Phys. Lett. A
   214 (1996)