Alcator C-Mod Weekly Highlights
			      December 22, 1997
 

	Alcator C-Mod continued plasma operation last week with runs
devoted to JET dimensionless similarity experiments, evaluation of
boronization effects, E-radial measurements during RF, and the collection
of probe and pedestal data during various H-mode regimes.  Excluding
conditioning shots during Monday's run, startup reliability approached
85% for the week.  91 plasma discharges were produced.  The RF transmitters
performed routinely at high power levels.
	The vessel walls were boronized over the weekend.  Approximately
1800 Angstroms of boron was deposited using our ECDC deposition technique.
Following the boronization, a day of ECDC in helium was conducted.
	On Tuesday, approximately half the run was spent conditioning the
newly boronized walls.  However, later in the day, EDA modes were
again obtained at RF power levels as low as 1 MW.  It was also discovered
that the argon injection into the machine used for ion temperature
measurements had been affecting the core radiation and H-mode development.
Reducing the level of injection much improved the H-modes and H-mode
power thresholds.
	Wednesday was devoted to the JET similarity experiments.  By
shot 007 good matches to the JET shape were obtained at plasma currents up
to 1.2 MA.  Later in the run, very good ELM-free H-modes were obtained with
190 kJ of stored energy, and nebar of up to 4.9 X 10^20 /m^3.  H factors of
approximately 2 were obtained.
	On Thursday a new diagnostic technique was tried in which a quarter
wave stub in the scanning probe circuit allowed probe characteristics during
high power RF heating to be measured.  Initial results look promising.
On two shots, the probe was actually inserted into the plasma during
L-mode and removed during H-mode.  Scans were also obtained during
EDA modes.  Analysis of the profile data is now in progress.
	Friday's run was supposed to be devoted to experiments with the new
scanning gas probe.  However, a problem with the gas probe shifted the run's
focus to one of collecting as much fast scanning probe and high resolution
profile data as possible during the various H-mode operation regimes.  A
great deal of data was obtained during ELM-free, ELMy, and EDA H-modes, over
a range of densities.  A major result of the day was confirmation from the
fast scanning probe that the pedestal is indeed 4 mm inside the LCFS; in
agreement with the high resolution edge x-ray array.
	Through very fast sampling (2 MHz) of poloidal field pick-up coil
signals, high frequency (580 kHz - 700 kHz) modes were observed during
Enhanced D alpha H-mode.  The modes appear well into the H-mode in discharges
that start with relatively low target density (nebar < 1.5 x 10^20 m^-3) but
reach normalized beta values exceeding 0.7.  In EDA discharges with much
higher target density and in ELM-free discharges, the high frequency modes
were not observed.  In several cases, the modes persisted for about 10 msec
after the ICRF was turned off though still in EDA H-mode.  This timescale is
about equal to the slowing down time of a fast ion tail driven by the ICRF.
These modes are probably driven by the fast ions and may be Toroidal Alfven
Eigenmodes or Energetic Particle Modes.
 	Development of the power supplies for the DNB for C-Mod continued.
The first stage in conditioning of the oil for the accelerator supply was
completed.  Oil breakdown is now at 35 kV, and conditioning has been halted
to determine whether this improved condition is stable.  Designs for
the cable terminations at the Modulator/Regulator cabinet are complete, the
machining is finished, and the cable ends of the terminations are ready for
assembly.  Detailed designs were completed for the safety interlocks that
restrict access to the area near the beamline.
	Gary Taylor of PPPL visited for several days last week, continuing
commissioning and tests of the new ECE grating polychromator.