Alcator C-Mod Weekly Hlghlights
				Dec. 15, 1997

Alcator C-Mod resumed plasma operations last week. Five run days were
scheduled and completed. Approximately 100 plasma shots were obtained. All
operation was at a nominal toroidal field of 5.4T, with plasma currents up to
1.2MA. Startup reliability was 75%. The RF system was available for the first
discharge and reliably delivered 2.5 MW by the third day of RF operation.

Operation was resumed on Monday following successful operation of the
alternator at full speed over the weekend. Full length, well-behaved 800kA
plasmas were produced from the first attempt, and after five shots the current
was increased to 1MA. The RF system began conditioning and tuning activities,
and checkout of several diagnostic systems was accomplished. Preparations for
using the impurity injector to introduce carbon for the purpose of generating
"cold-pulses" for a perturbative/non-local transport experiment were begun. It
was noted that the RF interference on the fast-scanning probe (FSP) which had
been a problem in the last campaign, was absent, even at high RF power
levels. The RF power transiently reached 3.9MW, and H-modes were readily
obtained.

Tuesday's run was devoted to continuation of RF conditioning and tuning. The
conditioning goal is 2.5 MW (total) reliably from both transmitters. A key
element of the run was re-establishing the plasma position feedback from the
RF loading signal, in order to maintain satisfactory loading across the L-H
transition. Impurity radiation was found to be high during H-modes, with
strong accumulation, similar to behavior in un-boronized conditions.

Physics runs began on Wednesday, with an experiment to investigate impurity
compression in the divertor (MiniProposal #167) using the divertor RGA and
core impurity diagnostics.  This experiment was carried out with ohmic 1MA
plasmas with the flattop extended to 1.5 sec. Data were obtained with Ar at 3
different densities (approximately 1.6, 2.4 and 3e20 /m3). With Kr we covered
the lower 2 densities. An analysis of the effect of impurity mass on scaling
awaits MIST analysis and further calibration of the RGA with Kr.  The Kr was
successful in that we had good signals on both the divertor RGA and the
McPherson spectrometer.  The spectrometer was set to view Mg and Na-like Kr
(Kr 24+ and 25+) that are at around 500 eV in the plasma.  We were not
successful in finding an appropriate line for B-like Kr (higher Te).  Divertor
detachment was obtained at densities of about 2.7e20/m3 in these 1MA plasmas.

Due to the high radiated power fractions obtained during H-mode operation
earlier in the week, a fresh boronization was carried out over Wednesday
night. Approximately 1000A of boron were deposited, followed by several hours
of ECDC (electron cyclotron discharge cleaning) in Helium. 

Thursday's run was devoted to H-mode pedestal measurements (MP# 188). Data were
obtained with electron cyclotron emission and the edge xray and XUV
arrays. The toroidal field was swept over about a 5% range to allow the ECE to
map the pedestal shape. Data were obtained at 1MA and at 600kA. 600kA H-mode
discharges have rarely been run on C-Mod; these had indications of enhanced
D_alpha behavior, including flat density and P_rad late in the H-mode. It will
be interesting to explore whether this is a reproducible effect at lower
Ip. At 1MA, the H-modes were mainly ELM-free and of relatively short duration,
with some Type III ELMy periods.  Density and Prad continued to rise rapidly,
despite the overnight boronization.

Preliminary analysis of ECE data shows similar widths to the EDA H-modes
measured in 1996, (~ 10-12 mm raw, 6-9 mm deconvolved) but the pedestals are
apparently further in from the LCFS by ~6 mm.  We need to check whether this
is a real effect or indicates a systematic error in the calculation of GPC
radii and/or EFIT flux surfaces. Pedestal heights, and confinement generally,
were not as good as in our best 1996 discharges.  Comparison of different
diagnostics and scalings with conditions will be carried out in the next few
weeks.

Some shots, early in the day, had relatively long delays before H-mode began,
despite 2.2 MW of RF.  There were indications of high gas/neutral loads, which
may or may not be relevant.  These will be examined further for threshold
purposes.

Friday's run was supposed to address control of high-q_parallel dissipative
divertor discharges (MP# 163). This experiment required the establishment of
high performance, steady-state (EDA) H-mode plasmas, into which N2 would be
puffed to detach the divertor. The goal was to determine the threshold amount
of impurity gas required for detachment, and to develop control (feedback)
techniques to maintain the detachment. No useful data in support of this MP
was obtained because of high core radiated power fractions and low H-mode
confinement. SOL powers were typically less than 1.5MW, even with > 2.5MW
of RF. Long duration H-modes (> 0.5sec) were obtained, but these generally had
type III ELMs and low H-factors. This run will be rescheduled later in the
campaign. 

While the machine generally ran well last week, it was apparent that the
radiated power fraction was high compared to our post-boronization 1996
campaign; H-mode behavior was similar to un-boronized cases. Several
differences between the recent boronization procedure and that followed in
1996 have been noted. To investigate the situation, and try to improve the
H-mode performance, a fresh boronization was carried out over this weekend,
employing higher fill pressures as had been used previously (5.5mTorr instead
of 3mTorr). We will continue to monitor radiated power and impurity
concentrations during this week's runs. 

The first get together of the MDSplus porting team took place on December
8-10, 1997 at MIT. The meeting was attended by David Schissel and Jeff
Schachter (GA), Bill Meyer and Jeff Moller (LLNL), Tom Gibney and Phyllis
Roney (PPPL), and Josh Stillerman and Tom Fredian (MIT).  During the meeting
the MIT members described the kernel porting of the MDSplus system including
the various libraries and applications, the file structures and the internal
data structures. Various porting issues were also discussed including the
various locations of source code and documentation, file sharing, and
interprocess communication schemes. Target platforms include four variants of
UN*X, plus WindowsNT/WIN95. The concurrent version system (cvs) will
be used to manage and distribute the mdsplus source code.  A breakdown of the
tasks involved in the port was presented and discussed during the
meeting. Priorities for the given tasks were discussed based on the needs of
targeted experiments: DIIID (GA), NSTX (PPPL), SSPX (LLNL) and CMOD (MIT).

The oil conditioning system for the DNB accelerator supply was tested, put
into regular operation, and is removing water from the oil.  Tests of the
fiber optic transmitters, receivers, and cable resulted in the identification
of upgrades for existing equipment and appropriate hardware for new
construction.  Assembly, wiring, and testing of components continues.

Roger Bengtson, David Winslow, and Keith Carter (all from UT-FRC) visited
this week to continue installation of the Turbulence Probe.

Dr. Gerd Schilling of PPPL is visiting during this campaign as part the PPPL
collaboration.  He is here to particitate in RF operations and experiments.

Stewart Zweben (PPPL) visited C-Mod on Dec. 5 to give a seminar on "Alpha
Particle Physics in TFTR".  He also worked with Earl Marmar on testing the
H-alpha fluctuation diagnostic, and talked with Brian LaBombard about optical
imaging of edge fluctuations on C-Mod, both as part of his collaboration
activity.

Vlad Soukhanovskii (Johns Hopkins) was visiting C-Mod on 12/11 and 12/12 to
evaluate performance of the JHU XUV Divertor Periscope.