Alcator C-Mod Weekly Highlights

			Dec. 05, 2005

FY2006 weeks of research operations:
   Planned: 12 weeks (tentative)
   Completed: 3.5 weeks

Operations
----------

Research operations continued at Alcator C-Mod last week. Four run days were
scheduled and completed. A total of 93 plasma discharges were produced with a
startup reliability of 86%. Experiments were carried out in support of
research under the Divertor/Edge, ICRF Physics, Transport, and Operations
topics.

The first full (overnight) boronization of the present campaign was
conducted on Wednesday night.

Plasma operations are scheduled to continue this week.

Physics and Run Details
-----------------------

Tuesday's run was devoted to MP#419 on wall pumping and hydrogen isotope
retention, from the Divertor and Edge studies group. This run concentrated on
1MA lower single null discharges with a range of densities between
0.8<nebar<2e20/m^3 . The effective wall-pumping was evaluated as a function of
density and plasma pulse-length. The level of pumping was also assessed in a
series of nominally identical discharges in order to determine saturation of
the pumping effect. Wall-pumping was observed to decrease over 3-4 full
length shots. The amount of gas puffed to maintain a given density was
observed to decrease with decreasing flattop duration; however, the amount
remaining also decreased, such that the net wall-pumping did not vary much
with pulse length. These data are being analyzed.

The run on Wednesday continued the work on localization and subsequent erosion
of boronization using ohmic H-modes (MP#440). Between-shot boronizations were
carried out using three different resonance locations of the ECDC discharge,
followed by a series of similar ohmic H-mode discharges (Ip~1.2MA,
BT~4.25T). The boronization effects were observed to persist for many shots,
in contrast to previous experiments with ICRF H-modes, in which the effect
typically lasted only one discharge. This result implies that the faster
degradation in the RF case is due to RF-specific processes, such as heating of
edge ions, sheath rectification, or ion orbit losses. There were small
differences in the effectiveness of the boronization, as measured for example
by the initial rate of rise of radiated power after the L-H transition, for
the different resonance locations tested, 0.9, 0.8, and 0.7m, with the last
exhibiting the best performance. However, since the duration of the effect was
not limited to single shots, a cumulative effect may have been responsible for
all or part of this apparent improvement, and further experiments may be
required.

Following an overnight boronization, Thursday's run continued our evaluation
of the operation of the four-strap J-port antenna with the Faraday screen
removed. The main focus of the day's experiments was to compare the
performance of the J-port antenna with that of D- and E-port in
post-boronization H-mode plasmas. Recovery of the J-port antenna from the
boronization was not as rapid as for the other two antennas (which still have
the Faraday screens in place), and sustained operation of J-port above 2MW was
still not possible by the end of the day.  Moreover, J-port antenna appeared
to be limited by neutral pressure at lower pressures, around 0.2mTorr, than in
previous campaigns with the Faraday screen. For the power levels achieved
during this run, the stored energy in H-mode was about 10% lower using J-port
than for the same power from D and E-port.

Friday's run was devoted to two miniproposals, MP#442 "Radial structure and
k-theta variation of the QC mode" and MP#437 "Field reversal experiments and
edge poloidal rotation", both from the Transport research group. The QC mode
experiment employed RF-driven EDA H-mode plasmas at Ip=800kA and BT=5.6T, with
the outer gap adjusted to place the Quasi-coherent mode location in the view
of the gas-puff-imaging (GPI) array. The gap was also swept rapidly across the
GPI view in order to obtain high resolution measurements of the mode
structure, as well as the pedestal profiles. The phase contrast imaging (PCI)
diagnostic was used to obtain k_R measurements of both the QC fundamental and
1st harmonic, at the top and bottom of the plasma. k-theta measurements for
both the fundamental and harmonic at the outboard midplane were successfully
made using the GPI fast diode system. Furthermore, the QC-mode fundamental and
several harmonics were observed on the fast magnetics diagnostics. The dynamic
scan of the outer gap was successful in moving the QC-mode across the radial
GPI array. Analyis of the radial structure is proceeding.

The portion of Friday's run dedicated to MP#437 was designed to establish
baseline results with the recently installed NeSoXs soft x-ray rotation
diagnostic in standard field discharges, for later comparison with reverse
field experiments. These experiments employed higher than normal levels of
neon and argon puffing in order to maximize the signal to noise of the
rotation measurements. The lower single null EDA discharges employed in the
first part of the day's run (without the dynamic gap scan) were also used for
the LSN portion of this experiment. Preliminary analysis suggests near-edge
poloidal rotation of ~40km/sec, with transient excursions of ~10km/sec
associated with L-H and H-L transitions. Upper null discharges were also
employed later in the day.

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

All of the klystron body current monitors have been tested using a test
current pulse of 90mA, and the fault response times tested up to the
Transmitter Protection System (TPS) HV enable comparator input. All channels
are now routed to the corresponding TPS fault circuits and digitizers. All
probe detector diodes and blocks have now been installed on the patch panel
and cables are connected to the CPS and new LH digitizer.  Set up work is
proceeding for the upcoming high power probe and directional coupler tests.

Diagnostic Neutral Beam System
------------------------------

The DNB resumed normal operations last week.  On 01 Dec 2005 the beam pulse
duration was gradually lengthened in order to verify that the new inboard wall
infrared temperature monitor could actually detect beam heating of the tiles.
This monitor has a minimum measurable temperature of ~300 C, and therefore
short 60 ms pulses do not register.  The longer beam pulses were fired into
the empty tokamak between plasma shot cycles.  At a full power beam pulse of
0.25 s duration, the inboard wall temperature just surpassed the 300 C floor.
A beam pulse of 0.30 s was observed to heat the tile surface to 344 C, as
measured by the IR monitor.  Details of the exponential cooldown (0.56 s)
indicate that the beam deposition produces predominantly 2-D surface heating,
not edge or corner hot spots.  Further thermal analysis should yield an upper
limit on the beam diameter.

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

  David Mikkelsen visited MIT all week to discuss the prospects for
experiments to probe the species dependence of transport and
turbulence.  Suitable RF heating scenarios have been identified for
plasmas with hydrogen and helium-4 as the majority species; these can
be run with the same toroidal field strength as H-minority heating in
a D-majority plasma. Plasmas with D and He majority can be run on the
same day, but the changeover to H majority is expected to take more
than a day. Using an upper-single-null configuration should produce
L-mode plasmas with density profiles that enable core reflectometry
with the planned correlation reflectometer (to be built in CY 06).
The use of TAE spectroscopy to constrain monotonic q profiles was
also discussed.


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