Alcator C-Mod Weekly Highlights
             May 7, 2007

FY2007 weeks of research operations:
     tentative target: 15 weeks
     Completed: 2.95 weeks

Operations
----------

Plasma operations continued at Alcator C-Mod last week. Just under two days of
experiments were completed. Time was lost last week in repairing power system
instrumentation and for software modifications required to compensate for the
loss of a magnetic flux loop used for both plasma control and equilibrium
reconstruction.  A total of 39 plasma discharges were produced with a startup
reliability of about 60%. Experiments on Tuesday,Thursday and Friday supported
research in the areas of Lower Hybrid and ICRF Physics, Operations (wall
conditioning), Edge physics, and diagnostic calibration.

Plasma operation is planned to continue this week.

Operation Details
------------------

The primary experiment on Tuesday was devoted to MP#412a, "Improved
hydrogen-to-deuterium changeover using radiative terminations and
disruptions". Intentional vertical displacement (VDE) disruptions were
programmed, with 1 MA target plasmas having stored energies up to 100kJ using
ICRF heating. Two downward-going VDE's released very little gas (~1-2 Torr-L
net release), indicating this region had been largely conditioned. Two
upward-going disruptions released much more gas, ~80 Torr-L net, with a
substantial (~40%) fraction of hydrogen. The start of this run was delayed by
repair of a shunt in the power system bus work, and the run was terminated
early when a flux loop in the upper region of the vacuum vessel developed a
short following one of the disruptions. Because an adjacent flux loop had
previously been damaged, compensation for the loss of this magnetic signal
proved difficult, and satisfactory modifications to the control and analysis
software were not completed until Thursday.

Despite the delays which limited the number of shots during Tuesday's run, the
piggyback experiment MP#488 "Evaluation of the Fast Ferrite Tuning System on
E-Port Antenna" succeeded in demonstrating feedback matching to the plasma
load, with less than 3% power reflection, with net power at the 200kW level.

Following software modifications taking into account the damaged flux loop,
plasma operation resumed on Thursday. About half the run was spent in
re-developing robust plasma startup programming using the modified control
algorithms. The remainder of the run was devoted to continuation of the
evaluation of the fast ferrite tuner (FFT) (MP#488) and to MP#468, "LH-driven
fast electron diffusion time measurement".  The FFT system ran successfully,
maintaining good matching during L->H->L transitions, and with density in the
range 0.6<nebar<1.6e20/m3 and currents in the range 0.5<Ip<0.8MA. With N2 gas
in the transmission line the FFT operated successfully with forward power
<0.5MW (circulating power <1.7MW). At higher power an arc occurred in one of
the FFT units, and the arc protection system promptly shut down the
transmitter, as designed.  Preparations are being made to pressurize the FFT 
with SF6 to increase its power handling capability.

Additional data were also obtained on Thursday in support of MP#468 on fast
electron diffusion during lower hybrid current drive; this work comprises part
of the doctoral reasearch of an MIT graduate student. Data were obtained at
Ip=800kA with 0.8<nebar<1.6e20/m3, and at Ip=500kA, with densities of 1.6 and
1.25e20/m3, using 90 degree phasing. Data were also obtained with 60 degree
phasing at 0.8e20/m3 and 800kA. These results, along with initial analysis
will be reported this week at the RF Conference in Clearwater, FL .

Two experiments were carried out on Friday. The morning session was devoted to
an extension of MP#429, "SOL turbulence in limited plasmas". This work, which
is being led by Stewart Zweben (PPPL), is intended to obtain high quality data
on SOL turbulence which are being compared with turbulence simulations 
obtained using the GEMR code (in collaboration with Bruce Scott, IPP,
Garching). Initial comparisons based on experiments last year have been
reported, and the new experiments are designed to test the predictions of the
simulation for the variation of the turbulence characteristics with magnetic
field and collisionality. Data were obtained at two densities at 5.4T, the
field of the original comparison, and also at fields of 6.75T and 2.75T.

The second experiment on Friday involved use of the cut-off of electron
cyclotron emission (ECE) for density calibration of the Thomson scattering
system (MP#296). High density ohmic discharges at toroidal fields of 4.1, 4.0,
and 3.9T were employed to update the calibration coefficients for the core
Thomson scattering system. The new calibrations have been entered in the
MDSplus model tree.

Time was also required on Friday for repair of power system instrumentation,
and replacement of fuses in one of the thyristor power spplies, reducing the
number of plasma discharges obtained during the afternoon session.

Physics
-------

Analysis by Dennis Whyte of a multi-device edge database found that the
gradient scale length of the separatrix electron temperature scaled linearly
with device major radius, R, and very weakly with other parameters including
total power P into the SOL. This indicates the separatrix resides at a
critical gradient, and as a result the upstream parallel exhaust power width
does not need to narrow as power increases.  As a result, the upstream power
exhaust requirement scales as P/R^2 while separatrix temperature scales as
(P/R)^(2/7). The scaling shows that exhibit upstream (midplane) power density
is ~ 1-3 GW/m2 in Alcator C-Mod, which is commensurate with power exhaust not
only in ITER, but also with ARIES-AT. Since this is not directly reflected in
divertor target heat loads, better understanding of the divertor response to
such upstream power power densities is required.

Collaborations
---------------

Last Tuesday and Thursday, Amanda Hubbard participated remotely in experiments
at MAST carried out in support of ITPA/IEA Joint Experiment PEP-16 in small
ELM regimes. The MAST experiments were led by Hendrik Meyer (UKAEA), with
assistance from Hubbard and Rajesh Maingi (NSTX). As was found on NSTX, H-mode
access was much easier in near-balanced DN discharges.  A power scan found
small ELMs at low power; their properties and type are being assessed.


ICRF Systems
-------------

As reported above, we successfully began plasma operation with the fast
ferrite tuner (FFT) matching network on the E antenna.  The system worked well
matching over the range the system was designed for with <1% reflected power
(<5% during fast transients).  To be able to match in real time will
significantly improve the discharge utilization because ICRF heating is not
lost due to antenna mismatch upon changes in the discharge target.

Both D- and J-port antenna systems were operated during plasma experiments
last week.

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

The fault condition reported last week as a tube arc was in fact determined to
be due to breakdown in external high voltage insulation. The damaged
insulation was repaired and the klystron successfully returned to service.

The lower hybrid system was operated into C-Mod plasmas in support of LH
physics experiments on Thursday.

Arcing in an external waveguide (6 bottom) resulted in damage to a gas
baffle. Repairs are underway. We are working on development of additional arc
detection systems to prevent damage to these baffles, and expect to begin
tests this week.

Long Pulse Diagnostic Neutral Beam System
------------------------------------------

The DNB was reliably operated into C-Mod plasmas during last week's
operations, in support of experiments.

The CXRS diagnostic was recalibrated during the weekend by Igor Bespamyatnov
(UTFRC).  Recalibration included refocusing the detector, and re-measuring the
dispersion and the instrument functions. This resulted in improved Ti profiles
and B VI profiles from CXRS measurements obtained during Thursday's
run. B V profiles from analysis of ambient profiles are also improved.


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

Rob Goldston and Jon Menard visited MIT on Monday (April 30). They had
detailed discussions with members of the C-Mod edge physics team, including
Bruce Lipschultz, Dennis Whyte, Brian LaBombard, and Jim Terry. They also gave
a PSFC seminar, entitled "The Path to Magnetic Fusion Energy: Crossing the
Next Frontier", which described the National High-power advanced-Torus
eXperiment (NHTX), a next-step tokamak concept.

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