Alcator C-Mod Weekly Highlights
			December 26, 2007

FY2008 weeks of research operations:
     Target: TBD
     Completed:  1.06 weeks

Operations
----------

Plasma startup and conditioning operations continued at Alcator C-Mod last
week. Four run days were scheduled and completed.  A total of 91 plasma
discharges were produced with a startup reliability of 75%.  In addition to
continuation of ICRF conditioning and machine cleanup, the runs supported
three research miniproposals, from the Lower Hybrid Physics, Boundary Physics,
and MHD groups.

This week will be a maintenance week. Plasma operations are planned to resume
after the first of the year.

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

The run on Tuesday concentrated on continuing conditioning of the ICRF system
and evaluation of the fast ferrite tuner matching system. The 6-inch coax
lines feeding the J-port antenna were purged with dry nitrogen to reduce the
relative humidity to <2%, which improved overall system performance. However,
we have not yet obtained long (>1sec) pulses through the J-antenna without
trips. The E antenna and the FFT system were tested into H-mode and we have
identified a clear voltage limit of ~33-34 kV, which corresponds to ~1.25 MW into
an ELM free H-mode (lowest edge density).  We intend to investigate installing
some prematching in the transmission line network to reduce the maximum
voltage in the FFT's for a given net power.

The majority of the run on Wednesday was devoted to testing of the recently
installed gas capillaries beside the lower hybrid launcher (MP#518), which are
intended to provide control over the local density in front of the waveguides
to improve coupling of the lower hybrid waves. Operation with lower single
null equilibria at up to 700kW at 5.6psi plenum fill (D2) was successful,
without any arcing in the waveguides on the vacuum side. This is one of the
key points of the MP since there was some experience at JET that placing a gas
capillary close to the LH antenna would cause arcing in the waveguides due to
the ionization of neutral gas.  The reduction in reflections was quite
pronounced at low power levels with power reflection coefficients falling from
 >35% to <25% for less than 300kW net LH power. As the power level increased,
the reduction in reflections seemed to lessen, although the noise level in the
reflections did still noticably decrease. Arcing in the waveguide was observed
for operation into upper null equilibria with the cryopump active, for LH
power above 500kW. In the USN configuration, the effect of the gas puffing
seemed to reduce reflections on the top of the grill while causing them to
rise of the bottom of the grill. Future experiments will further explore the
operational range, particularly to investigate the effect of increasing the
outer gap and pulling the launcher back further behind the limiter

The experiments on Thursday morning and on Friday were devoted to MP#419,
"Fiducial wall pumping measurements for comparing operating conditions".  On
Thursday wall retention (pumping) was measured during a density scan from
1<nebar<2e20/m^3, in 0.8MA upper null discharges, with and without the
cryopump active. Wall retention normalized to the amount of injected gas was
observed to initially increase with plasma density and then level off or
decrease at the highest density. Langmuir probe measurements indicated that at
the lowest density in the scan the inner (upper) divertor was already in the
high recycling regime, while the outer divertor probes indicated much lower
density. At higher values of nebar both inner and outer divertor densities
increased, with the inner leg detaching around nebar~1.6e20/m^3 and the outer
divertor clearly losing pressure relative to the SOL profiles at the highest
density. The run on Friday concentrated on lower single null equilibria, with
the goal of obtaining a density scan up through the detachment threshold. Wall
retention measurements were obtained as well as scanning probe measurements of
the SOL characteristics at three locations, for understanding the divertor and
SOL profiles. In addition, we obtained detailed measurements of the poloidal
phase velocity of fluctuations in the SOL.  The amount of retention
was a roughly constant function of the divertor fluence up until detachment;
roughly 1-2% of the divertor fluence was retained. At the highest densities
when the outer divertor fluence measure stopped increasing the retention kept
increasing as the density increased. This is likely due to ion fluence to
surfaces beyond the spatial limit of the divertor probes (the outermost
divertor probe which maps to the shadow of the main limiter had densities of
1.5e20 m-3).

The second experiment on Thursday was MP#450, "Gas jet disruption mitigation
with Ar/He gas mixtures". The primary purpose of this half-day experiment was
to determine the minimum amount of gas which would produce effective
mitigation using the previously identified optimum mixture of
85%helium+15%argon.  Mitigation effectiveness was measured by the amount of
halo current reduction, and by the radiated energy fraction. The pressure in
the plenum was varied to provide three different gas puffs, from the standard
270 Pa-m^3 (6.5e22 atoms) to about half that amount. Multiple shots were taken
at each level to check reproducibility. All three gas quantities resulted in
mitigation of halo currents, although the mitigation was not as good at the
lowest injection amount. The radiated energy fractions showed no clear
dependence on the amount of injected gas. A second experiment was also carried
out to test the idea of using the lower hybrid system to provide a seed of
fast electrons for studies of runaway effects during mitigated
disruptions. The data from these shots is being analyzed, but there was no
clear indication that the gas jet-induced disruptions resulted in
amplification of the runaway tail.

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

The lower hybrid system was operated into plasma during runs last week for
conditioning and in support of miniproposals. Reliable operation at up to
135kW per transmitter was obtained.


Diagnostics
-----------

Diagnostics continue to be brought back online in preparation for the 2008
campaign. The Compact Neutral Particle Analyzer (CNPA) is now operational.
The core Thomson scattering system began acquiring data using new digitizers;
a full calibration of the system will be carried out. The 50, 60, 75, and
88GHz reflectometer channels are connected and being tested. The soft xray
tomography arrays are acquiring data using newly installed cPCI digitizers.



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