Alcator C-Mod Weekly Highlights
                        September 7, 2010

FY2010 weeks of research operations
   Base Target: 13 weeks
   Base Completed: 15.3 weeks
   ARRA Target: 5 weeks
   ARRA Completed: 5.0 weeks
   Plasma Shots: 2521

Operations
-----------

Plasma operations continued at Alcator C-Mod last week. Four run days were
scheduled and completed. A total of 98 plasma discharges were produced with a
startup reliability of 84%. Runs supported research in the Lower Hybrid
Physics, ITER H-mode Scenarios, and Transport Physics area.

A fresh boronization was carried out on Thursday night.

Plasma operations are planned to continue this week.

Operations Details
------------------

The run on Tuesday was dedicated to MP#468 "LH-driven fast electron diffusion
time measurement", which comprises part of the thesis research of an MIT
graduate student. The goal for this day's experiment was to complete the
dataset at high relative phasing (n_parallel~3.1) with Ip=1MA,
nebar~8e19/m^3. The desired target conditions were established using constant
LH power, and modulation of the LH was successfully introduced.  These plasmas
proved to be susceptible to locked modes during the LH pulse, often resulting
in disruptions.  Attempts to prevent the locked modes by minor changes in the
target plasma or reductions to the LH power were unsuccessful.  The presence
of the locked mode significantly modified the plasma response, so data taken
in during periods with locked modes must be excluded from the primary
dataset. The remaining data are being analyzed.

Wednesday's run, devoted to MP#612 "Expanded exploration of LHCD Density
Limit", focused on the role of the plasma-inner wall gap in the LHCD density
limit on C-Mod.  Previous experiments showed a significant increase in
nonthermal electrons for inner wall limited discharges as compared to diverted
discharges.  Scans of the inner gap on Wednesday showed that nonthermal
electron cyclotron emission increased substantially as the plasma-inner wall
gap dropped below ~4 mm.

Thursday's run was dedicated to MP#577, "ITER-like Discharge Rampup
Studies". Scans of the plasma density during the current ramp phase were
carried out for ohmic, ICRF (L-mode), and LHCD assisted ramp-up
scenarios. The plasma density was in the range of 4e19<nebar<2e20, with at least
three density points obtained for each condition. Temperature profile data
show a direct (inverse) correlation with target density. This experiment
marked the first use of LHCD in ITER-like current ramp experiments in
C-Mod. The data are being analyzed, and will be compared with TSC simulations
including predictions of internal incuctance and volt-second consumption.

On Friday we carried out experiments in support of MP#565 "Power requirements
for high confinement H-modes and the role of radiated power spatial
distribution", proposed by Alberto Loarte of ITER. Impurity seeding with neon
and nitrogen was used to vary the net power conducted through the pedestal and
through the last closed flux surface. The target plasmas for this experiment
were 800kA, LSN EDA H-mode discharges with between 3 and 4.5MW of ICRF
heating. The L-H transition power threshold was measured with active neon
seeding to be Ploss~1.5MW, which agrees within 20% with the ITER scaling
law. The best performance was obtained with substantial seeding (either neon
or N2); decreasing the seeding resulted in increased Mo accumulation during
ICRF heating. Therefore, most of our data were obtained with fairly high
radiated power fractions. As in prior experiments, we were able to
demonstrate H98~1 operation with very small heat loads to the outer
divertor. Key diagnostic enhancements included improved coverage of kinetic
profiles with Thomson scattering, IR camera data on the outer divertor, and
divertor bolometry. The data are being analyzed, and are expected to be
reported in several upcoming presentations.

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

The ICRF system performed well in support of experiments on Thursday and
Friday last week.

Video images suggest that a significant fraction of metallic impurity
injections during high power ICRF heating originate in the antenna structure
itself. Spectroscopic views have been re-oriented to monitor injections from
the D- and E-port antennas. We also have an additional spectroscopic view of
J-port antenna to identify the specific metals associated with the injections
observed in the video images.

Several stainless steel test pieces for the new rotated ICRF antenna have been
produced to verify proper control and to allow analysis of the surface finish.

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

The lower hybrid system was successfully used in support of C-Mod experiments
on Tuesday, Wednesday and Thursday.

The LH-SOL reflectometer has produced its first density profiles in
differential mode.  There appears to be a region in the radial profile where a
relatively flat density profile is distorted due to density fluctuations.  The
full phase mode was tested and it was determined that a new delay line is
required.

DNB Systems
-----------

The DNB successfully operated as requested during all four run days last
week.

Following completion of plasma operation on Wednesday, a series of
beam-into-gas shots were taken to check the spatial calibration of the CXRS
and MSE diagnostics. The DNB was fired into the C-Mod vacuum vessel which was
backfilled with deuterium to an approximate pressure of 2 mTorr. The beam
emission spectra were acquired for selected view-chords of the CXRS and MSE
diagnostics with a high throughput, moderately high resolution
spectrometer. The Doppler shifted beam emission will be used with measurement
of the beam energy to determine the angle between the view chord and the
beam. This will be compared with the in-vessel spatial calibrations made
before the campaign began.

The effect of polarized background light on the accuracy of MSE pitch-angle
measurements has been analyzed by Steve Scott. Polarized background light is
observed on C-Mod due to reflections from metallic surfaces in the field of
view of the MSE optics, including ICRF antennas. While the signal to noise
ratio (SNR) with respect to the polarized background is ~20 in low density
L-mode plasmas, the SNR is of order 1.5 in medium density H-mode plasmas. The
resulting uncertainty in the field pitch angle can be expressed as
delta_theta(degrees) = 71.5 (f/SNR)*sin(theta_signal-theta_background) where f
is the fractional uncertainty in the background, typically ~0.05 in C-Mod. The
possibility of using a simultaneous measurement of the polarized light at
nearby wavelengths to improve the accuracy of background subtraction (reducing
f) is under investigation.


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

G. Wallace, A. Hubbard, and P. Bonoli hosted a visit by K. Indireshkumar
(PPPL) on Thursday.  The purpose of this visit was to facilitate the
integration of the GENRAY and CQL3D codes into TRANSP with the ultimate goal
of providing time-dependent simulations of LHCD with full ray tracing and
Fokker-Planck capabilities.  J.R. Wilson (PPPL) also visited on Wednesday and
Thursday to assist with the C-Mod runs.

Chuck Kessel (PPPL) visited MIT on Thursday and served as Session Leader for
the experiment dedicated to MP#577.

Publications
------------

Papers authored by two C-Mod physicists have been selected to the short list
for the 2010 Nuclear Fusion Journal award. "Plasma-surface interaction,
scrape-off layer and divertor physics: Implications for ITER", by B Lipschultz
et al. and "Inter-machine comparison of intrinsic toroidal rotation in
tokamaks" by J E Rice et al. were among the 11 short-listed papers
selected. Nominations are based on citation record and recommendation by the
Board of Editors. The recipient of the 2010 Nuclear Fusion Award will be
announced at the IAEA Fusion Energy Conference in Daejon, Korea, on 16 October
2010.
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