Alcator C-Mod Weekly Highlights
                        October 25, 2010

FY2011 weeks of research operations
   Target: 15 weeks
   Completed: 1.35 weeks
   Plasma Shots: 163

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

Plasma operations continued at Alcator C-Mod last week. Four run days were
scheduled, and 2.8 were completed. A total of 85 plasma discharges were
produced with a reliability of 89%. Experiments supported research in the
Lower Hybrid Physics, ICRF Physics, and Transport areas.

The run on Wednesday was curtailed due to a power system control problem
associated with excitation of the MIT alternator, which provides prime AC power
for the C-Mod magnet systems. Trouble-shooting identified a faulty 50V power
supply used in the control circuitry. The supply was replaced and, after
testing, C-Mod operation resumed on Thursday morning.

A fresh boronization was carried out over Thursday night.

Plasma operations are planned to continue this week.


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

Tuesday's run was devoted to MP#463, "Reaching Zero loop Voltage with LHCD".
A long standing goal in the lower hybrid program was attained by achieving
discharges with zero loop voltage, i. e., fully sustained by lower hybrid
current drive, for times in excess of the current diffusion time. Sawtooth
stabilization was observed, and preliminary analysis using EFIT constrained by
MSE measurements indicates that the central safety factor rose to ~ 2 and that
the shear was flat or slightly reversed. The new coupler performed extremely
well in these experiments, delivering a net power to the plasma of up to 1.0
MW . The keys to producing these discharges were operating at 0.5 MA, and in
an upper null configuration with the cryopump, which allowed good control of
the density at a value of n_bar ~ 5e19/ m^3. The normalized current drive
efficiency was n_bar*I*R/P ~ 2.3e19 m-2*MA/MW. This CD efficiency, in plasmas
with peak electron temperature of about 4 keV, is comparable to that assumed
for LHCD in ITER.

Wednesday's run was planned to support MP#619, "Effects of ICRF and LH power
on SOL density profiles". The goals of this experiment, which comprises part
of the research of an MIT graduate student, were to calibrate the new SOL
reflectometer diagnostic, which is collocated with the LH launcher, relative
to other density profile diagnostics, and to investigate the influence of ICRF
and LH on the SOL density in front of the launcher. Minimal data was obtained
before the run had to be suspended in order to trouble-shoot an alternator
excitation control fault. This experiment has been rescheduled.

The run on Thursday was devoted to MP#611, "Momentum Confinement and Lower
Hybrid Source Determination". The goal of this experiment, which comprises
part of the thesis research of an MIT graduate student, was to observe
rotation drive during LH injection with a variety of target plasma parameters,
and perform a current relaxation scan to attempt to identify the source
profile and time scale of the LH momentum drive. The start of this run was
delayed by 2 hours, but substantial progress was made in collecting the
desired dataset. Counter-current rotation was observed in conjunction with LH
current drive in nearly all conditions. A scan of LH phasing was carried out,
and we observed the momentum transport time changing with n_parallel, as had
been reported previously. N_parallel=1.6 was selected for the subsequent scans
of plasma parameters. A density scan was then carried out. At the lowest
target densities, locked modes were observed in ohmic target
plasmas. Application of ICRH or LH appeared to successfully unlock the modes,
even at quite low powers. Time scales of the rotation changes with ICRH and LH
were faster than previously observed, and these data are being analyzed. A
possible difference in the LH-driven rotation between lower and upper null
target plasmas was noted on the only USN shot attempted, at the end of the
day, and will be further investigated in a future experiment.

The run on Friday was dedicated to MP#595a "Analysis of the Radial Impurity
Transport at the Pedestal Region". Thomas Putterich (ASDEX-Upgrade) served as
Session Leader using remote conferencing tools. The purpose of these
experiments was to analyze the edge transport of boron and fluorine for a
variety of plasmas and parameters. The main focus of the day's experiments was
on EDA H-modes, although some L-mode discharges were also obtained. The
experimental technique employed dynamic scans of the plasma position in order
to enhance the spatial resolution of the profile diagnostics, including
DNB-based and gas-puff (NINJA)-based spectroscopic measurements, as well as
Thomson scattering and soft X-ray diagnostics. Laser blow-off (LBO) injection
of CaF2 was applied to provide the extrinsic fluorine impurity, and neon and
argon puffing were also applied. A current scan (600kA < Ip < 1MA) in EDA
H-modes was obtained. These data are being analyzed.

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

The ICRF system was utilized in support of C-Mod operations last week.  We had
difficulty with FMIT#2 due to low power control issues.  The system appears to
operate without demand for <1 ms and appears to be in a faulted state due to
faults from the antenna.  We duplicated the fault scenario into vacuum and
this should allow for a more efficient means to troubleshoot the problem.  The
symptom existed previously, but the new broad band solid state amplifier
responds more quickly and has made this an operational problem.  For FMIT#1
and #2, a problem identified from the previous week's operation at high power
was traced back to a source ready relay.  The screen voltage was dropping
below the level of its prove circuit and causing the source ready relay to
pull out.  With the replacement of the triode based IPA with solid state IPAs,
the final amplifier anode, screen, and grid supplies now limit the transmitter
output power into certain target plasmas.

Preparation for fabrication of the rotated, four strap antenna has been
proceeding.  Stainless steel straps have been cut and radiused to verify the
machine programming.  Inconel material is being prepared for machining of the
current straps, and the back plate material is being rolled in preparation for
manufacturing.

The Fast Ferrite Tuner (FFT) preliminary design review was completed with the
acceptance of the vendor report on the FFT preliminary design, thus meeting
the first milestone requirement (ARRA).  The main issues are shifting the phase
shift to higher frequency to ensure sufficient phase shift at high frequency,
and limiting the circulating power to 5 MW for 5 s.

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

The LH system was utilized for C-Mod runs on Tuesday, Wednesday, and Thursday
this week.  Tuesday's run achieved the highest net LH power to date into
plasma of 1.1 MW (average LH power of 1.0 MW during the 0.5 s pulse),
corresponding to a net power density of 41 MW/m2.  The high LH power, combined
with low density (nebar~5e19 m-3) and medium current (Ip~500 kA), allowed for
full non-inductive sustainment for several current relaxation times.  The LH
system operated smoothly at lower power levels (400-800 kW) during the runs on
Wednesday and Thursday.


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

Last week, Perry Phillips (U Texas) visited MIT to reinstall the FRCECE high
frequency receiver and to install a new data acquisition unit.  The high
frequency receiver had been repaired under warranty by the manufacturer and
returned. Last week, Perry Phillips installed the
repaired receiver unit and also installed a new data acquisition unit
(acquired by the PSFC) with the help of Josh Stillerman. The new data
acquisition unit provides data acquisition at 2 MHz for each of the 32
channels. This high frequency data is available simultaneously with a
down-converted 10 kHz version of the same data.  The latter is intended for
general use where the higher frequency version would be too cumbersome or
display is too time consuming. All 32 channels are now available and
calibrated.

Amanda Hubbard, Chuck Kessel (PPPL), and Randy Wilson (PPPL) attended the
meeting of the Integrated Operational Scenarios ITPA TG in Seoul,
Korea. Amanda made presentations on C-Mod contributions to IOS, on the
progress of IOS JE5.3 (on LHCD at high ne), on Pedestal Issues for ITER
Integrated scenarios (to a joint session) and on C-Mod I-mode results (to the
Transport and Confinement Group).

John Rice attended the ITPA TC group meeting in Seoul, Korea Oct. 18-20.

Jerry Hughes attended the ITPA Pedestal Topical Group meeting from 18--20 Oct
in Seoul, as well as a session of the Transport & Confinement group
meeting. He presented the status of several ITPA joint experiments in which
C-Mod is participating (TC-3, PEP-6, PEP-26, PEP-27, PEP-28) and reviewed the
status and plans for the ITPA H-mode threshold databases.

Bruce Lipschultz attended the ITPA SOL/Div group meeting in Seoul.  Bruce
chaired the committee meetings, which were held on October 18-20. The first
day consisted of joint meetings of the SOL/Div group with the Pedestal group
(on ELMs), with the MHD and EP groups on Disruptions, and with the Pedestal,
IOS and Transport groups on fueling.
_______________________________________________
Cmod_weekly mailing list
Cmod_weekly@lists.psfc.mit.edu
http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly