Alcator C-Mod Weekly Highlights

Research operations continued at Alcator C-Mod last week. Experiments in RF
Physics and Transport were carried out over three and a half run days.  Last
week's runs concluded the experimental campaign that began in the fall of
2003.

This week we are beginning preparations for a major vent, during which the new
Lower Hybrid Current Drive system will be installed.

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

The three and a half run days scheduled and completed last week brought the
total number of research days completed in FY04 to 75.5 days (18.9 weeks). A
total of 111 plasma discharges were produced with a startup reliability over
97%. A fresh boronization was carried out over Wednesday night.

The experiments scheduled for last week were those requiring ICRF at a
frequency of 50 MHz, including studies of RF Heating and Mode Conversion
physics, Internal Transport Barrier formation, H-mode threshold studies and
pedestal parameter scaling.

Runs on Tuesday and Wednesday were devoted to evaluation of the performance of
the ICRF system at 50 MHz, following installation of the new DC breaks, and to
studies of minority heating and mode conversion at this frequency. The new
breaks were found to be more effective at reducing RF leakage and pickup than
the original commercial breaks which they replaced, although some interference
with diagnostic signals remains. Difficulty in coupling power to the antenna
in vacuum and plasma at this frequency was found to be unrelated to the
leakage issue. Operation in vacuum remained problematic, but coupling to
plasma was obtained using FMIT#4 on Tuesday, and, after some retuning, using
FMIT#3. There appear to be problems with parasitic oscillations at this
frequency which prevented simultaneous operation of the two transmitters, so
the 50 MHz physics experiments last week were carried out using a single
transmitter at power up to 1.4 MW.  Additional testing to resolve the
oscillation issue is planned.

About one hour on Tuesday was also devoted to beam-into-gas calibration shots
for the MSE diagnostic. The main purpose of these tests, in which the gas
pressure was varied over about a factor of 8, was to determine whether
vertical fields in the beam duct were causing deflection of re-ionized beam
particles which subsequently re-neutralize and enter the vessel on different
trajectories, contributing to calibration errors. This effect was found not to
result in measurable errors under the conditions employed.

Scans of field and He3 concentration were carried out on Wednesday to study
the minority and mode conversion (MC) regimes. Clear signatures of mode
converted waves were observed on the PCI diagnostic, along with localized
electron heating observed on the ECE diagnostics. The location of the mode
conversion signals are consistent with theoretical expectations and the data
are under detailed analysis.

The run on Thursday was devoted to MP#370, "Large Volume ITB's at 2.8T". The
goal of this experiment was to extend the previous scalings of the location of
the internal transport barriers produced during off-axis ICRF heating with
toroidal field, using H-minority heating at 50 MHz with the resonance placed
on the high-field side. The run was successful in obtaining ITB's under these
conditions, and density profiles were obtained using Thomson scattering and
visible bremmstrahlung diagnostics. Analysis of these data is underway.

Two experiments were carried out on Friday, in support of MP#385 "Edge
rotation and the L/H threshold - dependence on known scaling parameters" and
MP#396 "Edge Pedestal Scaling at lower B". Both experiments made use of the 50
MHz ICRF from J-port for minority heating in the range 2.8<B<4 tesla, and 80
MHz from D and E-port antenna for comparison discharges B~5.4 T. MP#385
concentrated on measurements of the L-H power threshold and core rotation in
lower null and near-balanced DN discharges. As at higher fields, the threshold
power was found to be a strong function of SSEP, with a reproducible
sensitivity to changes in the magnetic balance of the order of 1mm. For
MP#396, H-modes were produced over a wide range of current and field,
(0.5<Ip<1.2 MA, 2.8<Bt<5.3 T); the range of safety factor was
2.9<q95<7. Pedestal data, principally from the edge Thomson scattering system,
are being analyzed. This experiment contributes to the thesis research of an
MIT graduate student.

ICRF System
-----------

The J antenna loops were converted to the 50 MHz configuration with the
commercial breaks replaced by straight coax and recently manufactured DC
breaks based upon a PPPL design.  The breaks have ~20 nF of capacitance (4
times that of the other breaks).  These breaks were tested in-situ during some
low power vacuum operation and were found to leak but at an order of magnitude
less than the previous breaks.  Although vacuum operation was unsuccessful,
plasma operation was commenced to complete the testing.  We were able to
couple 0.9 MW and 1.4 MW from FMIT#3 and #4 respectively and had limited
success operating simultaneously.  We appear to have a parasitic oscillation
that prevents robust operation.  Our current plan is to see if we can
reproduce the oscillation into our fixed load by varying the load reactance.
If successful, we will investigate vacuum operation as well.


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

An 11-window braze test using titanium waveguide and copper plated
alumina windows has been performed by our vendor.  The resulting
prototype is leak tight and displayed excellent braze properties.  A
second test using copper plated waveguide and copper plated alumina
windows will be performed early this week.  These tests will determine
if different wetting properties of the braze on titanium or copper can
be exploited to produce a better braze.  We expect a choice between
these two processes for the production of lower hybrid couplers to be
made as soon as the test results have been evaluated.

The fwg/rwg launcher assembly was disassembled this week to look for
the source of faults seen during high power tests last week.  No
tracking or other indications were seen around the primary rf gaskets
joining the rwg to the fwg.  However, measurements using a special
plastic material in place of the gaskets allowed an accurate
determination of the gasket compression to be made.  These
measurements indicated less than optimal compression of the gaskets,
which may have contributed to the faults seen earlier.  In addition,
some tracking over an rf gasket at the input to the rwg was
discovered.  This gasket, which may have been the cause of another set
of faults, has been replaced, and this rwg channel tested to full power
for 0.5 s.  We now plan to reassemble the fwg/rwg this week, with
better compression of the gaskets, and resume high power testing of the
assembly.

We continued work on the amplitude and phase control system.  The
vector modulator calibration is proceeding well after discussions with
the vendor indicated errors in some of the documentation.  The serial
fiber optic link (SFOL) boards are now working extremely well and
fabrication of the production boards will begin soon.

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

Gerd Schilling was at MIT 5/4-7 to participate in ICRF experiments involving
initial operation of the 4-strap antenna at 50 MHz.

In addition to the participants listed in last week's highlights, Stewart
Zweben (PPPL) attended the Transport Task Force meeting in Salt Lake City
and presented a paper based on turbulence imaging results from C-Mod and NSTX.