From: irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
September 7, 2001


The partial disassembly of Alcator C-Mod required for inspection of the
TF joints, OH coaxes, coax extensions, and bus connections is nearly complete.
Work continues on ICRF, LH, diagnostic, and inner divertor systems.


Physics
-------

Milestones 76 and 77 have been completed on schedule. A description of
each milestone, and the work completed is discussed below.

**** 76. Investigation of ITB control with multiple frequency ICRF.
(Baseline target AUG 2001)

Plasmas with Internal Transport Barriers (ITBs) show promise as the
goal for future advanced tokamak steady state reactor operation. Plasmas
with dual edge and internal, energy and particle barriers have been formed
in Alcator C-Mod with auxiliary radio wave heating, in the absence of
the usual neutral beam particle and momentum sources (which will be
unavailable in future reactors). The prescription for achieving these
ITBs is to lower the magnetic field, which causes the radio waves (at 80 MHz)
to be concentrated in the inner portion of the plasma, off-axis. Further
heating power at 70 MHz will be concentrated at the core of the plasma to
increase the temperature of plasma center, inside of the ITB, to hold the
density profile steady and to arrest impurity accumulation. The fundamental
role of plasma rotation in the ITB formation will also be investigated.

Milestone 76 was completed in August, 2001.

The ITBs, formed in conjunction with a reversal of the co-current central
toroidal rotation velocity, are apparent for both particles and energy, and are
confirmed by a dramatic reduction in the inferred core thermal diffusivity from
TRANSP modelling. Among the unique features of the C-Mod ITBs are the presence
of sawtooth oscillations, the monotonic q profile (with the ITB foot location
near q=1.5) and that they form in the absence of external momentum input. For
ITBs formed with single frequency ICRF waves, the particle and impurity
densities continue to increase at the plasma axis until the central radiation
exceeds the central input power (which is lower with the off-axis heating used
to create the ITB) and the barrier collapses. Additional central heating power
from a second frequency ICRF antenna has been found to arrest the particle and
impurity build-up at the same time as increasing the core temperature, and
double transport barrier plasmas have been held in steady state for as long as
six energy confinement times. Simultaneously with the arrest of the core
density and impurity buildup, the toroidal rotation reappears in the co-current
direction. Steady state double barrier plasmas have been achieved both at 4.5 T
(with 80 MHz ICRF on the high field side to form the ITB and 70 MHz ICRF for
the core heating) and 5.4 T (with 70 MHz ICRF on the low field side to form the
barrier and 80 MHz ICRF for the core heating). This final result shows that the
triggering of the barrier formation is not explicitly related to high-field
side ICRF heating, but rather the transition can be triggered by strong
off-axis heating on either the high- or low-field side. This, in turn, rules
out the primacy of icrf-induced ion orbit effects which formed the basis of
some of the theories proposed to explain the phenomenon. Further details of
these results will be elucidated by Steve Wukitch in an invited talk at the
upcoming APS-DPP meeting, and in its accompanying manuscript.

**** 77. Evaluate operation of the modified J-Port 4 strap antenna.
(Baseline target AUG 2001)

Operate the modified 4-strap antenna at 78 MHz with improved arc detection and
additional diagnostics up to the maximum power that can reasonably be achieved.
Using heating phase, evaluate the heating efficiency, power handling,
reliability, and impurity generation of the 4-strap antenna.

Milestone 77 was completed in August, 2001.

The operation of the modified J-Port antenna (see description below) was quite
successful in the past campaign, concluded at the beginning of August, 2001.
The modified strip lines were in excellent condition (no indication of arcing
on the strip lines or vacuum vessel) after the campaign. Furthermore, arc
damage was predicted to be located at the ground bridge between the bridge and
the strap because E||B and estimated to be ~15 kV/cm when the maximum voltage
on the transmission line was 25 kV @78 MHz (~50% higher than during the last
campaign) and 30 kV @70 MHz (consistent with expected frequency scaling). Arc
damage was found at this location particularly on straps #2 and #3. The
modified J-Port antenna was operated at 70 and 78 MHz and at power levels up to
3.0 MW without significant RF-plasma edge interaction at the antenna corners in
H-mode plasmas. From camera data, damage was expected on the BN tile fasteners
and this damage was found to be generic to all antennas where a BN-metal
interface was exposed to the plasma. This result suggests further
modifications to be implemented before the FY2002 run campaign, based upon
the empirical observations of limiting the E||B field to <15 kV/cm and
removing the plasma facing BN-metal interfaces.

The overall heating efficiency of the J-Port antenna is similar to that of the
D and E antennas. A phase scan showed that the nominal [0,pi,0,pi] was the
most effective heating phase and had little or no negative edge interaction.
An outer gap scan was also completed and suggested a gap of 1-1.5 cm was
better than larger outer gaps. Antenna performance was insensitive to
toroidal field from 5-5.6T.

The modified J-Port antenna differed from the previous version in the design of
the strip line components, front tiles, and back plate. Due to arc damage
found after the previous (winter 2000) campaign, the radial strip lines
elements were aligned with the magnetic field to eliminate as much as possible
E parallel to B arc paths. In addition, the electrode spacing was increased
from 1 cm to 1.5 cm. S-parameter measurements of this new configuration
indicated that the antenna was not significantly modified with respect to RF
electrical characteristics. These measurements confirm our attempt to
maintain a 50 Ohm transmission line while eliminating E parallel to B-field
arc paths. During the winter 2000 campaign, a strong RF plasma edge
interaction limited the injected power to ~2.5 MW into H-mode and the
interaction appeared to follow field lines. The BN tiles were aligned and
all metal surfaces except the Faraday screen were covered or removed.
Measurement of the antenna tile position on all antennas confirmed that the
J-Port antenna is at the same radial location as the D and E-Port
antennas. In order to interrupt long field lines across the antenna, an
insulating septum was installed. The back plane feedthrus were also modified
to reduce the E-field parallel to the B-field. In addition, four optical arc
monitors, six B-dot probes, and an MKS pressure gauge were installed. These
modifications have all contributed to the successful operation and
understanding of this antenna.

The optical arc monitor signals had recorded large transient signals that
correlated with reflected-to-forward power arc detection during some vacuum
conditioning shots. In plasmas operation, the optical arc monitor signals did
not correlate with most arc detect faults. This result suggests that faults
were occurring outside their field-of-view. In addition, the optical
monitors detected signal when the D and E-Port antennas were active.
Comparisons with the voltage data indicate the induced voltage from D and
E-Port antennas are low when the light signal is detected and high when no
light is observed. This result suggests that the low induced voltage is
sufficient to initiate multipactoring. This low power multipactoring does
not impact plasma operation, nor does it affect the subsequent operation of
the J-Port antenna.

The new B-dot probes indicated that the current in strap 4 was equal in both
the top and bottom half of the antenna strap. They also proved to be the most
sensitive to arcs in the antenna.

The new 200 kHz and 1 MHz digitizers allowed direct monitoring of the arc
protection system. During a particular experimental day, the fast data
indicated that some arcs survived for 30-80 micro-sec. Up to 100 J could be
available to dissipate in these arcs; therefore, we reduced the
reflected-to-forward power ratio necessary to generate a trip by 25% for all
future experiments. This change successfully limited the arcs to ~15 micro-sec
or ~15 J per MW injected.


Operations
----------

On Thursday of this week the 65,000 lb top dome that together with the
cylinder retains the TF magnets in position during a discharge, was removed
from C-Mod. On Friday, the 44,000 lb cylinder was removed, approximately
one week ahead of schedule. A picture of this operation may be seen at

http://www.psfc.mit.edu/cmod/operations/EngImages/Inspection_2001-2002/Disassembly_Assembly/P0002038.JPG

The TF finger joints are now exposed, and we will begin a careful inspection
and measurement process early next week.


Lower Hybrid MIE Project
------------------------

Work continues on the TPS PC board with a large portion of the work now
complete. The TPS front panel wiring design has been completed and released
for construction. A vendor has been selected and the PO released for the
waveguide switches that will allow the klystrons to be switched between the
launcher and the dummy load. The front panels for the transmitter equipment
racks have been received and installed. The HV junction box design is
nearing completion.

Work continues on the PLC programs needed to control the HVPS. The
supply will be shipped in the next few days and is expected in Cambridge
early next month. The fabrication of the dummy load needed to test the
supply continues in our shop.

The contractor continues to run the klystron cooling water piping. The run
between the power room and cell is now being fabricated.


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

We continue to refine the antenna simulation and use it to model changes we
plan to make to the J-Port antenna to increase its voltage handling
performance. We are also continuing to develop models of the transmitter
transmission and coupler systems.

On Friday of this week a meeting was held to discuss modifications to the
the boron nitride tiles required to suppress plasma interaction with the
tile support hardware. We expect to finalize and review these changes by
the end of this month.

Work continues on the new phase demodulators. Results from the prototype
indicates a very good linear response of the demodulator except within a
few degree of zero relative phase. We expect a faster ECL XOR chip will
improve the response, at which point we will be ready to go into
production of the demodulator boards. Work continued on documentation and
verification of all fault system electronics including fault indicator,
phase balance detector, and voltage limit detector boards and communication
links.


Inner Divertor
--------------

All 12 inconel rear girdle plates are now inhouse and along with the
c-plates are being prepared for invessel fitup. We hope to have all
inner divertor inconel components installed invessel next week for this
pre-installation fitup. Work has also continued on layout and fitups of
the divertor probes and preparation of the inner wall magnetics for
the divertor installation.


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

The high resolution ECE system has been moved back to UT for repairs
and upgrading. Two of the IF amplifiers are being tested before being
returned for repairs. All the other IF amplifier are also being
tested to check their gain. Work is starting on a temperature control
system and rf shielding is being improved.

The source of BES background light has been identified. It has
been verified via simulation that a small set of impurity lines will
produce the observed interference. The DNB penetration and diagnostic
signals (CXRS, MSE, BES) expected from several sets of beam parameters
were generated as a basis for improving the diagnostics for the next
run campaign.