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 21, 2001



The C-Mod inspection continued this week with detailed measurements of
TF joint resistances and the removal and documentation of spring plates.
Work continues on ICRF, Lower Hybrid, and divertor systems.


Physics
-------

Transient transport analysis of sawtooth perturbations has been
begun for ITB discharges. The analysis is based upon a time-to-peak
technique described in Fredrickson et al, NF 26 (1986). The primary data
is soft x-ray data since the ECE data is often cutoff. The radial profile
suggests a discontinuity in the time-to-peak data when an ITB is present.
The analysis also suggests that the barrier is initially near the plasma
edge and moves into the core stopping near r/a~0.5. Further analysis
and modelling is under way. The fluctuation data measured by the phase
contrast imaging diagnostic shows no significant change in the density
fluctuation level from 2 to 500 kHz. Since the PCI is a chordal
measurement, we conclude that the fluctuations are not suppressed over a
large cross section of the plasma during the ITB.

The TORIC code for the Ion Cyclotron Range of Frequencies has been used to
explore the interesting 2-dimensional structure of the mode conversion
region in H-D-He3 plasmas, where the launched fast wave converts to an
ion Bernstein wave near the center of the plasma. Results from the code can
be compared to the Phase Contrast Imaging (PCI) data of that region that
was obtained in last year's run campaign. High resolution TORIC
simulations using 161 poloidal modes in the Fourier expansion of the
RF electric field are now routinely run on the C-Mod cluster (using an
alpha workstation, run-time is 36 hours). Many different toroidal mode
numbers have been run, simulating the effect of the full antenna
spectrum in C-Mod. Each individual mode is asymmetric about the
midplane, and for toroidal mode numbers below ~15, the positive and
negative modes are not mirror images. The sources of asymmetry
include: the presence of Landau damping, which destroys the cold
plasma symmetry, a shift in the poloidal mode spectrum
due to evanescent gap at the edge, the size of which varies for
different poloidal mode numbers, and the presence of a poloidal field,
which causes the direction of the B field relative the gradient of
B to be different above and below the midplane.

In order to compare the TORIC results to the PCI data (which measures
density fluctuations), the RF fluctuating density must be calculated
using all three electric field components from TORIC. The most
important result from this calculation has been to realize the
dominance of the parallel electric field contribution to the
fluctuating density near the mode conversion region. Normally
these waves are studied considering only E+ and E-, the left
and right-hand circularly polarized components, which are much
larger than the parallel electric field. For this reason the
PCI results were puzzling at first, because they did not seem
to match the E+ and E- field pattern. However, by including
the effect of the parallel electric field, the TORIC results are beginning
to match the experimental measurements, and are pointing the way
to what should be done in future experiments to understand
the mode conversion process better.


Inspection
----------

TF magnet joint resistance measurements were completed this week with all
readings well within the acceptable range. Having completed these measurements,
the removal of the magnet spring plates began. The spring plates provide
the pressure required to maintain contact between the finger joints and the
feltmetal pads. A new hydraulic tool was used to extract the spring plates,
and record the force required as a function of plate insertion depth.
These measurements will allow us to assess the uniformity and magnitude of
spring plate pressure. Fixturing is now being setup to remove the TF arms
and legs. A direct inspection of the feltmetal pads will then begin.


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

Work continues on the LH water cooling system. Piping (6" and 8") has
been fabricated and installed from the basement into the power room. Holes
for the piping have been cored from the power room into the cell. Construction
of the concrete pad needed to support the HVPS has been completed by our
contractor. Delivery of the HVPS is still scheduled for early October.
Plans are being confirmed for transport of the supply from New York harbor
to Cambridge, and for a crane to install it on the pad.

Work continues on the TPS, optical transmitters and receivers, phase
and amplitude control system, and PLC software and hardware development.
Work is nearing completion on re-wiring the klystron filament power
supply controller. Work is proceeding on wiring the three transmitter
control and supply cabinets. Work is also proceeding on the wiring of the
three transmitter EMI/RFI control and protection cabinets. The HV junction
box for the HVPS has been mounted, including the interface box. The HV
junction box optical interface and relay control board was designed and sent
out for PCB fabrication.


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

Antenna modelling of modifications to the J-Port antenna suggests
a more aggressive approach is required to reduce the electric
field at the ground bridge.

Driver grid and screen spark gaps were installed and set in FMIT#1-#3
(already installed on FMIT#4). The spark gaps protect the tubes and
prevent the grid and screen capacitors from exceeding their design
voltage during a tube arc. FMIT#3 door solenoid was also repaired.

Work on the rf control and demodulator systems continues.

Our vacuum test stand has been moved into the rf lab where access to
rf power at the 100 kW level is available. Rf breakdown tests will
be performed on critical components using this facility.


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

We continue the invessel fitup and associated measurements. Results from
the fitup will be used to specify coating thicknesses for the C-plates.
We expect the fitup to be completed in the next few days, after which
installation of the new studs can begin. A small stud welder
has been developed to allow installation of the inner wall studs without
removal of the outer divertor. It is currently working well for new
stud installation, but needs some development to operate reliably for the
stud-on-stud application. We also continue work on new diagnostics for the
divertor and relocation of old ones.


DNB and Related Diagnostics
---------------------------

Design of a pumping system to lower the duct pressure and hence reduce
attenuation of the beam in the duct is underway. Some potentially
useful components have been identified, and modifications to the
F-port flange are being studied. The other occupants of the flange
have begun their redesign process to accommodate various proposed
modifications.

The BES/MSE optical system was returned to PPPL where Ron Bravenec
(UT-FRC) and Howard Yuh (PSFC) are working with PPPL engineers to
understand its performance during the C-Mod campaign and to identify
problems with the design. Work was done to measure throughput and focal
lengths of the invessel optics used in the last run campaign. Results
show significant degradation of internal optics throughput, assumed to
be caused by plasma disruptions and glass dust on optical
components. Howard and Ron discussed new upgrades/repairs
of all MSE/BES hardware with Russ Feder (PPPL), the new mechanical
engineer assigned to the project.


Trips and Visits
----------------

Martin Greenwald was in Santa Fe last week to attend a meeting of the ESnet
steering committee and to participate in a review of the ESnet
program.

>From Sept 5 to 7, Amanda Hubbard attended the 8th IAEA TCM on H-mode physics
and Transport Barrier Physics, at NIFS, Toki, Japan. She presented a poster
on "Evolution of Pedestal Profiles through the L-H and H-L Transitions in
Alcator C-Mod", and also one on "Double Transport Barriers in Alcator C-Mod"
on behalf of John Rice. On Sept 10-12, she attended the first meeting of the
International Tokamak Physics Activity (ITPA) on burning plasma transport,
also at NIFS.

Bruce Lipschultz was at JET from Sept 5 to 7 to work on JET data
related to SOL transport and main chamber recycling. This collaboration is
with Guy Matthews, Kevin Erents and Wojtek Fundamenski. He looked at SOL data
from a number of shots, discussed improvements in diagnostics and potential
joint experiments. He joined Brian LaBombard in Aix en Provence to attend the
IAEA Technical committee meeting on Divertor Concepts organized by the
Cadarache group. Brian LaBombard gave a talk on cross-field transport in the
SOL. Bruce Lipschultz gave a talk on the origin of neutrals in the main
chamber of C-Mod.