From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
July 7, 2003

Plasma operations continued at Alcator C-Mod last week. Four run days
were scheduled and completed. Six principal experiments were carried
out, in support of MiniProposals from the Transport and Edge/Divertor
topical science areas and the Advanced Tokamak thrust.

Plasma operations are planned to continue this week.

Operations
----------

A total of 112 plasma discharges were produced, with an overall
availability over 90%. All tokamak systems performed well, with the
exception of interference in the EF2U control circuitry that appears
to have been related to RF pickup.

Experiments conducted included half-day runs on Monday devoted to
MP#342 which studied density pedestals in L-mode plasmas and to the
completion of MP#335 on edge plasma flows in upper versus lower
x-point discharges. Tuesday's run was devoted to MP#347, a study of
performnce in limiter L-mode plasmas. An extended (10 hour) run day on
Wednesday was dedicated to two experiments: studies of ITB discharges
induced by off-axis (HFS) ICRF heating, and the initial experiments in
support of a proposal (MP#344B) to develop 100% bootstrap fraction
discharges. The second experiment was conducted with the Session
Leaders (P. Politzer and F. Perkins) participating via remote access
from GA in San Diego.

Finally, the run on Thursday was devoted to MP#334A, to investigate the
quasi-coherent mode in ohmic H-mode plasmas using a variety of
diagnostics. The target plasma for this run was similar to one used in related
experiments during the 2000 campaign, but this shot, which features TF
ramp-down to obtain low q and induce the L-H transition, proved difficult to
reproduce due to high disruptivity. Late in the day, use of the
non-axisymmetric control coils was found to reduce the field (and q) at which
disruptions occurred, though not to as low a value as obtained in the earlier
experiments. These results may indicate a change in the static error fields in
C-Mod since the 2000 campaign.

Physics
-------

A run was carried out to study performance issues by running inner-wall
limited discharges with strong RF heating. The RF power had programmed notches
designed to allow evaluation of the absorbed power via dW/dt analysis. The
overall confinement followed results from earlier years quite closely. Data
from different plasma currents overlaid (using ITER89 scaling). A degradation
in L-mode performance as density was raised was also reproduced. H factors in
these L-mode discharges varied from 1.3 at nebar ~ 1e20 to 0.7 at nebar >
2.5e20. Initial analysis of fast H-alpha data from the edge suggests that
there is no rapid change in the edge plasma associated with RF turn off (or
on). Analysis is ongoing.

New work by with the GYRO code simulates the C-Mod off-axis RF ITB H-mode
within a linear ITG/TEM flux-tube approximation. The linear, electromagnetic,
kinetic electron response simulations are in qualitative agreement with
earlier GS2 simulations. Quantitative differences are likely to be due to
different equilibrium models and the treatment of impurities.

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

ICRF Operations supported a number different miniproposals during the week.
Power levels of up to 3MW were coupled to a variety of target plasmas,
including limiter L-modes, H-mode and ITB discharges. Operation at neutral
pressure above 0.5mTorr during the L-mode experiments on Tuesday (MP#347) led
to consistent faulting of the J-port antenna, while the E-port antenna ran
without problem; this effect may be related to the longer vacuum transmission
line used in J-port. Some difficulties were encountered in coupling to current
ramp-down plasmas during the high bootstrap fraction (MP#344B) run on
Wednesday; at currents below 300kA the antennas would begin to fault and
attempts to eliminate the apparent arcing were not successful.

Problems with FMIT#1 vacuum breaker energy storage unit prevented running this
transmitter on Monday and Tuesday. A defective relay and transformer were
repaired, and the transmitter was returned to service for the Wednesday run.
On Wednesday, it was discovered that operation of transmitter #3 (J-port)
appeared to be associated with interference in the control signals for the
EF2U power supply, which resulted in several disruptions. The cause of this
interference is being investigated.

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

A draft schedule covering all aspects of the launcher rework necessitated by
the replating has been generated. This includes repair and replating of the
plasma-facing coupler, disassembly, replating and reassembly of the forward
waveguide, and comprehensive inspection and testing of all components before
installation on C-Mod.

The "old" copper and underlying base have been stripped off the
coupler. The stripping bath also etched away some of the braze alloy
sealing the ceramic windows to the coupler frame, resulting in vacuum
leaks. A coupler prototype is being used to duplicate this effect and
to verify the rebrazing, cleanup and replating processes prior to
repair and replating of the full coupler.

The forward waveguide assembly has been fully disassembled at PPPL.
All of the stainless steel plates comprising the stacked assembly have
been stripped of old copper and underlying base. They have been
carefully cleaned and measured to assure dimensional tolerances after
plating. All 50 plates have been replated and remeasured. 17 plates
have gone through a 200 C vacuum bake at MIT as part of verifying the
plating integrity.

Measurements of initial phase settings of the rear waveguide have been
completed at PPPL, and the unit with all its components was delivered
to MIT 7/2. The rear waveguide has been unpacked and placed in
position in the high power test area. The waveguide run to the
coupler from the klystron as well as tapers needed to couple into the
waveguide are nearly fabricated at MIT.


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

Miklos Porkolab attended the IPELS 2003 Conference (Interrelationship
between Plasma Experiments in Laboratory and Space) on June 29 -July 3
in Whitefish, Montana, and gave the invited talk "Mode Conversion
Processes at the Ion-Ion Hybrid Layer in Multi-Ion Species Plasmas in
Sheared Magnetic fields". The bi-annual international conference was
attended by approximately 100 scientists from around the world,
including space plasma physicists, laboratory plasma physicists, and
fusion scientists. The theme is cross-fertilization of ideas among
different branches of plasma physics. There was considerable interests
among space plasma physicists in the C-Mod ion cyclotron wave mode
conversion results for possible applications to explain energetic ion
formation in space plasmas.

Manfred Bitter, Ken Hill and Brent Stratton spent the week of June 30
working on the NSTX X-ray crystal spectrometer temporarily installed on
C-Mod. The crystal was rotated to bring the entire helium-like argon
spectrum onto the detector. This worked well and spatially-resolved
spectra were obtained during several days of operation. Unfortunately,
the spectral resolution of the present detector (on loan from the Korean
Basic Sciences Institute) is not sufficient to make a good ion temperature
measurement. A new detector due to be delivered soon from Brookhaven
should have much better resolution that will allow ion temperature profiles
to be measured.

Martha Redi was at MIT on 6/30 to discuss this year's EPS paper and poster
with Catherine Fiore.




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