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
Dec 8, 2003

Plasma operations continued on Alcator C-Mod last week. Four run days were
scheduled and three and a half were completed. Experiments were run in
support of MiniProposals from the AT, Transport, Divertor/Edge, and RF Groups.
Progress continued on the Lower Hybrid Project.

A meeting of the C-Mod Experimental Program Committee was held on Monday, Dec
1. Ten new MiniProposals for experiments to be run during the current campaign
were reviewed and approved. The proposals included research submitted by the
RF, Transport, Divertor/Edge, AT, and Burning Plasma Support Groups. Two of
these, MP#360 "Initial test of a load tolerant configuration on E-Antenna" and
MP#377 "Energy confinement in double-null discharges", are related to C-Mod
Level 1 (JOULE) milestones.

Operations are scheduled to continue this week.

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

Plasma operations were carried out Tuesday-Friday. A total of 74 plasma
discharges were produced with a startup reliabilty of 67%. High disruptivity
inherent in some of the discharge parameters required by the experimental
program contributed to the low startup fraction. A fresh boronization
was carried out over Thursday night.

The run on Tuesday was devoted to MP#374 "ITB dependence on
triangularity". The experiment was unsuccessful in obtaining satisfactory
plasma parameters because of deterioration of the boronization. This MP will
be rescheduled.

Wednesday's experiment was based on MP#367 "Interplay between confinement,
turbulence and magnetic topology". The point of this experiment is to study
changes in turbulence associated with transport barriers which may form near
low order rational q surfaces. Similar experiments on stellarators have shown
marked changes in the turbulence as iota_bar near the edge is varied. The
present experiment involved slow current ramps to vary the edge q from just
above 3.0 to just below, while turbulence was measured with PCI and
reflectometry, as well as scanning probes. Discharge development and initial
results were obtained, and the MP will require additional runtime for
completion.

Two related experiments concerning edge turbulence were carried out on
Thursday. The first was the completion of MP#332, which investigated edge
turbulence by looking at correlations between probe measurements and fast
diode signals viewing the same flux tube at different toroidal locations. This
run was primarily concerned with the H-mode part of the experiment. The
second half of the day was concerned with imaging of turbulence using the PSI
fast camera, which was also employed during the earlier turbulence
experiments, in density limit discharges (MP#319). A small number of
shots were obtained. Several discharges exhibited poloidal detachment, and
were not suitable for this experiment. In the cases which reached ~70% of the
density limit without detachment the He gas puff was not optimized and the
images were too dim to evaluate.

Friday's run did not begin until around noon due to a combination of a
computer crash, failure and replacement of a commutation bank charging supply,
and a gas main leak on Albany Street about a block away from the facility.
Despite the delay, good progress was made on the scheduled experiment in the
afternoon.

Following overnight boronization, and the morning delays, the run on Friday
was devoted to reconditioning of the machine and ICRF antennas (MP#355), and
to evaluation of the wall pumping under freshly boronized conditions
(MP#364). Re-conditioning was much more rapid than after the first
boronization on Nov 11. By the end of the day the ICRF power had been brought
up to >5MW, and EDA H-mode plasmas with stored energy over 200kJ (<P>~ 1.7
atmospheres) were produced.


Physics
--------

Initial analysis of data collected during recent runs (MP#335 "Edge plasma
flows in upper versus lower x-point discharges") has revealed an interesting
connection between scrape-off layer (SOL) plasma flows, toroidal rotation near
the separatrix, and toroidal rotation in the core. In single-null plasmas,
strong plasma flows along magnetic field lines are detected on the high-field
side SOL from both the inner-wall scanning probe and toroidal views of He+1
visible spectra looking at the same region. These flows approach Mach 1 in the
far SOL, corresponding to a toroidal velocity of about 50 km/s. The flows are
always directed from the low- to high-field side SOL regions along field lines
connecting between the two. The flows appear to be driven by a ballooning-like
cross-field particle transport since in double-null discharges pressure
e-folding lengths on the high-field SOL are reduced by a factor of 5 and the
flows reduce to zero. Conversely, when magnetic topology is switched from
lower to upper null, these strong flows persist and change from the co-current
to the counter-current direction. At the same time, the toroidal rotation of
plasma near the separatrix (monitored by three different Mach probes) and in
the core (monitored by doppler shift of Ar+17 x-ray spectra) is seen to
exhibit approximately the same ~20 km/s shift, co-current for lower and
counter-current for upper x-point topologies. Thus the toroidally-directed
momentum of the flows in the SOL appears to couple across the separatrix into
the confined plasma. One might expect that a stronger (weaker) co-current
plasma rotation of the SOL plasma near the separatrix would be associated with
a stronger (weaker) radial electric field in that region. Probe data support
this trend; plasma potentials near the separatrix are systematically higher
(lower) in lower (upper) x-point discharges.

Further analysis of the ITER-shape high normalized current (I=1.6MA, B=5.3T,
I_N=1.4) discharge developed under MP#363 on 11/26 indicates that the
terminating disruption was due to a locked mode. The mode appeared despite use
of the A-coil to compensate the intrinsic error field. Analysis based on our
present model of the intrinsic non-axisymmetric field on C-Mod indicates that
while the total 2/1 component was quite small (<2e-4 T), the 1/1 sideband
evaluated at the q=2 surface was large, and may have been sufficient to induce
locking. Further analysis is ongoing, future experiments may be undertaken to
determine whether the observed effect is attributable to a sideband effect or
to deficiencies in the present model of the intrinsic error field.


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

Final machining of the Lower Hybrid launcher Forward Wave Guide assembly has
been completed, and the assembly has been received by PPPL. Vacuum leak
checking is in progress as of 12/4, and this will be followed by final
low-power RF testing before fit-up to the vacuum port extension. The forward
waveguide is expected to be shipped to MIT late this week following an
acceptance inspection by MIT personnel. In preparation for rebrazing of the
front Couplers, 131 ceramic windows have been delivered to the industrial
vendor, sufficient for 4 units.

The refurbished klystron which had been returned to the vendor last week has
been evaluated by their engineers. The problem is with the cathode/heater
component of the tube, and the klystron will be repaired under warranty.

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

Peter Politzer (GA) visited MIT and presented a seminar on Non-Inductive
Discharges in DIII-D. He also consulted with C-Mod physicists on his
evaluation of prospects for 100% bootstrap experiments on C-Mod (MP#344B).

Stewart Zweben(PPPL) was at C-Mod this week to take more data on edge
turbulence using the new Princeton Scientific Instruments PSI-5 camera. Many
good 300 frame movies were obtained in Ohmic and L-mode conditions. Gerd
Schilling (PPPL) was at C-Mod 12/2-5 to participate in RF experiments.




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