From:
wolfe@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator
C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization:
MIT
Alcator C-Mod
Weekly Highlights
June
25, 2001
Plasma operations continued at Alcator C-Mod last week. Four
run days were
scheduled and 3 1/2 were completed. Run time was devoted to
ICRF conditioning,
cross-calibration of diagnostics, density limit
studies, and imaging of edge
turbulence. A recurrence of the arcing in the
phase shifter for ICRF
transmitter #4 resulted in the loss of a half
day.
Plasma operations are scheduled to continue this week.
Physics
and Operations
----------------------
A half-day run was
devoted to study of the density limit. The goal was to
collect data on the
changes in turbulence and transport which have been linked
to the density
limit. In the previous experiments it
was observed that the
regime of large coherent fluctuations which normally
exists in the far
scrape-off, moves inward to the separatrix and intrudes
into the closed field
line regions as the density limit is reached. In
this experiment, we inserted
the probe more deeply into these plasmas at
high densities in order to follow
the evolution of the profiles and
fluctuations, to assess how far into the
main plasma this phenomenon
reaches. With the probe inserted about
50 msec
before the density-limit disruption, we observed a break in
profiles,
turbulence and autocorrelation function about 1cm inside the
LCFS. The
temperature profile was seen to shrink from the plasma edge as
the limit was
approached even with Prad << Pinput. Several shots were identified for
further
analysis.
A half-day was dedicated to a cross-calibration of the
Thomson scattering
density response based on cut-off of the ECE signal in
high density, reduced
field discharges. Corrected calibration factors were
obtained, and are being
applied.
Two experiments involving the
fast CCD cameras were carried out in the
half-run on Friday. The original
camera was used in conjunction with the fast
scanning probes to study edge
turbulence (MP#281). On the same shots, the new
million frame/sec PSI
camera was used to image the ablation cloud from lithium
pellets.
ICRF
Systems
------------
The J-port antenna was tested into vacuum
and plasma after repair of the phase
shifter. It was vacuum conditioned to 32 kV and then was operated
into
plasma. J-port reached 2.5 MW
without significant RF-plasma edge interaction
at the antenna
corners. However, some interaction was
observed on a couple of
discharges on the K-side of the septum at the
midplane. Comparing the faults
with
the optical monitor, we conclude that the system reliability was limited
by
arcs in the transmission line, at the phase shifter, Into plasmas, the
voltage
reached ~22-24 kV during H-mode which is ~50% higher than during the
last
campaign. Given the continuing problems
with the phase shifter, we have
removed the teflon support from the phase
shifter (arcing is going across the
teflon surface). Repair of the phase
shifter controller was completed on
Saturday. The phase-shifter will be
tested during vacuum conditioning on
Monday, and is expected to be
available for plasma operation on Tuesday.
E-port was operated upto
1.5 MW into plasmas quite reliably.
D-port
however was less reliable.
It was tested into vacuum and arcing was
detected at relatively low
voltage. It is awaiting inspection and
repair.
Diagnostic Neutral Beam Systems
-------------------------------
The
DNB did not operate into plasma this week.
A failure in the arc supply
was traced to overheating of a
supply. Repair consisted of replacing a
failed
fan and replacing the air filters.
Following repair of the arc, a second
problem appeared with the HV
timing. The testing that we were able
to do each
day before the runs, between shots, and on the bench eliminated
a number of
possibilities for the problem. The timing of the suppressor is now working
normally. Trouble shooting of the HV controls
continues.
BES filter
functions as a function of filter rotation angle were completed and
included
in the computational model for the BES signal.
Inner Divertor
Fabrication Project
----------------------------------
Parts
continued to arrive for the new Probe Box assembly. Mock-up building is
moving
forward, and a new Stainless Steel Wall was installed. Stud welding
procedures
for shooting long studs and welding studs-on-studs were developed
and
documented; testing indicates that 5,000 Lbs tensile strength can be
obtained
consistently.
The vendor has finalized the manufacturing process for
the Rear Girdle plate
and is proceeding with production. Our other vendor
has completed all twelve
inconel C-plates; final inspection, including
100% dimensional check is
proceeding.
Lower Hybrid MIE
Project
------------------------
The modifications and wiring
for the klystron carts are basically complete.
Some additional
modifications may be necessary later to facility the
circulator
mounting. Work continues on the
equipment rack wiring and PC board
assembly.
Travel and
Visitors
-------------------
Stewart Zwebwen (PPPL) spent the
week at C-Mod. He brought and helped set up
the Princeton Scientific
Instruments PSI-3 million frame/sec camera, got new
Gas Puff Imaging data
during the density limit experiment, and worked on data
analysis. He also
served as co-session leader for Friday's experiment on edge
turbulence
imaging.
Gerd Schilling (PPPL)
was at MIT 6/19-22, working on ICRF antenna conditioning
and
attempting
to help with the diagnostic neutral beam (which was down this week
with an
electrical problem).
Yi-Jung Yang, one of this years' recipients of
the National Undergraduate
Fellowship in Plasma Physics, began her summer
internship at C-Mod this
week. She will be here for a total of nine weeks,
and will be working with
Brian LaBombard and Sanjay Gangadhara on plume
image analysis.
J. Snipes, I. Hutchinson, D. Mossessian, C. S.
Pitcher, and J. Rice attended
the EPS meeting in Madeira, Portugal from 18
- 22 June 2001. Four papers on
C-Mod results were presented.