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
July
2, 2001
Plasma operations continued at Alcator C-Mod last week. Four
run days were
scheduled and completed. A total of 61 plasma shots were
produced, with a
startup reliability of 85%. Three runs concentrated on
conditioning and
performance characterization of the four-strap
"J-port" ICRF antenna. The
fourth was an experiment to test the
peeling-mode model of ELMs.
Plasma operations are scheduled to
continue this week.
Physics and Operations:
-----------------------
Following
completion of repairs to the phase shifters, ICRF antenna
conditioning
resumed on Tuesday and continued Wednesday. The J-port antenna
was tested
up to 3.0 MW into plasma.
A fresh boronization was carried out over
Wednesday night, with an average
deposited thickness of approximately
2000A. The J-port antenna conditioning
and testing were completed on
Thursday. The results of these
experiments are
detailed in the ICRF section below.
Friday's
run was in support of MP#282A, to investigate the effect of edge
current
density on ELM behavior in high beta_N (1.2<beta_N<1.6) C-Mod
plasmas.
EDA H-mode plasmas were produced at 800kA and 1MA, with RF power
between
2.5 and 3.5MW. These discharges were in the regime where "grassy"
ELM
activity begins to be seen on the H-alpha and other diagnostics, along
with
the QC mode typical of the EDA. The edge current profile was
transiently
altered by imposing current ramps, raising and lowering the
surface loop
voltage by about 2V.
The goal was to test the theoretical prediction that the
consequent
increase in edge current density would make the peeling modes more
unstable,
leading to increased ELM activity, while decreasing Jedge with
reverse
current ramps would stabilize the modes. This experiment was
technically
challenging, as it was difficult to maintain good RF coupling
during these
transients; there were also some perturbations to the equilibrium
shapes
during the current ramps, which may complicate the analysis. However,
the
preliminary result is that the observed changes in ELM behavior were
opposite
to the expectation: increasing the current seemed to suppress
pre-existing
ELM activity, while reverse ramps resulted in short bursts of
ELMs in a
previously quiescent discharge. A
number of shots have been
identified for detailed analysis.
Piggyback
experiments this week continued to exploit the capabilities of the
new
ultra-fast PSI camera. On different days, the camera was used to image
filamentary
striations in the edge plasma and lithium pellet ablation
clouds. The
latter technique can be used as a q-profile diagnostic. Using the
ablation
light detected on a separate photodiode as a trigger for the camera,
it
was possible to obtain 12 separate Li^+ ablation trails with time
resolution
of 20 microseconds, corresponding to radial spatial
resolution of about 2
cm.
ICRF System
------------
As mentioned above, the
J-port antenna was tested to 3.0 MW into plasma. It
heated well and reached ~26 kV briefly. The antenna had numerous faults when
the
voltage reached ~20 kV and degraded over a series of discharges. The fast
data indicated that some arcs
survived for 30-50 usec. Up to 100 J
could be
available to dissipate in the arc; therefore, we reduced the
reflected-to-
forward power ratio necessary to generate a trip by 25% for
Friday's run.
This successfully limited the arcs to ~15 usec or ~15 J per
MW injected. We
suspect arcing in
the antenna strap is responsible for these arcs, because no
signal
corresponding to the arcs were observed in the optical monitor signals,
and
the antenna "paper clip" geometry could allow an arc on one antenna
strap
half without resulting in a phase balance or reflected power fault
in the
transmission line. We often
observe an interaction at the antenna midplane
near the septum and side
limiters at about the location of the short.
This
interaction was more severe when running inner wall limited
L-mode discharges,
and was weakly dependent on plasma current. 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. Boronization recovery did require
a few discharges even
though the antenna was voltage conditioned to 32 kV
before the run. False phase balance faults are still
occurring on retry when
other antennas are operating. We will be examining a fix for this in the
next
week.
The D-port antenna was out of service the previous
week due to arcing in the
phase shifter.
There were arcs both across the teflon surface and across the
gap. Additional arc tracks were found near the
push/pull rod (but not on it),
suggesting that arcing was occurring across
the conductor gap. We had been
operating
up to 40 kV with 20 psig of N2, which is close to the design
maximum. The arc location was also an area with
enhanced E-fields due to the
geometry.
Thus, the breakdown was probably a result of exceeding the phase
shifter
operating limits in N2. Future
operation will be with 20 psig SF6,
which will raise the power handling by
a factor >10 and result in a safety
factor of about 2 in voltage. Once the phase shifter was repaired on
Monday,
D-port operation resumed.
During operation early last week, the D-port
antenna experienced
numerous false phase balance faults due to a bad
demodulator. Another demodulator was installed for
Friday's run and the
antenna fault protection behaved properly.
Diagnostic
Neutral Beam System
-------------------------------
The beam
was operated at full voltage and current.
There was an apparent
heating problem on the suppressor on
Wednesday. The last two days of the
week
were devoted to parameter scans to improve the beam component mix and
to
identify any remaining problems with the beam. Based on these, we plan to
instrument
the arc to assure that the arc current is shared among the
filaments and
to assure that the arc SCRs are receiving appropriate trigger
signals. This did not prevent us from continuing the
development of CXRS,
MSE, and BES diagnostics.
Lower Hybrid MIE
Project
------------------------
The construction of the Lower
Hybrid launcher's test set up for the
reflected-power probe and for the
"water dump" has been completed at PPPL. It
is being assembled now and a probe is being fabricated;
testing is expected to
begin next week.
The launcher design is proceeding well, with the FDR
tentatively
scheduled for July 31 at PPPL. The
launcher will have radial
movement capability via vacuum bellows, and a
vendor visit concerning these
bellows was very encouraging. Plating of copper to provide a high
electrical
conductivity skin on the titanium waveguide is proceeding at
another vendor.
Ceramic windows with a larger radius of curvature at the
corners will be
received this week for brazing tests.
Inner
Divertor Fabrication
--------------------------
All ribs (upper section and main body) on 12
Inconel Rear Girdle Plates were
completed on Wednesday, June 27th. There
are two more operations: Milling the
bottom of the plates and wire EDM
inside radius + remove corners.
Estimated
delivery date of all 12 Rear Girdle Plates is July
31st.
All 12 Inconel C-Plates were completed and 100% inspected at
the manufacturer.
The C-Plates are expected to arrive in-house this
week.
Travel and Visitors
-------------------
Gerrit
Kramer (PPPL) was at MIT 6/26-28, working on MSE data. Gerd Schilling
was at MIT 6/26-29,
helping with the DNB operation and attempting to improve
the beam species
mix.
Drs. H. Kishimoto, A. Kitsunezaki, and T. Ogawa, of JAERI,
visited Alcator on
Friday. They toured the facility and met with
Professors Porkolab, Parker, and
Hutchinson.
Dr. Michael
Holland from the Office of Management and Budget visited the PSFC
on
Friday, June 30. He had discussions with scientists and students from the
center,
and toured the C-Mod facility. He also presented a talk on the budget
scenario.
From
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2001
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From:
harold.hansen@hs.utc.com (harold hansen)
Newsgroups:
sci.physics.plasma
Subject: langmuir probe data analysis
Date: 6 Jul
2001 07:14:45 -0700
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Status: O
Does anyone know of a program
that can take the raw langmuir IV curve
and determine the plasma
parameters (perferably low cost/shareware).
Thanks for any
help.
Harold Hansen