From:
irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod
Weekly Highlights
Organization: MIT
Approved: plasma@cs.uml.edu
Alcator C-Mod Weekly Highlights
June 9, 2003
Alcator C-Mod successfully completed all four scheduled runs
last
week. Work progressed on the lower hybrid and ICRF systems. Plasma
operation will continue this
week.
Operations
----------
Tuesday's
run was devoted to recovery from boronization that had been
applied the
previous Friday. Wednesday's run
investigated ICRF
induced fast ion loss and its effect on edge radial
fields and the
H-mode transition.
We also studied the mapping between probes and the
gas puff imaging
system. Thursday was spent developing
high power rf
operation scenarios, with up to 5 MW of power coupled to the
plasma
for 0.5 s and over 6 MW for shorter periods. On Friday we continued
our JET/C-Mod
similarity studies.
Startup reliability was 96% this week, excluding
Tuesday's boronization
recovery day.
Startup reliability was 100% on both Thursday and
Friday during
which 64 physics quality discharges were produced.
The successful
operation of the non-axisymmetric control coils has
motivated us to add
more capability to this new system. A
second TMX
supply is being installed in the power room that is expected to
become
operational before the end of the current campaign.
Physics
-------
Analysis
of fast dynamics at the L-H transition, carried out in
collaboration with
Benjamin Carreras, ORNL, during his recent visit,
has revealed some
interesting results. The primary
diagnostic was the
high resolution, low noise FRCECE diagnostic (U.
Texas). By using
discharges with
off-axis RF heating, the effects of sawtooth heat
pulses were
minimized. On several channels, there
is a two-stage
transient; an initial, near-linear 'jump' in Te over a
period of order
1 ms, followed by a slower, apparently diffusive response
at all
radii. Somewhat
unexpectedly, the biggest jump in Te is at about 87.9
cm, 1.5 cm inboard
of the separatrix; the prompt response extends from
86-89 cm. A model by Carreras and del-Castillo-Negrete
gives
qualitatively similar behavior.
The simulations indicate that the
scaling of the size and rate of
the initial 'jump' with P-P(thresh)
would be the best test of the model;
as power is raised above the
threshold, the size of the jump should get
larger, and the duration
shorter.
It is therefore planned to investigate the dynamics more
systematically
using a controlled power scan.
A miniproposal on "Edge Minority
Heating for H-mode Control" was run
by Stewart Zweben, PPPL, on
C-Mod. A scan of the H-minority
resonance
position was made from the low-R edge to the high-R edge at 78
MHz
(i.e. B = 3.7 - 6.7 T) with RF power varying in the range 0.2-2
MW.
Analysis of the results is in progress. Ultra-fast camera
measurements
of the 2-D radial vs. poloidal structure of the edge
turbulence using
the PSI-4 camera were also obtained. Good images of the radial and
poloidal
propagation of edge blobs were obtained at 250,000 and
500,000
frames/sec.
Lower Hybrid Project
--------------------
A
high power testing facility is being planned for the lower hybrid
launcher. A major part of this facility will be an
absorber lined
chamber into which 60 kW of power from the launcher can be
safely
dumped. Simulations of the
coupler and its radiation pattern in the
chamber are being performed and
will be used to design the chamber.
More equipment was moved into
the Lower Hybrid Active Controller
racks.
The ethernet connections were made for both the CPCI and PXI
chassis,
and communications for both systems were successfully tested.
Mounting
shelves and wireways arrived and were installed. The CAMAC
timing system was moved to the new racks and fiber
optics and cables
for the timing system were reconnected to allow tests to
continue.
Work continues in building the LH TC/Heat/Cooling system PLC
rack.
Wiring continued on heater control chassis. SFOL Tx board layout is
nearing
completion. Labeling convention issues
for the LH patch panel
were resolved for the directional couplers. The probes will be
installed in
accordance with our existing convention, as confirmed in
discussions with
Nevell Greenough, PPPL.
ICRF Systems
------------
Steady
(~0.5 sec) 5 MW ICRF EDA H-modes were achieved this week for
the first
time on Alcator C-Mod. Even higher RF
power H-modes were
achieved for shorter pulse lengths. RF pulse lengths
and the total
power level were paced by D and J-Port antennas. The D-Port dipole
antenna can currently
be used for reliable long-pulse operation at
power levels up to about 1
MW. An issue with the D-Port control
circuitry, which will be resolved
during the next maintenance day,
prevented operation at higher powers. The
E-Port dipole antenna is
operating reliably at powers levels of 1.5 MW or
above for long
pulses. The J-Port 4-strap antenna is now sufficiently
conditioned to
allow for 2 MW, long-pulse operation, and a new record of
3.2 MW was
coupled through this antenna for shorter (80 ms) pulses. The
J-Port
antenna continues to improve as more conditioning is done.
We
continue to make improvements to the ICRF control system. Power
calibration measurements have
been completed for both the D-Port and
E-Port transmission line
systems. The measured powers from
several
diagnostics now agree to within 5-10%. The calibration factors
involved in predictive tuning have
also been refined and shot-to-shot
tuning has become more reliable. Noise problems in the CPCI
digitizers
have been isolated and are being eliminated as maintenance
time becomes
available.
Travel and Visits
-----------------
Martin
Greenwald attended a workshop in Reston, Virginia "DOE Science
Networking". The workshop was charged by OASCR to draft a
five year
roadmap for networking for Office of Science programs.
Manouch
Farkhondeh, Jan Van der Laan, G. Townsend Zwart, and William
Graves from
the MIT Bates lab visited Peter Koert and Charley Schwartz
to discuss some
applications of ferrite tuners. They have some
interest in the development
of tuners on a free electron laser
project. Our discussions may develop
into a shared knowledge base for
the materials needed for our ICRF
tuners.
Paul Bonoli visited PPPL on June 4-6, 2003 to attend a
Program
Advisory Committee Review of the OFES Sci-Dac Projects. Included in
this meeting was a review
of the rf Sci-Dac Initiative. Paul
Bonoli
also had discussions with Cynthia Phillips, Stan Kaye, and J.
Manickam
on the implementation of new Fokker Planck and ICRF heating
modules in
TRANSP.
Brent Stratton, PPPL, was at MIT last week,
working on the 2-D imaging
high resolution X-ray spectrometer system which
is on loan from
NSTX. The PC-based control and data acquisition system was
integrated
into the C-Mod network, and synchronization with C-Mod shot
cycles was
successfully implemented.
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