From:
irby@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups:
sci.physics.plasma
Organization: MiT
Alcator C-Mod Weekly Highlights
August 25, 2003
We
continued work on lower hybrid and ICRF systems this week. The
alternator inspection
continues.
Physics
-------
Data from a
June experiment focusing on fast dynamics at the L-H
transition are being
analyzed. In this experiment, RF power
was
turned on quickly at varying power levels ranging from the
threshold
(1.4 MW) to over 5 MW.
Heating was off axis (80 MHz, 6.1 T) to
minimize sawtooth heat
pulses. Systematically, a small, fast
jump is
seen on the edge temperature and its gradient, followed by a
slower,
apparently diffusive response.
Fitting edge channels of the FRCECE
diagnostic, we find a
characteristic time of ~0.6 ms and delta Te of
~30 eV. These values did not change significantly
with input power.
At higher powers, the transition occurs close to the RF
turn on, when
dW/dt is large.
After correcting for the dW/dt term and other terms
in the power
balance, we find the net power across the LCFS varied by
only about 50% at
the transition, indicating that it occurs quickly
once the instantaneous
conducted flux reaches the threshold value.
In
low power shots, a long period of 'dithering' L-H-L cycles
is
observed. These observations
are being compared to modeling by
Benjamin Carreras of ORNL. Edge Thomson Scattering profiles of ne
and
Te before the L-H transition are also being compared to
theoretical
thresholds.
Analysis of data from the deuterium DNB
run last month continues. The
goal of this run was to investigate the
slowing down and/or loss of
fast beam particles by looking at their
signature on the DD fusion
neutron signal. This analysis is quite relevant to RF heating on
C-Mod,
since these beam deuterons presumably undergo the same physics
as the
H-minority tail generated by ICRF.
Making the simplifying
assumption that the deuteron slowing down
can be represented as a
single exponential decay time, tau_s, allows for
the determination of
tau_s by fitting to the temporal behavior of the 2.45
MeV neutron
signal. Since the
slowing down times are comparable to the timescale
for variations in the
beam current, the fit must actually use a
convolution integral. The principal findings are that a single
tau_s
does indeed fit the neutron rise, fall, and flattop behavior for
each
shot, and that the slowing down times agree with classical values
in
magnitude and scaling. Further
analysis of absolute neutron rate is
continuing, in order to put further
limits on the prompt loss
fraction.
More detailed analysis will be done with the TRANSP code.
Operations
----------
We
continued work on new amplifiers for the PCI (Phase Contrast
Interferometer)
diagnostic. Work also proceeded on a
new
high power amplifier for the MHD antenna.
Work on the
alternator inspection has moved from the stator to the
field rotor. UT inspections of the bearings have been
completed
successfully. UT
inspections of the rotor are in progress.
ICRF Systems
------------
Modifications
needed for operation of the J-Port antenna at 50 MHz
continued. We also continued inspections of rf
transmission line,
phase shifter, and stub tuner components in preparation
for the next
run campaign.
The AFT fast-ferrite-tuner prototype
arrived this week.This system had
been on loan from GA to ASDEX-UPGRADE,
and its return was facilitated
by T.V. GEORGE, OFES. It will be used in ICRF experiments at
C-Mod,
allowing the match between the transmitter and the antenna to be
adjusted
automatically on a millisecond timescale.
Lower Hybrid
Systems
--------------------
PPPL made good progress this week
reworking the Forward Waveguide
[FWG] plates. Polishing of the copper
plating on the FWG plates is now
complete. One stacked assembly has been
re-assembled and satisfactory
rf losses were achieved during low power
tests. The second stacked
assembly
will be completed next week. We plan to
begin high power
tests at MIT on one of the stacks in about one
week.
Prototype testing and analysis [by MIT and PPPL] continues on
the
titanium couplers. Emphasis by PPPL this week was placed on
developing
an acceptable method of removing the ceramic bricks brazed in
the
titanium couplers. Two removal methods were being developed:
chemical
etching/fluid flow and thermal/pressure. However, the chemical
etching/fluid
flow method has proved to be very successful, and is
the most likely
method to be chosen for use on the actual couplers.
A purchase order
was placed at PPPL for a full set of new metalized
ceramic bricks.
Delivery is expected in late September.
The initial high power tests
of the prototype circulator were
completed this week. Full 250 kW, 5 second long pulses were
achieved
into a properly matched load.
Under these conditions the circulator
showed very low insertion
loss and better than specification return
power to the klystron. A plate was then used to cover the output
port
and simulate the short circuit conditions that might occur during
a
waveguide arc or when the coupler is badly matched to the plasma. The
circulator performed well for 5 s
at power levels of up to 125 kW
under these conditions. Progressively
shorter pulse lengths were
obtained as the power was increased up to the
specification value of
250 kW. In
all cases the circulator provided excellent protection for
the
klystron.
Travel and Visits
-----------------
On
8/19 Bob Childs visited PPPL to inspect the forward wave guide
assembly
and gather engineering info.
Wolfgang Weiser, AFT, was here this
week to witness the high power
circulator tests.
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