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W Tabler
03/31/2003
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Driver
Cost is Strongly Dependent on Beam Fill Factor
The design and cost of a multi-beam driver for heavy ion fusion varies
strongly with the size of the beam compared to the size of the beam bore.
We quantify this in terms of the beam fill factor, the ratio of the maximum
beam radius to the bore radius. As the beam is transported from quad to
quad, its cross-section changes from an ellipse to a circle and back to
an ellipse rotated by 90 degrees. The maximum beam radius (i.e., semi-major
axis of ellipse) is nearly 1.3 times the average for typical driver parameters.
Some clearance is needed between the beam and the bore to allow for beam
halo and possible misalignment. A series of IBEAM driver systems code calculations
were completed in order to determine the driver cost as a function of fill
factor.
The results, shown here, indicate a strong dependence on fill factor.
The reference point is the robust point design, where we set the bore radius
= 1.25 x max beam radius + 0.5 cm.
This gives a bore radius of 2.91 cm for the maximum beam radius of 1.93
cm and a fill factor of 66%. Increasing the clearance by 0.5 cm decreases
the fill factor to 56% and increases the driver cost by ~30%, while decreasing
the clearance by 0.5 cm (fill factor = 80%) decreases the driver cost by
~20%. [Experiments on the HCX at LBNL have shown that a beam can be transported
through 10 quadrupoles at fill factors of 60% and 80% without degradation,
see HIF News Jan./Feb., 2003.
~ Wayne Meier
Beam profile simulations and experiments in the quadrupole-focusing subsystem for NTX
The Neutralized Transport Experiment, NTX, Final Focus System transforms
the high brightness potassium ion beam from the NTX Source to a converging
beam at the entrance to the neutralized drift section, where it will be focused
to a small spot. The final focus lattice consists of four pulsed quadrupole
magnets. We have started to characterize the quadrupole lattice by comparing
experimental results with particle simulations using the particle-in-cell
code WARP.
Figures 1 and 2 show the beam profiles for several quadrupole
strength configurations as calculated by WARP and as measured, respectively.
(The good agreement was achieved by using a beam energy 3% higher than measured,
which is within the measurement accuracy, for each of the calculated profiles.)
For each quadrupole, the corresponding row shows the profiles for a change
of -5%, 0% and +5% from the nominal quadrupole strength. This comparison
shows the good agreement that we have obtained so far, between measurements
and calculations.
~ Enrique Henestroza.
Low pressure RF argon plasma source for NTX
A
pulsed plasma source, developed at PPPL, uses 3.5 kW of RF power at 17.6
MHz to generate argon plasmas. It was delivered to LBNL in December 2002
and was integrated into NTX in March 2003. This source produces a local plasma
density of 1011 cm-3 while having an instantaneous neutral argon pressure
in the 10-6 Torr range. The plasma source is placed at the downstream end
of NTX in order to simulate the volumetric plasma that will be present in
the target chamber of a reactor after the detonation of a previous target.
The source consists of a 6-way cross that is inserted into
the NTX beamline. The gas inlet and the window through which a planar spiral
RF antenna looks are mounted to the top face. A turbopump is mounted below,
and diagnostics such as Langmuir probes and pressure gauges may be mounted
to the two unused transverse faces.
The plasma source achieves both high plasma density and low neutral
pressure by taking advantage of the difference in the times-of-flight between
the eV-energy plasma and the room temperature argon. Within 1 ms after
breakdown, the plasma density remains in the 1011 cm-3 range while the neutral
argon pressure rises to almost 0.1 mTorr. By varying the time at which the
~10 ms NTX beam passes through the plasma, the beam particles can experience
greatly different neutral gas pressures while experiencing essentially the
same plasma density.
~ Erik P. Gilson
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