High fill-factor experiments on HCX
Recent High-Current Experiment (HCX) measurements
with a K+ (0.2 A, 1 MeV) beam and the first 10 electrostatic transport
quadrupoles show no emittance growth through the lattice, within the
diagnostic sensitivity (~±10%). Currents collected on quadrupole
electrodes indicate particle losses of <0.5%, while Faraday
cup current monitors indicate ~1% losses in the entire distances. Sensitive halo
measurements show that halos as small as 10 -3 of the beam core can be probed with present
diagnostics. PIC simulations presented at the Moscow HIF Symposium
predict that matched beam excursions filling 80% of the quadrupole bore
would result in negligible emittance growth, if perfect alignment and
envelope control were maintained. The figures show a snapshot of the
horizontal beam phase space in the converging plane (so the beam is
smaller at the diagnostic station) after the last quadrupole for two
fill-factors: where the beam filled 60% and a more aggressive 80% of the
46mm-diameter aperture. Since the transportable current scales as the
square of the fill factor, determining its maximum will have a large
impact on the cost of multi-beam induction accelerators for HIF. This
year, further fill factor, matching, halo and diagnostic development
experiments will be carried out in the 10 electric quadrupole lattice,
and measurements will begin with 4 magnetic quadrupoles. Next year,
planned electric lattice extensions promise better resolution and first
systematic looks at the collective evolution of beam inhomogeneity.
~ P. Seidl and L. Prost
Beam
Aperture Experiments at NTX
A high brightness ion source is an essential component of the
Neutralized Transport Experiment (NTX). An ion beam extracted from a
Pierce-type diode suffers from spherical aberrations as evidenced from
the phase space distortion (emittance growth) and from the non-uniform
density profile.
Since the source of these aberrations are the high
order field components, the particles at the edge of the beam are the
most affected. One way to generate high brightness beams is to remove
the edge of the beam after it is generated in the diode; but scraping
the beam also generates secondary electrons that must be controlled to
prevent them from perturbing the beam.
We have designed a beam scraper
system that includes an electron trap for the control of secondary
electrons. Figure 1 shows the beam as it is generated in the diode and
after passing through the scraper system and Figure 2 shows the
slit-integrated density profile for the apertured beam. The measured
emittance is approximately the expected emittance coming from the
emitter temperature and the slit-integrated density profile shows the
expected shape for a uniform density beam; thus showing that scraping a
beam from a typical diode and controlling the secondary electrons
provides a mechanism to generate high brightness beams.
~ Enrique Henestroza
Robust
Point Design for a HIF Power Plant An updated, self-consistent point design for a heavy ion
fusion (HIF) power plant has been completed. The design, which is based
on an induction linac driver, indirect-drive targets, and a thick liquid
wall chamber, meets all known physics and technology constraints.
Conservative parameters were selected to allow each design area to meet
its functional requirements in a robust manner, and thus this design is
referred to as the Robust Point Design (RPD-2002). The driver energy is
7 MJ, with 5.25 MJ from 72 Bi+ main pulse beams at 4 GeV, and 1.76 MJ
from 48 Bi+ foot beams at 3.3 GeV. This study includes a target that,
through detailed simulations, provides a gain of 57. The accelerator has
a calculated efficiency of 38%. Operating at 6 Hz, the plant has a net
electric power of 1 GWe. The final focusing magnets are shielded by a
combination of flowing molten salt jets and vortices, magnetic dipoles
and solid shielding material, resulting in lifetimes of 30 years or
longer based on 3D particle transport simulations. The superconducting
quadrupoles that constitute the final focusing magnets have very large
apertures, stored energy and forces, but the fields required have been
obtained in other particle accelerators. The final spot size based on
our analytic and numerical understanding of neutralized beam transport,
meets the target requirement of ~2 mm. FigureÊ1 illustrates the layout
of the final focus magnet arrays (60 beams from each side) and the
chamber.
~ Wayne Meier
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