QUICKSILVER Simulations of the Applied-B Ion Diode at Sandia National Laboratories
A new class of ion diode simulations has been completed using the
three-dimensional, electromagnetic, particle-in-cell code
QUICKSILVER. These calculations, which assume top-bottom symmetry
as in the past, model the new anode surface cleaning hardware that
has recently been installed on the PBFA II accelerator. In addition,
we are simulating a larger azimuthal section
( /2 and
) of the
PBFA-II diode hardware compared to the previous
/8 section.
It is important to model a large portion of the hardware for two
reasons. First, long-wavelength diocotron instability modes have
lower frequencies, giving them the potential to induce larger ion
beam divergence. Second, long-wavelength, low-frequency fluctuations
are seen in PBFA-II experiments. The
/4,
/2, and
simulations show that
the most significant contribution to beam divergence is from an
m = 8 (eight wavelengths around the diode) ion mode.
The /2 and
simulations also
have a smaller amplitude m = 4 diocotron mode that leads to
interesting mode coupling effects; however, the horizontal beam
divergence is not significantly affected. Hence, a
2 simulation would
not be expected to reveal different behavior. In addition, one
of these simulations illustrates an important new result: diode
geometries with initially nonuniform emission of lithium ions
from the anode surface can cause an early transition to the ion
mode instability. This latter result has implications for the
performance of extraction diodes, since it is more difficult to
achieve uniform beam current density in an applied-B diode in
which ions are axially, rather than radially, focused. A key
contributor to this difficulty, however, is the passive LiF
source: simulations with active ion sources show that we can achieve
uniform current density and a low divergence phase in extraction
geometry. Furthermore, extraction diodes are more amenable than
barrel diodes to the use of electron limiters to shape the virtual
cathode surface and therefore control the beam divergence.
Extraction diodes will be required for high-yield inertial
confinement fusion experiments because of the need to protect
diode hardware from intense bursts of x-rays and gamma rays by
transporting the ions a distance of two to four meters to the
target.
Other Reports on High Energy Density and Inertial Confinement Fusion
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