Background
-
Spherically Imploding Ion Beams: Inertial Electrostatic
Confinement (IEC) is achieved by accelerating ions into a highly transparent
cathode-grid, concentrically placed inside a larger vacuum vessel of the
same geometry. Typical IEC devices are usually spherical, although many
cylindrical ones have also been studied. Ions generated near the wall of
the vessel are accelerated into the cathode-grid, due to the strong electric
field produced by the potential difference (~50 kV) placed between the
cathode-grid and the grounded vessel wall. Different schemes are employed
to generate the ions, e.g., glow discharge, electron-impact from a low
density electron cloud that is confined near the vessel wall via an extra
grid and produced from hot electron emitters, and externally mounted ion
sources.
-
Multiple Potential Well Formation: Ion confinement
time in the hot plasma can be significantly increased via the formation
of a series of multiple potential wells inside the cathode-gird. These
"virtual wells" appear as "virtual" anodes and cathodes to the ions, confining
them but not absorbing them like the "real" cathode-grid. With this formation
of wells the only loss mechanism for the ions is upscatter (an interaction
in velocity space, not physical space) and fusion. These wells, modeled
with computational codes and observed experimentally (described below),
form due to a complex interaction between the ions circulating through
the cathode-grid and electrons, emitted as secondary products from the
cathode-grid when an ion is absorbed and from ionizations of the background
gas inside the cathode-grid.
-
Encouraging Experimental Data: Numerous studies, spanning
almost four decades, have reported exceedingly high neutron yields, as
a function of plasma density and energy, and direct measurements of potential
well formation,,.
-
The Early Pioneers: The approach to fusion using IEC
was originally conceived by P. Farrnsworth (inventor of electronic television
in the US), and later studied experimentally by Hirsch (including neutron
production, direct plasma measurements, and computational modeling)1.Then
during the late 60’s and 70’s, Verdeyen conducted many experiments on the
potential well depth and formation, but not neutron generation. R. W. Bussard
and G. H. Miley renewed studies in the late 80’s, looking at electrical
power production, space propulsion, and neutron generation.
Attractiveness
-
Non-Maxwellian Ion Energy Distribution: Since the
ions are accelerated, reaching the center of the cathode-grid with near
uniform kinetic energy, the resulting plasma is non-Maxwellian. This energy
distribution and consequent beam-beam type reactions, plus lack of cyclotron
radiation due to the elimination of B-fields makes the IEC attractive for
burning advanced fusion fuels, like D-3He and p-11B.
-
Non-Linear Scaling of Reaction Rate with Ion Current:
Computational studies have shown that potential well formation and ion
confinement time is a function of ion density,. Since ion density is dependent
on ion confinement time, non-linear scaling of fusion reaction rate with
ion current is considered possible. Scaling on the order of current cubed
would lead to high reactor power densities and attractive reactor efficiencies.
-
Plasma Target Fusion: The greatest number of fusion
reactions is occurring inside the cathode-grid; in most devices operating
today, the majority of reactions are beam-background in nature, that is,
between the fast moving ions in the plasma and the neutral background gas.
Since there is no solid target upon which ions are directed there is not
a solid target that will deteriorate from plasma interactions. This is
advantageous for it places no upper limit on confined ion density and fusion
reaction rates.
-
Non-Ignited Plasma: Most IEC reactor concepts employ
a direct energy conversion scheme, wherein the high energy fusion products
are allowed to escape the potential well "trap" and slowed down in an external,
high voltage electric field. Such a configuration does not require an "ignited"
plasma where the fusion reaction products are keeping the plasma hot. Therefore,
reactor sizes smaller than conventional fusion devices are possible to
demonstrate reactor breakeven.
-
Compact Size and Low Reactor Weight: IEC devices do
not need magnetic fields, hence eliminating the need for large, heavy magnets.
In addition, IEC devices are relatively simplistic and small, making for
an easy to transport inexpensive device.
Potential Applications
-
Terrestrial Power Source: Several approaches are under
consideration for earth-based power production: Q >1 fusion reactor, employing
direct energy conversion; and a fusion-fission hybrid with a Q < 1 IEC
reactor and a keff < 1 fission assembly.
-
Deep Space Power and Propulsion: IEC is presently
being investigated for use in deep space power and propulsion. The low
weight, compact nature of the IEC makes it attractive for use in propulsion
and remote site power production.
-
Radioisotope Production: A Q < 1 IEC device producing
either high energy neutrons or protons, would be ideal for producing radioisotopes
for everything from industrial uses to medical diagnostics and therapy.
-
Medical Therapy: A high output IEC device has been
considered in therapies involving neutrons such as Boron Neutron Capture
Therapy.
-
Nondestructive Evaluation/Neutron Activation Analysis:
Thermalizing neutrons from an IEC device, or using 14-MeV neutrons from
D-T fusion in an IEC, is advantageous, considering the attractive attributes
considered above, i.e., low weight, compact nature, and most importantly,
plasma target. Indeed, Daimler-Chrysler has commercialized IEC technology
for small, compact neutron generators (this is the first commercial application
of a confined, fusing plasma!).
Key Issues
-
Multiple Potential Well Formation: Further study is
required to understand potential well formation and the scaling of such
formation with ion current, energy, etc. This is the central key to developing
IEC technology for advanced applications beyond that of small scale neutron
activation analysis.
-
Scaling of Reactor-Power with Current: Operation with
present IEC devices is in the mode of linear scaling of neutron output
with cathode-grid current. It is believed that too high of a background
operating pressure is truncating ion confinement, preventing the formation
of deep, multiple potential wells. Construction of next-generation IEC
devices are needed to reduce background neutral gas pressure, increase
ion injection currents, so as to measure reaction rates as a function of
cathode-grid current.
-
Power Conversion: Given the advantage and possible
use of advanced fuels, such as D-3He and p-11B, research is needed to develop
a conversion scheme that will allow for high efficiency extraction of useful
energy from the IEC device.
-
Cathode Grid Life Expectancy and Efficiency: It is
unlikely that present-day cathode-grid technology is sufficient to make
a grid that can contain a Q > 1 IEC plasma. Investigation is needed to
optimize grid design, or even to see how to eliminate the cathode-grids
altogether.
Copyright Notice
For Scientific and Technical Information Only
© Copyright 2006 Los Alamos National Security,
LLC All rights reserved
For All Information
Unless otherwise indicated, this information has been
authored by an employee or employees of the Los Alamos National Security,
LLC (LANS), operator of the Los Alamos National Laboratory under Contract
No. DE-AC52-06NA25396 with the U.S. Department of Energy. The U.S. Government
has rights to use, reproduce, and distribute this information. The public
may copy and use this information without charge, provided that this Notice
and any statement of authorship are reproduced on all copies. Neither the
Government nor LANS makes any warranty, express or implied, or assumes
any liability or responsibility for the use of this information.
|
