AAWSAP DIRD, Inertial Electrostatic Confinement Fusion, March 2010

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experimental parameters were obtained. The parameters which were varied include the
background pressure, the ratio of electron to ion circulating currents, the applied
potential difference between the grids, and the inner grid variables such as measured
current, transparency, and construction error. When estimates of each of these
parameters which simulate the ion injection mode experiment were inserted into the
model, the resulting potential profile exhibited no more than a shallow potential well.
This result is consistent with that of beam defection measurements in the ion injection
mode experiments. In spite of these assumptions, the model is extremely valuable in
determining the relative (sensitivity of the potential well profile and depth to effects of
many of the system parameters.
It has been found that for the "medium" level of ion currents under discussion, the
most critical factors which inhibit deep well formation are inadequate spherical focusing
and charge neutralization. The focusing is determined to a great extent by the degree
to which the grids are spherical potential surfaces. The grid must approach a spherical
shape within a few percent before other factors such as grid transparency and
background pressure play an important role. However, as the current is increased, the
requirements for sphericity are somewhat relaxed. For a grid construction error of less
than 5 percent, increasing the grid transparency and decreasing the pressure will also
lead to significant improvement in well depth."
These results were somewhat encouraging. However, they showed that grid
deformation could be very harmful. This deserves several comments. First, the present
author (G. Miley) later showed that design of grids with larger openings provided the
STAR mode where ion beams passed through the center of the openings, avoiding grid
collisions and making sphericity of the grid itself less important. This is important for
small neutron/proton source type devices. However, the assumption of grids fails to
address the question of how a grid could survive in a power reactor or if they could be
eliminated to use a potent ial well with virtual electrode formation. The use of grids
cannot be completely ruled out for power reactors. Magnetic field protection techniques,
active cooling, etc. are conceivable.
Another question relates to the role of background gas in the IEC. The point is this:
When Miley moved to simplify the device for small neutron sources, he used the
discharge between the grid and vessel to form the ions needed for acceleration and
fusion. This inherently forces use of a modest background neutral gas pressure of the
fuel (typically deuterium) inside the reaction vessel. That in turn results in ion reactions
with the background gas becoming a dominant process in these IECs. Such interaction
includes fusion itself, scattering, charge exchange, etc. This greatly changes the plasma
physics of the IEC as opposed to the ideal of a potential well with "zero" background
pressure. Some key differences in the physics of such IECs were brought out by Tim
Thomson in his experimental study described in Reference 1.4. He noted that: "In
present gridded systems, convergence is not important since beam-target fusion
reaction dominate the reactivity of these devices, as evidenced by the linear scaling of
reactivity of these devices, as evidenced by the linear scaling of reactivity with the
cathode current. In fact, convergence may reduce the reactivity by forming a virtual
anode that limits the central ion density. However, this space charge effect can be
overcome by proper introduction of electrons. Good convergence is required to achieve
optimal beam-beam reactivity scaling for the application that require higher fusion
reaction rates, and the importance of symmetry in determining convergence places a
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