AAWSAP DIRD, Inertial Electrostatic Confinement Fusion, March 2010

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constraint on any Spherical !EC device planned for these applications. The observed
loss of convergence with decreasing pressure and increasing current makes achieving
significant beam-beam scaling far less favorable."
In addition to the issue of beam-background fusion dominating in the gridded systems
at higher pressures, Thomson pointed out the importance of energetic ions undergoing
charge exchange and being lost from the system. This work and others on the small
gridded systems at that time showed several important problems which are best
understood by considering beam-background vs. beam-beam fusion scaling. The former
scales with density and pressure as nb • nok<av> ~lilb • p <av> while beam-beam
fusion goes as nb2<av> .
Here: nb = beam ions per cm3; nbk= background atoms per cm 3; p = background
pressure; <av> is the fusion reactivity averaged over the appropriate beam ­
background (or beam-beam) distribution functions.
If ions are produced as done in most small gridded experiments by electron ionization
collisions with neutral gas during a plasma discharge between the grid and vacuum
vessel wall, reduction of background gas pressure will also reduce the ion source,
reducing the reaction rate. Thus, it becomes apparent that to get the favorable beam­
beam scaling needed to go into the power reactor regime, ions must be produced
externally while the main reaction chamber is keep at very low background pressure to
avoid charge exchange losses. Indeed, without explaining that this was the reason,
Hirsch used external ion '\guns" in his early experiment at Farnsworth labs. The present
author (G. Miley), however, went back to the internal discharge ion source technique to
simplify the device for portable neutron source appli,cations. Power devices will need to
go back to external production of some type however. Again, this issue will be
addressed further later.
While earlier workers sought small grid openings designed to provide uniform ion flows
for good core plasma convergence (stressed in the earlier papers already noted), Miley
disclosed in a paper in Reference 1.5 that the STAR mode could be produced with wider
grid openings. In fact, Miley noted that three key modes can be formed in gridded IECs
depending on the pressure and grid openings. These are described as:
"Glow discharge operation of the IECGD is categorized by three distinct discharge
"modes": Star, Central Spot, and Halo (illustrated in Figure 1.3). These names are quite
descriptive of the visual appearances of the visible light emitted from the discharges. All
three modes are reproducible and stable; each is associated with a different potential
well structure, hence neutron production rate. The star mode was used extensively in
recent experiments. It is distinguished by microchannels or "spokes" radiating outward
from a bright center spot (Figure 1.4). As verified by magnetic deflection experiments,
the spokes are primarily composed of ion beams aligned so that they pass through the
center of the openings delineated by the grid-wires. This mode is very efficient for
neutron production, since the large effective grid transparency allows numerous passes
of ions through the center spot before being intercepted by the grid or being ion by
charge exchange. The Star mode is typically obtained at lower operating pressures
(<10 mTorr) and higher voltages (>30 kV), using a carefully formed grid with good
sphericity and high transparency (>95 percent). The halo (or "jet") mode occurs when
one of the grid openings is slightly enlarged compared to the others."
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