AAWSAP DIRD, Inertial Electrostatic Confinement Fusion, March 2010

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was obtained from recirculation beam-beam reactions in a potential well such as in
Figure 2.1 (but without multiple structures). Indeed, to further confirm the existence of
a potential well, Hirsch did both collimated neutron and gamma measures. As shown in
the paper, he found structure for both consistent with well formation. One possible
explanation is that the ion-electron densities obtained were high enough to "burn out"
(completely ionize) the background neutrals in the potential well. There is no direct
evidence to support this view however.
These important results have never been fully explained. Attempts to reproduce his
experiments were done by Gardner and co-workers at Brigham Young University
(Reference 2.1) who borrowed the original device used by Hirsch. However, despite
many months of effort, the neutron production they obtained was significantly lower
than that reported by Hirsch. They attributed this problem to a failure to regain the gun
alignment necessary to have a highly converged plasma "core" in the center of the
device. A major hurdle to this appears to have been that no provision was made to
allow precision alignment of the gun ions entering the device (although the
investigators did not mention this explicitly). Later when Miley reinitiated gun
experiments, his first gun design followed many of the design elements used by Hirsch,
but incorporated electrostatic beam steering. This worked well, but the design was
eventually discarded to move to RF guns with much higher beam currents. In addition,
the gun design of Figure 1.5 uses a magnetic nozzle for reducing the exiting beam
diameter and to allow strong differential pumping (not used in the prior Hirsch
experiments).
It should be stressed again here that the terms "injector" and "gun" are misleading.
The objective is to simply "flow" low energy ions into the device such that they are then
accelerated to fusion energies by either the grid or the virtual electrode structure. Thus,
a loss of "excess" energy after injection is needed trap the ion, i.e. prevent it from
simply passing through the potential well and hitting the opposite wall. A biased
reflector on the opposite wall can be introduced to help prevent this, but this only works
well if the entering ions have little excess energy. To further understand this problem,
the reader is advised to study the design of the Hirsch chamber of Reference 2.2 which
uses an auxiliary biased grid ("reflector") near the wall. Indeed the issue of how to best
introduce ions into the potential well so that their energy falls below that required to
escape the well is a key for proper design of the IEC. In addition to designs to cause an
initial ion energy loss to "drop" them into the potential well, designs with ion sourced
"imbedded" in the well such that ions are born trapped are discussed later.
Gridded devices for near-term applications such as neutron activation analysis (NAA) do
not rely on virtual well potential traps. Rather, the negative bias of the grid forms a
potential trap, and ions are born within the potential trap by ionization collisions in the
internal plasma discharge.
Note that the electron injected case faces the same problem of getting ions into the
potential trap. The approach used with the Polywell employs an embedded ion source
plus relies on "burn out" densities to eliminate neutrals. Success with this technique,
after many problems, was the key that lead to the "breakthrough" reported by R. W.
Bussard (as noted earlier) just before he passed away.
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