Design and characterization of a neutralized-transport experiment for heavy-ion fusion Page: 2 of 68
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 favored a baseline case using 30 kA of 10-GeV U+1 ions in a moderate vacuum of 10-4
- 10-3 Torr. Numerical and experimental studies indicate that ballistic transport could, in
fact, be feasible. A 1998 scaled experiment  based on the HIBALL-II final-focus
design was performed at Lawrence Berkeley National Laboratory (LBNL), obtaining
excellent agreement between theory and experiment. Also, detailed numerical
simulations of driver-scale systems show good spot sizes for the Olson parameters .
Since the HIBALL-II study, however, several significant shifts in conceptual designs for
HIF drivers have made ballistic chamber transport unfeasible. One change has been the
development of indirect-drive targets , which give a more symmetrical energy
deposition on the deuterium-tritium capsule than the direct-drive HIBALL-II target but
which require more energy to heat the cylindrical metal "hohlraum" enclosing the
capsule. At the same time, driver economics has pushed the HIF program toward beams
with lower kinetic energy, at the cost of requiring higher total beam current. This higher
current can be partly offset by using more beams, but even with an ion mass of 200 amu,
more than 450 beams would be required to reduce space-charge effects sufficiently to
allow ballistic transport. A third development has been the adoption of "thick-liquid"
walls as the mainline US approach to protecting the fusion chamber. In HYLIFE-II 
and more recent designs , jets of molten salt fill much of the volume between the
target and the chamber interior wall and absorb much of the blast wave and the radiation.
The vapor from these jets has a pressure about 0.6 mTorr and consists mainly of Be2F
and LiF. Due to collisional stripping with this gas, a singly charged ion beam develops a
wide spread of charge states by the time it reaches the target, with the average charge
state being about two. The effective perveance of the beam is not increased by a similar
factor because the liberated electrons propagate along with the beam, but the beam itself
becomes much more sensitive to the net space-charge field. Finally, the distributed-
radiator targets [10, 11] developed in recent years exacerbate the problem of beam
focusing by requiring that beam energy be deposited in a narrow annulus on the hohlraum
ends, rather than over the entire end surfaces. The gain for these targets degrades if focal
radii of the beams exceed two millimeters, in effect requiring beams with a low
transverse temperature and net charge.
A recent HIF driver study  reconciles these stringent final-focus requirements. A
pivotal feature of this design is the use of low-density plasma in the beam line between
the final-focus magnets and the chamber to neutralize much of the beam space charge.
[12, 13, 14] If there are enough electrons in the volume swept out by a beam, they can be
trapped by space-charge potential well of the beam and provide neutralization during the
final transport to the target, thus allowing substantial beam currents while still
maintaining the low net charge needed for good focus. In addition, after the target has
been pre-heated by lower-current "foot" pulses, thermal radiation from the target will
photoionize the nearby background gas, providing additional neutralization as the beam
The Neutralized Transport Experiment (NTX) has been designed to study, on a reduced
scale, the physics of the final focus and neutralized transport of beams with high space
charge. The experiment was designed as part of the research program developed by the
Virtual National Laboratory for Heavy-Ion Fusion, a formal collaboration of Lawrence
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Henestroza, E.; Eylon, S.; Roy, P.K.; Yu, S.S.; Anders, A.; Bieniosek, F.M. et al. Design and characterization of a neutralized-transport experiment for heavy-ion fusion, article, March 14, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc787004/m1/2/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.