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I. SUMMARY OF RESULTS
Results from calculations of focal spot size for heavy ion beans
transported through a gas-filled reactor are summarized below. Three
transport modes have been studied. The first is the ballistic node Where
beams are injected through large portholes (M5 cm radius) Co converge at
the target. The second is the self-focused mode where beams are injected
through small portholes (sub-cm), arid propagate in their self-generated
pinch field. The third mode is a variant of the second: an electron beam
of modest current is injected from the opposite wall to aid the ion beam
pinching.
The basic tools for our analyses are & set of 1-D codes which
calculate the r.m.s. radius of a beam during its passage through the
reactor. Analytic formulae for the r.m.s. radius of the beam in the target
region were derived on the basis of several approximations which were shown
by the 1-D codes to be reasonable in the range of parameters of interest for
HIF. We have also checked 1-D code results against simulations and 2-D
codes. These codes include only classical effects (no instabilities), and
the predicted focal spot size represent the minimum disruption the beam is
expected to experience.
Ballistic Mode
We find an enlargement of the focal spot size due to multiple
scattering and classical self-field effects. The magnitude of these effects
seem large enough to be of concern for target requirements. However, there
is still sufficient uncertainty in the theory, particularly in connection
with the calculation of beam-generated electrical conductivity, that firm
predictions are not yet possible. Detailed calculations of the electrical
conductivity with Boltzmann codes are in progress. Calculations up to this
point indicate the sensitivity of final focal spot size to various beam and
reactor parameters, and suggest several ways for minimii’.ng the spot size
enlargement:
1. Small reactors. The neck radius of the beam is a very
sensitive function of the reactor size. To keep the neck radius small, the