Focal spot size predictions for beam transport through a gas-filled reactor Page: 5 of 12
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total path of propagation in gas should be kept to a minimum (zq < 5
meters). In addition, the neck radius is reduced by maintaining large
portholes- The geometric factor R#/«o should be maximized, where
is the beam radius at reactor wall.
2. Proper choice of gas medium. To minimize multiple scattering
effects, one is restricted to a lov-Z material below a few torr. Self-field
effects could be significantly reduced by choosing a good conducting gas,
e.g., Ne and are better conductors than He or Li. One might also
consider mixtures involving more complex gases, e.g., H^O. Self-field
effects are relatively insensitive to gas pressure.
3. Some pulse shaping in the radial direction would be helpful.
Emittance growth is minimized by having a nearly square current profile-
The magnitude of the current in a given beamlet has relatively little effect
on the emittance increase in the classical calculations. Since the change
in beam emittance in the reactor is independent of its initial value, little
is gained by improving beam quality at entrance. Overall, the effects of
initial beam parameters on focal spot size enlargement due to classical
phenomena are secondary. One should note, however, that the beam energy
cannot be lowered much below 10 Gev, since multiple scattering, which is pro-
portional to fS , would become intolerable.
Whereas the reactor geometry and reactor environment are constrained
in the ballistic mode of propagation, the self-focused mode is insensitive
tc the reactor Bize, and can accommodate a large Tange of gaB types and
pressure (‘'-1 to 50 torr). In addition, the small portholes associated with
the self-focused mode ease pumping requirements. The constraint for this
mode is on beam quality. The self-focused beam characteristically expands
at the beam head, and la pinched at the tail. The amount of beam head lost,
and the final pinch radius at the tail are both sensitive to the initial
beam emittance. Our code indicates that with initial emittance of 1 mrad-cm,
Che beam is pinched to a radiua of M nm with only a small portion of the
beam head lost. But the ion beam pinching rapidly degrades as the initial
emittance is increased. Beam radius at injection is also an important
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Yu, S.S.; Lee, E.P. & Buchanan, H.L. Focal spot size predictions for beam transport through a gas-filled reactor, article, January 23, 1980; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1087464/m1/5/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.