Focal spot size predictions for beam transport through a gas-filled reactor Page: 6 of 12
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parameter. The optimal beam size at entrance is a few millimeters.
Generally, the more current there is in a given beaalet, the tighter is the
final pinch. However, beam current is not as critical a parameter as the
beam quality in determining the final beam radius.
The requirement on beam quality could be significantly eased by the
injection of an electron beam from the opposite wall simultaneously with or
prior to the injection of the heavy ion beam. Our code indicates that vast
improvements in the ion beam pinching is achieved with a modest electron
beam current. With a 3 kA/co e-beam, a 3 tarad-cm ion beam is pinched to
slightly larger than 1 ran at the beam tail with little loss of the beam
head. The stable propagation of such a e-beam has been demonstrated
The basic ansatz for our beam transport studies is the envelope
equation which describes the evolution of the r.m.s. radius R of a beam
segment (characterized by x = (Jet, the distance from the beam head) as a
function of z, the distance from the reactor vail:
The envelope equation is quite general, and is derived by taking appropriate
moments of the particle equations of motion. The complications lie in
evaluating the evolving emittance E, and the pinch force, characterized by
the average betatron oscillation = I Jl., where I „ and
P net A’ net
1^ are the net current and Alfven current, respectively. Since the
physics of the ballistic mode and the pinched modes are quite different,
they will be discussed separately.
Only the effects of multiple scattering and classical pinch fields are
included in. the present study. The classical field effects arise from
incomplete current neutralization, which results in self-magnetic fieldB
that deflect beam particles. Degradation of the focal spot size happens
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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/m1/6/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.