Centroid and Envelope Eynamics of Charged Particle Beams in an Oscillating Wobbler and External Focusing Lattice for Heavy Ion Fusion Applications Page: 4 of 20

                                
Abstract

Recent heavy ion fusion target studies show that it is possible to achieve ignition
with direct drive and energy gain larger than 100 at 1MJ. To realize these advanced,
high-gain schemes based on direct drive, it is necessary to develop a reliable beam
smoothing technique to mitigate instabilities and facilitate uniform deposition on
the target. The dynamics of the beam centroid can be explored as a possible
beam smoothing technique to achieve a uniform illumination over a suitably chosen
region of the target. The basic idea of this technique is to induce an oscillatory
motion of the centroid for each transverse slice of the beam in such a way that the
centroids of different slices strike different locations on the target. The centroid
dynamics is controlled by a set of biased electrical plates called "wobblers". Using
a model based on moments of the Vlasov-Maxwell equations, we show that the
wobbler deflection force acts only on the centroid motion, and that the envelope
dynamics are independent of the wobbler fields. If the conducting wall is far away
from the beam, then the envelope dynamics and centroid dynamics are completely
decoupled. This is a preferred situation for the beam wobbling technique, because
the wobbler system can be designed to generate the desired centroid motion on the
target without considering its effects on the envelope and emittance. A conceptual
design of the wobbler system for a heavy ion fusion driver is briefly summarized.

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