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Abstract
Current density compression of intense ion beams in space and time is required for heavy ion fusion, in order to achieve the necessary intensities to implode an inertial confinement fusion target. Longitudinal compression to high current in a short pulse is achieved by imposing a velocity tilt upon the space-charge-dominated charge bunch, and a variety of means exist for simultaneous transverse focusing to a coincident focal plane. Compression to the desired levels requires sufficient neutralization of the beam by a pre-formed plasma during final transport. The physics of current density compression is studied in scaled experiments relevant for the operating regime of a heavy ion driver, and related theory and advanced particle-in-cell simulations provide valuable insight into the physical and technological limitations involved.
A fast Faraday cup measures longitudinal compression ratios greater than 50 with pulse durations less than 5 ns, in excellent agreement with reduced models and sophisticated simulations, which account for many experimental parameters and effects. The detailed physics of achieving current density compression in the laboratory is reviewed. Quantitative examples explore the dependency of longitudinal compression on effects such as the finite-size acceleration gap, voltage waveform accuracy, variation in initial beam temperature, pulse length, intended fractional velocity tilt, and energy uncertainty, as well as aberration within focusing elements and plasma neutralization processes. In addition, plasma evolution in experimental sources responsible for the degree of beam neutralization is studied numerically, since compression stagnation occurs under inadequate neutralization conditions, which may excite nonlinear collective excitations due to beam-plasma interactions.
The design of simultaneous focusing experiments using both existing and upgraded hardware is provided, and parametric variations important for compression physics are investigated. Current density compression factors from 10 3 to over 105 can be realized, depending on the optimization of various sensitive components for a given set of experimental constraints and system parameters. Since a heavy ion beam driver becomes more compact and cost-effective than previously envisioned as the amount of compression increases, the physics foundation of simultaneous beam focusing has near-term applicability to warm dense matter studies, and heavy ion fusion in the longer term.
