Abstract
Heavy ion drivers for warm dense matter and heavy ion fusion applications use intense charge bunches which must undergo transverse and longitudinal compression in order to meet the requisite high current densities and short pulse durations desired at the target. The neutralized drift compression experiment (NDCX) at the Lawrence Berkeley National Laboratory is used to study the longitudinal neutralized drift compression of a space-charge-dominated ion beam, which occurs due to an imposed longitudinal velocity tilt and subsequent neutralization of the beam's space charge by background plasma. Reduced theoretical models have been used in order to describe the realistic propagation of an intense charge bunch through the NDCX device. A warm-fluid model is presented as a tractable computational tool for investigating the nonideal effects associated with the experimental acceleration gap geometry and voltage waveform of the induction module, which acts as a means to pulse shape both the velocity and line density profiles. Self-similar drift compression solutions can be realized in order to transversely focus the entire charge bunch to the same focal plane in upcoming simultaneous transverse and longitudinal focusing experiments. A kinetic formalism based on the Vlasov equation has been employed in order to show that the peaks in the experimental current profiles are a result of the fact that only the central portion of the beam contributes effectively to the main compressed pulse. Significant portions of the charge bunch reside in the nonlinearly compressing part of the ion beam because of deviations between the experimental and ideal velocity tilts. Those regions form a pedestal of current around the central peak, thereby decreasing the amount of achievable longitudinal compression and increasing the pulse durations achieved at the focal plane. A hybrid fluid-Vlasov model which retains the advantages of both the fluid and kinetic approaches has been implemented to describe the formation of pedestals in the current profiles. The comparison between the experimental measurements and the various theoretical models is excellent.
Highlights
The final transport section leading to the target offers a significant challenge for the development of heavy ion drivers for applications such as high energy density physics, warm dense matter, and heavy ion fusion [1,2,3]
This paper has demonstrated that reduced theoretical models can be effective in describing the longitudinal compression of an intense charge bunch in neutralizing background plasma
II) as a tractable computational tool for investigating nonideal effects associated with the longitudinal compression of intense ion beams to high current densities
Summary
The final transport section leading to the target offers a significant challenge for the development of heavy ion drivers for applications such as high energy density physics, warm dense matter, and heavy ion fusion [1,2,3]. Intense ion beams [4 –6] are transversely focused to final diameters less than a few mm [7,8] and longitudinally focused to pulse durations less than 10 ns Maximizing such compression offers the potential of compressing heavy ion beams to very high current densities. Longitudinal focusing is achieved by applying a time-dependent velocity tilt to an intense charge bunch and subsequently allowing it to drift through a neutralizing background plasma. The relative power, of the charge bunch grows in the intermediate region of the applied tilt, where the decelerated head meets the accelerated tail, while the beam drifts through a background plasma, the phrase ‘‘neutralized drift compression’’ explicitly refers to this process.
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