Rapid charge redistribution leading to core hollowing in a high-intensity ion beam

K Ruisard,A Aleksandrov

doi:10.1103/physrevaccelbeams.24.014201

K Ruisard, A Aleksandrov

Open Access

https://doi.org/10.1103/physrevaccelbeams.24.014201

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Abstract

Recently, the first direct measurement of a full 6D accelerator beam distribution was reported [1]. That work observed a correlation between energy and transverse coordinates, for which the energy distribution becomes hollowed and double-peaked near the transverse core. In this article, a similar structure is shown to emerge in expansion of an initially uncorrelated, high density bunched beam as the result of velocity perturbation from nonlinear space charge forces. This hollowing is obscured when the 6D phase space is projected onto one- and two-dimensional axes. This phenomenon has not been widely recognized in accelerator systems, but parallels can be drawn to observations of laser-ionized nanoclusters and electron sources for diffraction. While this effect provides insight into the origin of the measured core correlation, it does not provide a complete description. A better reproduction of the measured structure can be obtained via self-consistent simulation through the radio-frequency quadrupole. [1] B. Cathey, S. Cousineau, A. Aleksandrov, and A. Zhukov, PRL 121, 064804 (2018).

Highlights

Understanding beam dynamics in the early stages of capture and acceleration is crucial in high-intensity accelerators
Another parallel can be drawn to the application of laser-wakefield accelerators (LWFA) as electron injectors to colliders or FELs [16,17], where very small emittance leads to significant space charge effect during bunch expansion between plasma boundary and rf structure
Previous studies at the Spallation Neutron Source (SNS) Beam Test Facility (BTF) observed a nonlinear correlation between energy and transverse coordinates

Summary

INTRODUCTION

Understanding beam dynamics in the early stages of capture and acceleration is crucial in high-intensity accelerators. The measurements in [4] revealed a hollowed, bimodal energy distribution for particles near the beam core This feature, which was observed to scale with beam current, was characterized as new and unexpected. It will be shown that the energy splitting leads to “pile up” of excess density at the beam edge as a result of an outwardly-propagating density perturbation This perturbation is launched by nonlinear space charge in the initial distribution which causes the core to expand more rapidly than the edge. As observed in [4], for a 40 mA BTF beam the hollowed core distribution is obscured in the full projection, which has smooth and monotonically decreasing profiles This illustrates that standard characterization of the 2D phase spaces may neglect significant core features, for high-charge bunches (∼6 × 108 ions/bunch in the BTF). VI addresses the implications to simulation and measurement of high-intensity bunched beams

OBSERVATIONS AT THE SNS BEAM TEST FACILITY

SELF FIELD DRIVEN HOLLOWING IN CHARGED PARTICLE BUNCHES

HOLLOWING OF AN INITIALLY GAUSSIAN MEBT BUNCH

RELEVANCE TO MEASUREMENTS

Findings

DISCUSSION