Analysis of the blowout plasma wakefields produced by drive beams with elliptical symmetry

TL;DR

This paper investigates how elliptical drive beams create asymmetric plasma wakefields in PWFA, revealing elliptical bubble shapes and focusing fields, and extends symmetric theory to account for these asymmetries.

Contribution

It introduces an extension of plasma wakefield theory to elliptical blowout regimes, analyzing asymmetric bubble shapes and focusing fields.

Findings

Elliptical plasma bubbles are produced by asymmetric drive beams.

Focusing fields in the elliptical bubble are linear in both transverse directions.

The study extends symmetric wakefield theory to elliptical geometries.

Abstract

In the underdense (blowout) regime of plasma wakefield acceleration (PWFA), the particle beam is denser than the plasma. Under these conditions, the plasma electrons are nearly completely rarefacted from the beam channel, resulting in a nominally uniform ion column. Extensive investigations of this interaction assuming axisymmetry have been undertaken. However, the plasma blowout produced by a transversely asymmetric driver possesses quite different characteristics. They create an asymmetric plasma rarefaction region (bubble) which leads to asymmetric focusing in the two transverse planes. This is also accompanied by an undesired non-uniform accelerating gradient. The asymmetric blowout cross-section is found through simulation to be elliptical, and treating it as such permits a simple extension of the symmetric theory. In particular, focusing fields linear in both transverse directions exist in the bubble. The form of the wake potential and the concomitant matching conditions in this elliptical cavity are discussed in this paper. We also discuss bubble boundary estimation in the long driver limit and applications of the asymmetric features of the wakefield.