Low Emittance Growth In a LEBT With Un-Neutralized Section

TL;DR

This paper demonstrates that maintaining an un-neutralized section in a LEBT with proper transition points can significantly reduce emittance growth caused by space charge effects, especially during beam chopping.

Contribution

It presents an experimental scheme at Fermilab's PXIE to preserve low beam emittance by implementing an un-neutralized section in the LEBT, a novel approach for beam transport.

Findings

Low emittance growth achieved in un-neutralized LEBT section

Transition point chosen where the beam has low transverse tails

Experimental validation at Fermilab's PXIE

Abstract

In a Low Energy Beam Transport line (LEBT), the emittance growth due to the beam's own space charge is typically suppressed by way of neutralization from either electrons or ions, which originate from ionization of the background gas. In cases where the beam is chopped, the neutralization pattern changes throughout the beginning of the pulse, causing the Twiss parameters to differ significantly from their steady state values, which, in turn, may result in beam losses downstream. For a modest beam perveance, there is an alternative solution, in which the beam is kept un-neutralized in the portion of the LEBT that contains the chopper. The emittance can be nearly preserved if the transition to the un-neutralized section occurs where the beam exhibits low transverse tails. This report discusses the experimental realization of such a scheme at Fermilab's PXIE, where low beam emittance dilution was demonstrated.