Low Emittance Growth in a LEBT with Un-Neutralized Section

TL;DR

This paper proposes and demonstrates a method to maintain low beam emittance in a LEBT by keeping a section un-neutralized, reducing emittance growth caused by space charge effects during beam chopping.

Contribution

It introduces a novel scheme to preserve beam quality in LEBT by selectively un-neutralizing sections, supported by experimental validation at Fermilab.

Findings

Low emittance dilution was achieved in experiments.

The scheme effectively reduces emittance growth during chopping.

Experimental results confirm the physical model's predictions.

Abstract

In a Low Energy Beam Transport line (LEBT), the emittance growth due to the beam's 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 usually 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 introduces the rationale for the proposed scheme and formulates the physical arguments for it as well as its limitations. An experimental realization of the scheme was carried out at Fermilab's PIP2IT where low beam emittance dilution was demonstrated for a 5 mA, 30 keV $H^{-}$ beam.