Stripping Mechanisms and Remediation for H$^-$ Beams

TL;DR

This paper analyzes various stripping mechanisms affecting H$^-$ beams in accelerators, discusses their implications for beam loss and activation, and suggests that many stripped particles are safely transported to linac ends, potentially easing loss limits.

Contribution

It provides a comprehensive analysis of stripping mechanisms in H$^-$ accelerators and evaluates the impact of stripped particles on beam loss and machine activation.

Findings

Stripping due to residual gas, blackbody radiation, and electromagnetic fields is significant.

Most stripped H$^0$ particles are lost at linac ends, not near the beam pipe walls.

Understanding stripping pathways can help relax beam loss limits.

Abstract

Negative hydrogen ions are often used for injecting protons from linacs to storage rings via charge-exchange injection. In this process, the two electrons are stripped by a foil or laser to produce protons which can be merged with an existing beam without significantly affecting its dynamics, allowing high intensities of protons to be accumulated. However, this capability comes with the drawback that the outer electron of an H$^-$ ion has a low binding energy and can easily be stripped away prior to injection. This paper addresses the following stripping mechanisms: interactions with residual gas in the beam pipe, blackbody radiation from accelerator components, and electromagnetic fields from accelerator optics (Lorentz-force stripping) and particles within the bunch itself (intrabeam stripping); with a discussion on how to avoid excessive activation from stripped H$^0$ particles and protons. We also demonstrate that the proportion of stripped H$^0$ colliding with a nearby beam pipe or machine-element walls presents only roughly 10\% of those lost in stripping; the remaining stripped particles traverse to the end of a linac or local straight section, which may relax the limits for allowable stripping-based beam loss in H$^-$ accelerators.