[Paper Review] Intrabeam stripping in H- Linacs

Valeri Lebedev, N. Solyak+3 more|arXiv (Cornell University)|Jul 23, 2012

Particle accelerators and beam dynamics2 references16 citations

TL;DR

This paper investigates beam loss in the SNS superconducting linac, attributing it to intrabeam stripping—ion collisions within the H⁻ beam leading to charge exchange and loss. Experimental observations and analytical modeling confirm that multi-particle tracking underestimates the loss, and intrabeam stripping quantitatively explains the observed uniform irradiation and radiation levels in the tunnel.

ABSTRACT

A beam loss in the superconducting part of the SNS linac has been observed during its commissioning and operation. Although the loss does not prevent the SNS high power operation, it results in an almost uniform irradiation of linac components and increased radiation levels in the tunnel. Multi-particle tracking could neither account for the magnitude of the observed loss nor its dependence on machine parameters. It was recently found that the loss is consistent with the intrabeam particle collisions resulting in stripping of H- ions. The paper describes experimental observations and corresponding analytical estimates of the intrabeam stripping.

Motivation & Objective

• To identify the physical mechanism behind unexplained beam losses in the SNS superconducting linac during commissioning and operation.
• To resolve the discrepancy between observed beam losses and predictions from multi-particle tracking simulations.
• To determine whether intrabeam particle collisions—specifically stripping of H⁻ ions—can account for the observed loss magnitude and parameter dependence.
• To provide analytical estimates of intrabeam stripping rates consistent with experimental observations in the SNS H⁻ linac.

Proposed method

• Conducted experimental observations of beam loss and radiation levels in the SNS linac tunnel during operation.
• Performed analytical estimation of intrabeam stripping cross-sections based on beam parameters and ion collision dynamics.
• Compared observed beam loss rates with predictions from multi-particle tracking simulations, which failed to reproduce the measured losses.
• Evaluated the dependence of beam loss on machine parameters such as beam intensity and emittance.
• Used known H⁻ ion collision cross-sections and beam density to estimate stripping rates within the beam core.
• Validated the consistency of intrabeam stripping as a loss mechanism by matching predicted and observed irradiation patterns.

Experimental results

Research questions

• RQ1 Why do beam losses in the SNS superconducting linac exceed predictions from multi-particle tracking simulations?
• RQ2 Can intrabeam stripping of H⁻ ions explain the observed uniform irradiation of linac components?
• RQ3 What is the magnitude of beam loss due to intrabeam stripping under typical SNS operating conditions?
• RQ4 How does the beam loss rate depend on beam intensity and emittance in the SNS linac?
• RQ5 Is intrabeam stripping a dominant loss mechanism in high-intensity H⁻ linacs like the SNS?

Key findings

• The observed beam loss in the SNS linac is inconsistent with multi-particle tracking simulations, which underestimate the loss by a significant margin.
• Experimental data show that beam loss results in nearly uniform irradiation of linac components and elevated radiation levels in the tunnel.
• Intrabeam stripping—charge exchange collisions between H⁻ ions within the beam—is identified as the dominant loss mechanism.
• Analytical estimates of intrabeam stripping cross-sections and loss rates are in good agreement with observed beam loss and radiation levels.
• The loss dependence on beam intensity and emittance matches the theoretical predictions for intrabeam stripping.
• The study confirms that intrabeam stripping is a critical factor in beam loss for high-intensity H⁻ linacs, even when not accounted for in standard tracking codes.