Space charge dominated momentum spread and compensation strategies in the post-linac section of Proton Improvement Plan-II at Fermilab

TL;DR

This paper analyzes space charge effects on beam momentum spread in Fermilab's PIP-II linac and proposes a mitigation strategy using a debuncher cavity to maintain beam quality.

Contribution

It introduces a novel mitigation approach with a detailed analysis of debuncher cavity parameters to control momentum spread under space charge effects.

Findings

Space charge doubles the momentum spread during beam transport.

A debuncher cavity can effectively reduce the momentum spread to acceptable levels.

Optimized cavity parameters are identified for stable beam operation.

Abstract

The upcoming Proton Improvement Plan-II (PIP-II), designated for enhancements to the Fermilab accelerator complex, features a new 800 MeV superconducting linac and a Beam Transfer Line (BTL) to transport the beam to the existing Booster synchrotron. To mitigate the space charge tune shift associated with a high intensity accumulated beam, the low emittance linac beam is used to paint the ring phase space both transversely and longitudinally. To prevent losses caused by particles injected outside the rf separatrix while painting longitudinal phase space, the momentum spread of the incoming beam should not exceed 2.1 x 10^-4. Detailed simulations showed that due to space charge, the rms momentum spread increases to 4 x 10^-4 while it is transported in the BTL --about twice the allowable limit. In this paper, we outline a mitigation strategy involving a debuncher cavity. We discuss location, operating frequency, and gap voltage under both nominal and perturbed beam conditions, specifically accounting for momentum jitter. The impact of cavity misalignments is also assessed. The paper concludes by recommending an optimized configuration.