Space Charge Driven Emittance Growth and the Effect of Octupoles in IOTA

TL;DR

This paper investigates space charge effects on emittance growth in IOTA, comparing theoretical models and simulations, and explores how octupoles can mitigate emittance growth in high-intensity proton beams.

Contribution

It provides a detailed analysis of emittance growth mechanisms and evaluates the effectiveness of octupoles in mitigating this growth within the IOTA accelerator.

Findings

Emittance growth mainly occurs immediately after injection.

Matching at the nonlinear insertion region can reduce emittance growth.

Octupoles in quasi-integrable configurations can mitigate emittance growth.

Abstract

The Integrable Optics Test Accelerator (IOTA) at Fermilab is a small machine dedicated to a broad frontier accelerator physics program. An important aspect of this program is to investigate the potential benefits of the resonance free tune spread achievable with integrable optics to store and accelerate high intensity proton beams for which space charge is significant. In this context, a good understanding of proton beam emittance growth and particle loss mechanisms is essential. Assuming nominal design parameters, simulations show that for a bunched beam, the bulk of emittance growth takes place immediately following injection, typically within tens of turns. We attempt to account for this growth using a simplified RMS mismatch theory; some of its limitations and possible improvements are briefly discussed. We then compare theoretical predictions to simulations performed using the PIC code pyORBIT. Further exploring ways to mitigate emittance growth and reduce particle loss, we compare two beam matching strategies: (1) matching at the injection point (2) matching at the center of the nonlinear (octupole) insertion region where $\beta_x = \beta_y$. To observe how nonlinearity affects emittance growth and whether it dominates growth due to mismatch, we track two different distributions. Finally, we explore the potential of using octupoles in a quasi-integrable configuration to mitigate growth using a variety of initial distributions both at reduced and full intensities.