IOTA Experiment for Proton Pulse Compression at Extreme Space-Charge

TL;DR

The paper addresses the challenge of compressing high-intensity space-charge-dominated proton bunches for future muon colliders. It proposes the FAST/IOTA Bunch Rotation Experiment (FIBRE) at Fermilab's IOTA to test snap-bunch rotation via adiabatic capture and rapid RF-voltage ramping, supported by turn-by-turn diagnostics and benchmarks against 3D space-charge simulations in ImpactX. Simulations suggest the central portion of the bunch can be shortened by a factor of about two even under strong space-charge, but longitudinal defocusing and large space-charge tune depressions complicate optimization, especially at higher currents and for larger compression factors. The work highlights the interplay between RF manipulation, phase-slip behavior, and space-charge dynamics, offering insights for designing compressor rings in a potential muon collider and suggesting mitigation pathways like inductive inserts for future improvements.

Abstract

The longitudinal compression of high-intensity, space-charge-dominated proton bunches is a critical requirement for future proton-driven muon colliders. We propose a proton bunch compression experiment at the Integrable Optics Test Accelerator (IOTA) storage ring at Fermilab to investigate optimal radio-frequency (RF) cavity parameters and lattice configurations. IOTA is a compact, fixed-energy storage ring dedicated to beam physics Research and Development and capable of circulating a 2.5 MeV proton beam under extreme space-charge conditions. Using the ImpactX code with its 3D space-charge solver, simulations indicate that the bunch length can be rapidly reduced by at least a factor of two without appreciable degradation of transverse beam quality--even in the strong space-charge regime. However, longitudinal defocusing due to the space-charge remains a significant challenge in short-pulsed intense proton bunches, and the optimization of compression under these conditions is discussed.

Figures (14)

• Figure 1: Schematic layout of the Muon Collider complex (adapted from boscolo2019future).
• Figure 2: IOTA storage ring and proton injector line layout (from Jinst).
• Figure 3: Time evolution of RF cavity voltage: during adiabatic capture ($0 - t_1$), stationary bucket to allow beam to reach equilibrium ($t_1 - t_2$), and finally snap rotation ($t_2 - t_3$).
• Figure 4: 2D projections of the initial 6D distribution of the beam along with 1D projections on each axis. Panel (a.) is the horizontal phase space $\Delta p_x - \Delta x$, panel (b.) is the vertical phase space $\Delta p_y - \Delta y$, and panel (c.) is the longitudinal phase space $\Delta p_t$$/p_0 - \Delta \beta ct$. The color indicates density, which has been normalized to 1.
• Figure 5: Evolution of the longitudinal rms bunch length $\sigma_{z,95\%}$, computed over adiabatic capture, a period of stationary RF, and then snap rotation. The vertical dotted line in the section on the left marks the transition from adiabatic capture/bunching to stationary RF, followed by snap rotation, which occupies the section on the right.