Proton Dynamics Scenarios in the Integrable Optics Test Accelerator (IOTA) at Fermilab

TL;DR

This study analyzes proton dynamics in the Integrable Optics Test Accelerator (IOTA) at Fermilab using a $2.5$ MeV beam to explore space-charge, impedance, and nonlinear optics effects in two lattice configurations: the Danilov-Nagaitsev nonlinear integrable optics setup and an ECOOL electron-cooling lattice. It combines injector parameter modeling, linear optics design, and self-consistent space-charge simulations (2.5D PIC in PyORBIT and IBS in MAD-X) to estimate emittance-growth rates and beam lifetimes across varying gas pressures, lattice apertures, and beam intensities. The results show residual-gas scattering as a baseline emittance-growth and loss mechanism, with lifetimes sensitive to vacuum composition and aperture, while IBS and space-charge dominate at higher intensities, leading to rapid deterioration unless mitigated. The findings provide concrete guidance for commissioning protons at IOTA, inform strategies to mitigate heating and loss (e.g., bake-out, electron cooling, and resonance compensation), and support planning for future high-intensity hadron accelerator concepts.

Abstract

The Integrable Optics Test Accelerator (IOTA) at Fermilab provides a versatile platform for studying the interplay of space-charge, impedance, and non-linear optics in high-intensity hadron beams within synchrotrons and storage rings. This report examines the parameters and dynamics of 2.5~MeV proton beam operations in two configurations of the bare IOTA lattice: one for demonstrating Non-linear Integrable Optics with the Danilov-Nagaitsev magnet, and the other for use with electron cooling. We offer order-of-magnitude estimates of the transverse emittance growth rate as a function of beam intensity, highlighting contributions from residual gas scattering, intra-beam scattering, and space-charge effects. Under nominal conditions, the beam lifetime is projected to be less than 7~minutes at low intensity with the current vacuum quality, and fewer than 100,000~turns at high intensity due to strong space-charge effects. The calculations presented here will guide strategies to mitigate emittance growth and inform future IOTA experiments.

Figures (15)

• Figure 1: Layout of the Integrable Optics Test AcceleratorAntipov2017 without special non-linear magnets in sections BL and BR. The beam moves clockwise. The blue arrow indicates the propagation of electrons in the proposed cooler to be situated in section DR.
• Figure 2: Normalized rms emittance of protons as a function of beam current as measured in a previous test of the duoplasmatron source and the Low Energy Beam Transport section. Figure reproduced from Wai-Ming Tam's PhD thesisTam2010.
• Figure 3: Twiss functions for two sets of lattice configurations. Panels (a) and (b) depict horizontal and vertical betatron amplitudes, while panels (c) and (d) display horizontal and vertical dispersions as functions of position, respectively. The black solid lines represent the DN lattice, the orange lines denote the reference ECOOL lattice, and the blue lines illustrate other members of the ensemble of ECOOL lattices. Panel (e) shows the aperture radius as a function of position in the ring.
• Figure 4: Equilibrium momentum spread ($\sigma_{\delta,\mathrm{eq}}$) as a function of final rf voltage ($V_1$) in the DN lattice after completing the adiabatic capture process. Each gray band provides the range of $\sigma_{\delta,\mathrm{eq}}$ as a function of $V_1$ corresponding to the span of injected beam energies. The blue shaded region encompasses all configurations where the final RF voltage exceeds the minimum capture voltage. The star denotes the reference values of capture voltage and final momentum spread.
• Figure 5: Partial pressure of residual gas as a function of molecular weight as measured by a Residual Gas Analyzer mounted in IOTA.