High resolution phase space measurements with Allison-type emittance scanners

Abstract

Allison-type emittance scanners are widely used to measure projected 2D phase space distributions of low energy beams. This paper extends the conventional data analysis model to introduce three significant corrections that commonly arise in the pursuit of high resolution measurements. First, effective longitudinal asymmetry in the E-dipole placement (typically resulting from directional choice of relief cuts in thick slit-plates) causes deviation from the ideal voltage-to-angle conversion relation. Second, finite slit thickness generates variation in weights of data points that should be compensated. Third, when the interval between data points is smaller than the device resolution (ordinary in the angular data accumulation), a detailed account of the phase space region contributing to each data point can be used to resolve the beam distribution more accurately. These findings are illustrated by simulations with numerically generated phase space distributions. The improved model is applied to experimental measurements of an Ar ion beam with an Allison scanner operating at the front-end of the Facility for Rare Isotope Beams (FRIB) at Michigan State University. Results show that the improved model obtains better agreement among a set of measurements and modifies beam moments significantly (can be ∼10% relative to conventional methods, with larger deviations at increasing angular divergence), thus rendering the corrections important for accurate high resolution phase-space characterizations. Python code tools that implement the improved analysis described are made available. These tools are readily applicable to any Allison scanner given a specification of the device geometry and scan ranges associated with each measurement.