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 $\ensuremath{\sim}10%$ 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.
Highlights
Allison-type emittance scanners [1], or Allison scanners for short, are widely used to efficiently measure projected 2D phase space distributions of low-energy beams
The scanner assembly is translated mechanically to change the slit position, and the dipole voltage V0 is swept at each step to select transmittable angles by varying the bending strength
Assuming that the phase space distribution is uniform over the acceptance region, an ensemble of incident particles are generated under the following conditions
Summary
Allison-type emittance scanners [1], or Allison scanners for short, are widely used to efficiently measure projected 2D phase space distributions of low-energy beams. The scanner samples one grid point in phase space for each coordinate and “E-dipole” voltage setting, with the beam. The entire projected distribution is measured by recording currents collected over a range of position and voltage values that samples the full phasespace projection. After designating the beam region, all exterior data points are used to characterize the background. In this example, the background noise has an average μ 1⁄4 −0.778 nA with standard deviation σ 1⁄4 0.079 nA.
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