Abstract
Optical transition radiation (OTR) has proven to be a versatile and effective diagnostic for measuring the profile, divergence, and emittance of relativistic electron beams with a wide range of parameters. Diagnosis of the divergence of modern high brightness beams is especially well suited to OTR interference (OTRI) techniques, where multiple dielectric or metal foils are used to generate a spatially coherent interference pattern. Theoretical analysis of measured OTR and OTRI patterns allows precise measurement of electron beam emittance characteristics. Here we describe an extension of this technique to allow mapping of divergence characteristics as a function of transverse coordinates within a measured beam. We present the first experimental analysis of the transverse phase space of an electron beam using all optical techniques. Comparing an optically masked portion of the beam to the entire beam, we measure different angular spread and average direction of the particles. Direct measurement of the phase-space ellipse tilt angle has been demonstrated using this optical masking technique.
Highlights
Transition radiation (TR) is produced by a charged particle as it passes between media with different dielectric constants, for example, a metallic or dielectric foil in vacuum
Two important features can be measured from the OTR interference (OTRI) angular pattern: (i) the interference fringe visibility characterizes the divergence distribution width for a given spatial portion of a charged particle beam profile, in analogy to beamlet expansion in the pepper-pot technique; (ii) a centroid shift of the OTRI angular pattern represents a change in average direction for a given beam cross section, in analogy to pepper-pot beamlet separation
The mask was positioned in the image plane of the second Optical transition radiation (OTR) foil, and its aperture defined the portion of the beam whose TR light was used to measure the interference pattern
Summary
Transition radiation (TR) is produced by a charged particle as it passes between media with different dielectric constants, for example, a metallic or dielectric foil in vacuum. Two important features can be measured from the OTRI angular pattern: (i) the interference fringe visibility characterizes the divergence distribution width for a given spatial portion of a charged particle beam profile, in analogy to beamlet expansion in the pepper-pot technique; (ii) a centroid shift of the OTRI angular pattern represents a change in average direction for a given beam cross section, in analogy to pepper-pot beamlet separation. Measurement of both the divergence width and divergence centroid as a function of transverse spatial coordinates can be used to completely reconstruct the transverse phase-space ellipse.
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