Abstract
Investigations of the effects of optical transition radiation (OTR) polarization components on beam profiles are presented. The transverse profiles are examined using the OTR perpendicular and parallel polarization components with respect to the dimension of interest. We observed $\ensuremath{\sim}15%$ projected profile size reductions with the perpendicularly polarized components on a $65\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{m}$ beam image size case at 14 MeV, a $150\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{m}$ beam image size at 4.5 GeV, and a $1100\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{m}$ beam image size at 7 GeV. These effects are all several times larger than expected (and anomalous in this sense) when compared to the standard OTR point-spread function calculations. We propose the time-averaged induced-current distribution which generates the OTR represents the actual beam size more faithfully with the perpendicular polarization component and recommend its routine use and subsequent deconvolution.
Highlights
The characterization of the transverse beam size of relativistic electron beams using optical transition radiation (OTR) imaging [1,2,3,4] has been implemented at many accelerators in the past two decades
The fundamental issue is whether one can detect a measurable difference in beam-profile sizes if one uses the perpendicular component of OTR, and if so, what is the magnitude? We used focusing by upstream quadrupoles to generate narrow vertical and horizontal stripes at the prototype station
(4) The polarizer reduces light transmitted, and the digital processing may underestimate the size. We evaluate this as a small effect and note the lack of any detectable polarization effect with YAG : Ce scintillator light taken with the same system as OTR [19], and the clear increase of the beam size with the parallel component in the Jefferson National Lab (JLAB) data with the initially roundish beam spot
Summary
The characterization of the transverse beam size of relativistic electron beams using optical transition radiation (OTR) imaging [1,2,3,4] has been implemented at many accelerators in the past two decades. We examine empirical evidence [15,16,17,18,19] that the utilization of the polarization component orthogonal to the dimension of interest results in a noticeably smaller observed projected image profile than theoretically predicted [5,6,7,8,9]. In those calculations the OTR point-spread-function width was in the few-micron range for 0.10 rad collection angles so only experiments that involve beam sizes with sigma
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