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Schematic of laser and target geometry (a) and positron emittance measurement setup (b) (not to scale). Positron (and electron) source refers to those particles that emerge from the rear surface of the target. In (b), the magnetic field of the spectrometer is in the plane defined by the axes of the laser beam and the spectrometer, and perpendicular to the spectrometer axis. The emittance measurements were made at various positions (denoted A, B, and C) to cover half of the angular divergence of the whole beam, while for a given position each slit covers only a small portion of the beam.
Angular divergence of the positron beam measured at Titan laser with laser energy of ∼300 J, and pulse duration of 10 ps. The positron energy at the peak of distribution was 12 MeV. The inset figure is the angular divergence of positron beam as a function of laser energy. The data for laser energies ∼300 J were obtained at Titan (circles and squares depict data taken in two separate experiments), and for ∼800 J at Omega EP.
A section of image plate showing the electron spectrum taken with a slit array with variable slit-width of 200, 1000, 50, 100, 250 μm (from top to bottom). The slit array was oriented parallel to the magnetic field of the electron-positron spectrometer.
Lineout of the image of the positron beam through a slit array with variable slit-width of 200, 1000, 50, 100, 250 μm, from left to right. The average of δ1 and δ2, taken between 12% and 88% height of the maximum amplitude, was used to infer the source size. The inset shows the measured positron source size as a function of positron energy (taken at the peak of the positron energy distribution) for several laser shots at Titan under various laser conditions.
Summary of the measured emittance of laser-produced positrons in comparison to that of SLC. The open squares and open circles denote the upper limit emittance determined from beam divergence and source size measurements on Titan and Omega EP lasers, respectively. The solid square and solid triangle were derived from 1-D pepper-pot analysis of the Titan data at ∼12 MeV using the measured data points and using a continuous fit of these data points, respectively. The latter two data points were arbitrarily shifted to 11 MeV to make the figure more legible. The horizontal error bars represent the FWHM energy span of the positron beam. The SLC positron emittance and its expansion range are plotted as dashed line and surrounding shadow.
Coupled LSP/ITS simulation of the positron beam angular distribution function versus transverse position without (a) and with (b) electromagnetic fields in the simulation. The plane where the emittance phase diagrams were calculated was 0.5 mm behind the target. Both images are normalized in amplitude to the same color scale.
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