Dispersion Calibration for the National Ignition Facility Electron Positron Proton Spectrometers for Intense Laser Matter Interactions

TL;DR

This paper calibrates the dispersion of the NIF Electron Positron Proton Spectrometers using electron beams and Monte Carlo simulations to improve measurement accuracy in intense laser-matter experiments.

Contribution

It introduces a calibration method for the EPPS dispersion curves using experimental data and simulations, enhancing measurement reliability.

Findings

Established accurate relationship between magnet current and electron energy.

Determined energy distributions of electron beams at the spectrometer slit.

Provided improved dispersion curves for the EPPS.

Abstract

Electron-positron pairs, produced in intense laser-solid interactions, are diagnosed using magnetic spectrometers with image plates, such as the National Ignition Facility (NIF) Electron Positron Proton Spectrometers (EPPS). Although modeling can help infer the quantitative value, the accuracy of the models needs to be verified to ensure measurement quality. The dispersion of low-energy electrons and positrons may be affected by fringe magnetic fields near the entrance of the EPPS. We have calibrated the EPPS with six electron beams from a Siemens Oncor linear accelerator (linac) ranging in energy from $2.7$--$15.2$ $\mathrm{MeV}$ as they enter the spectrometer. A Geant4 TOPAS Monte-Carlo simulation was set up to match depth dose curves and lateral profiles measured in water at $100$ $\mathrm{cm}$ source-surface distance. An accurate relationship was established between the bending magnet current setting and the energy of the electron beam at the exit window. The simulations and measurements were used to determine the energy distributions of the six electron beams at the EPPS slit. Analysis of the scanned image plates together with the determined energy distribution arriving in the spectrometer provide improved dispersion curves for the EPPS.