Dispersion calibration for the National Ignition Facility electron–positron–proton spectrometers for intense laser matter interactions

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 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 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.

Confinement of relativistic electrons in a magnetic mirror en route to a magnetized relativistic pair plasma

“…as derived from Tanaka et al 44 Two super-Gaussians are fitted to the background and signal on each image plate in order to remove the background and determine the projected slit width. 45 Calibrations of the absolute dose 46 and dispersion with measurements in the expected trapped particle energies, 47 3-15 MeV, provide absolute electron energy spectra. Without the magnetic field, the electrons exit mostly radially, normal to the target surface.…”

The influence of laser focusing conditions on the direct laser acceleration of electrons

“…The range of peak vacuum intensities was (0.15-9.0) × 10 20 Wcm −2 , corresponding to an a 0 range of 3.4-27. The electron energy distributions were measured along the laser axis using a magnetic electron-positron-proton particle spectrometer (EPPS) [22] with an energy coverage from 1-150 MeV and the energy uncertainty range from 2% at low energy end to up to 30% at the high energy end [23].…”

Annihilation-gamma-based diagnostic techniques for magnetically confined electron–positron pair plasma

“…Magnetic spectrometers have been used to diagnose the energy distribution of relativistic pair beams (von der Linden et al. 2021 b ). With high magnetic fields and temperatures, measuring cyclotron emission may be possible.…”

“…With no partially ionized species it will also not be possible to collect passive emission from plasma constituents (although spectroscopy of the neutral bound states of positronium may be possible; Mills 2014). Magnetic spectrometers have been used to diagnose the energy distribution of relativistic pair beams (von der Linden et al 2021b). With high magnetic fields and temperatures, measuring cyclotron emission may be possible.…”