Proton deflectometry with in situ x-ray reference for absolute measurement of electromagnetic fields in high-energy-density plasmas

Abstract

We report a technique of proton deflectometry that uses a grid and an in situ reference x-ray grid image for precise measurements of magnetic fields in high-energy-density plasmas. A D3He fusion implosion provides a bright point source of both protons and x-rays, which is split into beamlets by a grid. The protons undergo deflections as they propagate through the plasma region of interest, whereas the x-rays travel along straight lines. The x-ray image, therefore, provides a zero-deflection reference image. The line-integrated magnetic fields are inferred from the shifts of beamlets between the deflected (proton) and reference (x-ray) images. We developed a system for analysis of these data, including automatic algorithms to find beamlet locations and to calculate their deflections from the reference image. The technique is verified in an experiment performed at OMEGA to measure a nonuniform magnetic field in vacuum and then applied to observe the interaction of an expanding plasma plume with the magnetic field.

Proton imaging of high-energy-density laboratory plasmas

“…A variation on the beamlet technique is to use an object (such as a mesh, mask, or pepperpot) to subaperture the proton beam into many beamlets (Sokollik et al, 2008;Johnson et al, 2022), down to a few "pencil" beamlets (Lu et al, 2020), or even just down to a single beamlet (Chen et al, 2020). This allows one to probe areas of specific interest in a limited fashion that is more easily detectable (in terms of deflection) or to streak the beamlet in time.…”

“…Determining the exact projection of the initial profile in the absence of any deflections is not always a trivial matter, because confounding factors such as imperfect target fabrication can mean that a deflectometry grid's position is not always consistent from shot to shot. Blurring of the mesh due to the ablation of actual physical grids by strong x-ray radiation that inevitably arises during the course of laser-plasma experiments can also inhibit successful tracking of the grid's distortion (Johnson et al, 2022;Malko et al, 2022). In some circumstances, the grid itself can become charged, resulting in apparently distorted grids when there is in fact no interaction of the proton beam with plasma electromagnetic fields (Palmer et al, 2019).…”

“…Another key opportunity is to combine information from both x-ray and proton images (Orimo et al, 2007;Johnson et al, 2022). In this technique, the last piece in an RCF or CR-39 detector stack is an image plate (see Fig.…”

Design of proton deflectometry with in situ X-ray fiducial for magnetized HED systems

“…2 (top) and features the 3 MeV proton image (left), the 15 MeV proton image (center), and the X-ray image of the mesh (right). A detailed explanation of the data analysis of this diagnostic technique is presented in [17], and we summarize the main components here.…”

“…Here we report the design and implementation of a platform at the Omega laser facility for measuring non-uniform magnetic fields using proton deflectometry that includes an in situ X-ray reference of the mesh [17]. The technique is based on using X-rays and protons generated from the same source.…”