Proton deflectometry of a capacitor coil target along two axes

Abstract

A developing application of laser-driven currents is the generation of magnetic fields of picosecond–nanosecond duration with magnitudes exceeding $B=10~\text{T}$ . Single-loop and helical coil targets can direct laser-driven discharge currents along wires to generate spatially uniform, quasi-static magnetic fields on the millimetre scale. Here, we present proton deflectometry across two axes of a single-loop coil ranging from 1 to 2 mm in diameter. Comparison with proton tracking simulations shows that measured magnetic fields are the result of kiloampere currents in the coil and electric charges distributed around the coil target. Using this dual-axis platform for proton deflectometry, robust measurements can be made of the evolution of magnetic fields in a capacitor coil target.

Measuring magnetic fields in laser-driven coils with dual-axis proton deflectometry

“…Here we present additional results and analysis from an experiment conducted on the Vulcan laser and previously reported in [17]. The dual-axis experimental scheme can be seen in figure 2.…”

“…Figure 4 shows how d v varies with proton energy and coil magnetic field. The magnetic field at the coil centre is estimated via B 0 = µ 0 I/2R and the white contour lines represent the range of void widths observed on the Vulcan experiment in [17]. Since the TNSA protons generated during the experiment had energy ϵ p < 15 MeV, the figure suggests that wire currents and on-axis magnetic fields were below ∼25 kA and ∼30 T respectively.…”

“…Spatial dimensions are grossly exaggerated in this image. The proton foil-target distance was 12 mm and the target-RCF distance was 70 mm for a coil magnification of ≈7-this is corrected from a magnification of 10 in [17]. Inset on the right-hand side is a diagram of the coil target.…”

“…When a proton beam passes axially through a current loop, the loop magnetic field causes the beam to rotate through a fixed angle. If a high-Z grid is used to imprint a mesh structure in the beam spatial profile, the rotation of the grid shadow can be measured and an estimate of the magnetic field extracted [17,20,24]. Crucially, grid rotation is independent of radial electric fields and beam divergence angle.…”

“…David possessed an uncommonly clear intellect that helped produce novel measuring instruments [43][44][45] and his ideas have been influential in the fields of ion acceleration [46][47][48], electron dynamics and sheath formation [49,50], charged particle radiography and high-frequency radiation sources [1,37,[51][52][53]. Many of these advances are directly applicable to the physics of 12 We would like to take this opportunity to correct the magnification of the proton deflectometry diagnostic reported in [17]. The magnification was 7not 10 -which means the inferred currents and charges are slightly larger than previously observed.…”