A quasi-monoenergetic short time duration compact proton source for probing high energy density states of matter

Abstract

We report on the development of a highly directional, narrow energy band, short time duration proton beam operating at high repetition rate. The protons are generated with an ultrashort-pulse laser interacting with a solid target and converted to a pencil-like narrow-band beam using a compact magnet-based energy selector. We experimentally demonstrate the production of a proton beam with an energy of 500 keV and energy spread well below 10[Formula: see text], and a pulse duration of 260 ps. The energy loss of this beam is measured in a 2 [Formula: see text]m thick solid Mylar target and found to be in good agreement with the theoretical predictions. The short time duration of the proton pulse makes it particularly well suited for applications involving the probing of highly transient plasma states produced in laser-matter interaction experiments. This proton source is particularly relevant for measurements of the proton stopping power in high energy density plasmas and warm dense matter.

Figure 1

Scheme of the experimental setup. It can be divided into three stages: proton generation (i), energy selection using magnet based proton selector (ii) and proton energy measurement with the magnet spectrometer (iii).

Figure 2

MCP traces in three configurations: (a) Full proton beam with no magnetic selector in (full TNSA spectrum). (b) Partial selection magnetic selector in with only the 20 $\mathrm{\mu }$m entrance slit. (c) Full selection with 20 $\mathrm{\mu }$m entrance slit and 20 $\mathrm{\mu }$m exit pinhole). (d) Selected beam (c) after passing through a solid 2 $\mathrm{\mu }$m Mylar foil.

Figure 3

Spatial sampling of experimental spectra measured with magnet spectrometer. (a) Several example spectra obtained in configuration of no selection featuring reference initial proton spectrum with cut-off energy of $\sim$ 4 MeV (3000 V MCP Voltage). (b) The selected proton beam spectra (light-grey curves) obtained in full selection using 20 $\mathrm{\mu }$m entrance slit and 20 $\mathrm{\mu }$m exit pinhole (5000 V MCP Voltage). The chosen shot represents the typical spectrum (red solid curve). (c) Comparison of the chosen shot with synthetic proton spectrum obtained with MC FLUKA simulations at the exit pinhole (green dashed curve) and at spectrometer position (black dashed curve) in corresponding experimental geometry.

Figure 4

(a) Sketch of the point like pinhole magnification system. (b) Typical proton signal on MCP obtained with configuration of 20 $\mathrm{\mu }$m slit and 20 $\mathrm{\mu }$m pinhole that corresponds to 44 keV energy bandwidth (FWHM) and proton trace width of $\sim$ 0.98 mm. (c) Proton signal on MCP obtained with FLUKA simulation reproducing an experimental result of energy bandwidth of 44 keV (FWHM) and width of $\sim$ 1 mm using initial source size of 150 $\mathrm{\mu }$m.

Figure 5

Scheme of maximum and minimum allowed energies due to horizontal divergence. Estimated values using simple gyroradius model in a square B field are given.

Figure 6

The experimental initial proton spectrum (red curve) and the downshifted proton spectrum after passing through 2 $\mathrm{\mu }$m Mylar coated with 40 nm Aluminum (black curve). The additional spectra (grey curves) are shown to demonstrate the repetitive behaviour. The simulated downshifted spectrum (dashed blue curve) is obtained with FLUKA MC simulation.

Figure 7

Monte-Carlo simulations performed with the FLUKA MC code. (a) The protons enter through the entrance slit, are deflected by the $B_y=1.2$ T magnetic field, and 500 keV protons are selected by the exit pinhole. (b) Propagation of protons and deflection by the $B_x=0.2$ T magnetic field of MCP spectrometer. (c) and (d) Equivalent simulations including a 2 $\mathrm{\mu }$m Mylar target. Scattering in the target increases the proton divergence and only part of the beam is sampled by the entrance pinhole of the spectrometer.

Figure 8

Scheme of the energy selection of 0.5 MeV proton beam with the energy spatial correlation.

Figure 9

The design of magnet with entrance slit for the energy selector. 2D plot of the magnetic field generated by the dipole magnet.

Figure 10

Sketch of the spectrometer. Simple expression of dispersion is given as a guide for the reader using a constant magnetic field gyroradius method.