Angular-Resolved Thomson Parabola Spectrometer for Laser-Driven Ion Accelerators

Abstract

This article reports the development, construction, and experimental test of an angle-resolved Thomson parabola (TP) spectrometer for laser-accelerated multi-MeV ion beams in order to distinguish between ionic species with different charge-to-mass ratio. High repetition rate (HHR) compatibility is guaranteed by the use of a microchannel plate (MCP) as active particle detector. The angular resolving power, which is achieved due to an array of entrance pinholes, can be simply adjusted by modifying the geometry of the experiment and/or the pinhole array itself. The analysis procedure allows for different ion traces to cross on the detector plane, which greatly enhances the flexibility and capabilities of the detector. A full characterization of the TP magnetic field is implemented into a relativistic code developed for the trajectory calculation of each pinhole beamlet. We describe the first test of the spectrometer at the 1PW VEGA 3 laser facility at CLPU, Salamanca (Spain), where up to 15MeV protons and carbon ions from a 3μm laser-irradiated Al foil are detected.

Keywords: charged-particle spectroscopy; instrumentation; ion beams; plama diagnostics.

Figure 1

Multi-pinhole TP spectrometer design.

Figure 2

Measured magnetic field distribution. Ions propagate in z-direction through the field towards the MCP, deflected on the y-direction by the Lorentz force.

Figure 3

Multi-pinhole Thomson parabola traces obtained from a single laser shot at VEGA 3. The origin of the coordinate system corresponds to the zero-deflection point of the central beamlet. ${\mathrm{P}}_{\mathrm{L}}$, ${\mathrm{P}}_{\mathrm{C}}$, and ${\mathrm{P}}_{\mathrm{R}}$ corresponds to the tracks related to the left, central, and right beamlets, respectively. ${\mathrm{C}}^{\mathrm{n}+}$ indicates the three $\mathrm{n}$-charged ion traces. The halo at the bottom left corner is originated by a leak of white light from plasma emission, which reaches the MCP through the junction of MCP structure and shielding.

Figure 4

Left trace proton spectrum in logarithmic scale. Blue: corrected spectrum. Red: same spectrum showing the peaks subtracted, corresponding to trace crosses. Several representative error bars are plotted, picturing the estimated energy resolution.

Figure 5

Left, center, and right proton beamlets spectra in the logarithmic scale. Dashed lines show the reconstructed spectrum range. It is important to note that the crosstalk between traces lays at different energy locations for each beamlet. This effect is clear from Figure 3; the relative horizontal position of each proton trace makes the crosses appear at different energetic levels. Note for instance that the proton right trace does not suffer any crosstalk; thus, no correction is applied.

Figure 6

Left, center, and right ${\mathrm{C}}^{4+}$ beamlets spectra in the logarithmic scale. Dashed lines show the reconstructed spectrum range.

Figure 7

Left beamlets spectra for protons and ${\mathrm{C}}^{4+}$, ${\mathrm{C}}^{5+}$, and ${\mathrm{C}}^{6+}$ ions in the logarithmic scale. Dashed lines show the reconstructed spectrum range.