Figure 3: Effect of helical coil on TNSA beam.

In comparison with the typical proton-beam profile obtained from a flat-foil target in the experiment shown in (a), (b) shows the beam profile obtained from the helical coil target. Au foils (10 μm thick) were used as the laser interaction target in both cases. The coil was made of 100 μm aluminium wire and had internal diameter, pitch and length of 0.7, ∼0.28 and 8.7 mm, respectively. The RCF stack was placed at 35 mm from the target. RCF images of 50 mm × 50 mm size are shown in a, which are five times larger compared with the RCF images shown in b (the black scale bar on the last piece of RCF in a and b correspond to 10 and 2 mm respectively), in order to account for the large divergence of the TNSA beam produced from the flat foils. The black-dashed circles on the first RCF layer for both a and b correspond to the projection of the exit ring of the coil on the RCF (∼2.8 mm diameter circle corresponding to ∼5° full cone angle). (c) shows the comparison between proton spectra from the reference flat-foil target and the helical coil target shown in b obtained by spectral deconvolution of the RCF signals described in refs 37, 38. The error bars were estimated from the possible error in dose conversion²¹ and uncertainties in background substraction. Since the flat-foil proton signal at ∼8 MeV was below the detection threshold, the solid black circle shows the upper bound for proton signal calculated by considering the detection threshold of the RCF (∼10⁵ protons per MeV mm⁻²) and an overly generous beam size (10 mm diameter on the RCF, which is similar to the beam size at 6.6 MeV shown in a) for protons at that energy. (d) Three-dimensional profile of the pencil beam of protons obtained in the RCF corresponding to ∼9 MeV, where the inset shows the two-dimensional dose map of the central part of the beam. The white dashed circle in the insert represents the internal diameter of the helical coil.