Fig. 1. (a) An example side profile image of a helical coil target. The inset shows the same coil viewed through the coil axis. For consistency, the first and final turns of the coil were kept on the same side for all targets. (b) Schematic of the experimental setup. Helical coils, 8 mm in length, with a delay line attached between the target foil and coil, were positioned at the laser focus. A Thomson parabola spectrometer (TPS) was positioned approximately 67 cm from the interaction point. A stack of radiochromic film (RCF) with very coarse energy resolution and a 3 mm diameter pinhole drilled through the centre was placed in front of the TPS pinhole, to determine beam pointing across the large distance.

Fig. 2. (a)–(d), (e)–(h) Dose-converted scans of the RCF stack in front of the TPS from two helical coil shots (labelled as shots 1 and 2, respectively) with similar laser and target parameters, as described in the text. Bragg peak energies are displayed for each layer. The hole in the centre was drilled to allow protons to reach the TPS. The red circles highlight the regions (half-angle divergences of ~1.5° and ~1°, respectively) from which the dose was taken to obtain the coarse proton spectrum displayed in (k). The blue circles represent the 2° half-angle aperture subtended by the coils. (i), (j) PSL-converted scans of the image plates used in the TPS for the same shots as in (a)–(d) and (e)–(h), respectively. Ion energies increase going down the image, towards the point of zero deflection. The insets show a zoomed-in section of the IP, with green arrows indicating the narrowband proton bunches. (k) Proton energy spectra obtained for these shots from the RCF (data points) and the TPS. The grey line indicates the spectrum from the fourth scan of the IP in (j), rescaled to the flux level detected in the unsaturated regions of the initial first scan (red). The green curve is the spectrum obtained from the data shown in (i), which did not have any saturated region in its first scan. Saturated regions of shot 2’s first scan (red) are shown as dotted lines. A reference TPS spectrum from a flat foil target (with no HC attached) is shown in black. The 3 noise level is indicated by the dashed grey line.

Fig. 3. (a)–(d) Dose-converted scans of the RCF stack in front of the TPS for another helical coil shot, showing a ‘direct hit’ of the proton bunch into the TPS. Bragg peak energies of each layer are indicated. The red circle denotes a cone of half-angle divergence approximately 0.75°. (e) PSL-converted IP scan of the same shot; the inset shows a zoomed-in section of the IP, with a green arrow highlighting the narrowband proton bunch. (f) Proton spectrum obtained from RCF and the corresponding TPS data. Data was taken from a second IP scan and rescaled following the same procedure as in Figure 2(k). Nevertheless, the dotted region shows significant saturation still present, which could not be removed via further scanning. The RCF data point at approximately 50 MeV serves as an approximate reference of the peak proton flux (and solid angle subtended by the protons) around that point, and a Gaussian fitting (red dashes) has been performed on the unsaturated data, to show a spectral bunch width of 8.5 MeV (FWHM), centred at 52 MeV.

Fig. 4. Beam pointing with respect to the TPS pinhole. Each position is calculated as the centre of the proton dose on the RCF, in the layer corresponding to peak proton flux for each respective shot. The radii of the data points represent the maximum half-angle divergences (in mrad) subtended by the proton beams in the same layer. The colour scale reflects the maximum proton energy measured by either the RCF or TPS data. The angle subtended by the 3 mm diameter hole to allow protons through to the TPS is indicated in blue. The plot area is with the same size as the RCF layers. The shots shown in Figures 2 and 3 are indicated in the plot.

Fig. 5. PTRACE simulations of helical coils showing the effects of the final turn on long-distance beam pointing. (a)–(f) Virtual RCF images from the initial HC, at a plane 8 cm from the source. Every RCF layer is emulated to match the size of real RCF in the experiment – 50 mm × 50 mm squares. Energies of each RCF layer are shown as labelled, with an energy range of ±2 MeV. (g)–(j) Long-distance virtual RCF at the maximally focused energy (32 MeV), 70 cm from the source, for four different final turn positions. In each successive simulation, the azimuthal position of the end of the HC, , has changed by one quarter turn. Negative numbers imply the coil is shortening slightly compared to the original simulation. The - and -axes are converted to angles to aid interpretation of the beam pointing. The blue circles show the size of the hole in the real RCF used in the experiment, to allow access to the TPS. (k) Proton spectra contained inside a half-angle divergence of 0.5°, for each simulated coil. The slight successive shortening of the coils by one quarter turn has little effect on the final spectrum.