Fig. 1. (a) shows a schematic of the experimental setup for transverse proton probing of the EM pulse propagating along a helical coil (not to scale). (b) Shows the front view (looking from the detector side) of the target. (c) and (d) Show the radiographs of the helical coil obtained by 5.5 and 3.0 MeV protons, respectively. The dotted lines show the axes of the proton beam and the intersection point is the approximate centre of the proton beam. The spatial scale shown in (c) corresponds to the image plane.

Fig. 2. (a) and (b) show the schematics (side and top view, respectively) of the arrangement for transverse probing of the helical winding. F and B represent, respectively, the front and back principal probing points on a winding chosen for analysis. The difference in probing times for the two principal points arises due to the different proton time of flights. (c) shows the temporal profile of the pulse travelling along the wire of the helical coil as obtained from the data shown in Figure 1.

Fig. 3. (a) Image of the target used for longitudinal probing. The coil had ${\sim}900~\unicode[STIX]{x03BC}\text{m}$ diameter and 1.9 mm long and consisted of 8 windings with average pitch of ${\sim}260~\unicode[STIX]{x03BC}\text{m}$. (b), (c) and (d) show experimental, spatially resolved dose profiles of the proton beam for energies 3.0, 4.4 and 5.5 MeV, respectively. (e) shows the percentage reduction of diameter of the central part of the beam with respect to proton energies (MeV), as obtained from the experimental (black) and simulated (red) RCF images.

Fig. 4. (a) shows a schematic of the setup used for the shot taken with a 3.3 mm long helical coil with RCF placed at 70 mm from the interaction foil. (b), (c) and (d) show the raw RCF images for 3.0, 6.6 and 9.6 MeV protons respectively, where the pronounced focusing of the channelled beam of 3 MeV protons can be seen in (b), in contrast to the geometrical projection of the exit winding of the coil at the RCF plane shown by the red dashed circle.