Figure 2: Helical coil working principle.
From: Guided post-acceleration of laser-driven ions by a miniature modular structure
(a) Schematic representation of the target designed for optimizing the beam parameters of laser-driven protons. In this configuration a helical coil, made of a metallic wire, is attached to the laser-irradiated thin foil at one end and grounded at the other end. The helical coil design guides the EM pulse carrying the neutralizing charge around the proton-beam axis and allows synchronizing its longitudinal propagation (that is, along z) with protons having a given energy within the beam. (b) Schematic representation snapshot showing the electric field configuration inside the coil. The red section of the coil represents the segment charged by the travelling pulse at a given moment of time, where the red arrows represent the electric field (E) lines originating from the coil, the black and blue arrows represent radial (Er) and longitudinal (Ez) components of the electric field, respectively. The length of the black and blue arrows represents the relative strength of the field at different locations. (c,d) Ez|(r=0) and Er|(z=0) profiles inside the coil at a given time, where z=0 corresponds to the location of the peak of charge density along the coil at that time. The field profiles are calculated by the subroutine that defines the input electric field configuration for particle tracing in the PTRACE simulation (see Methods). Dynamics are modelled using an asymmetric Gaussian pulse profile of 5 ps rise and 10 ps decay, as obtained in the experiment shown in Fig. 1e, travelling along a helical coil with the same dimensions as the one used in the experiment illustrated in Fig. 3.
