Fig. 1. (a) Top view of experimental setup for laser-driven ion acceleration using helical coil (HC) targets. (b) Schematics of a single HC and an HC with a tube. The tube is axially symmetric. The HC is called “short-circuited” when Δr = 0 (i.e., the HC is in contact with the tube).

Fig. 2. Microscope images of HC: (a) without target; (b) with cut-out target filed and pasted on the HC.

Fig. 3. (a) Comparison of the proton spectrum obtained using an HC of radius a = 0.6 mm, pitch h = 0.35 mm, length L = 5 mm, and wire thickness 200 mm and the spectrum obtained using standard TNSA (2 μm tantalum foil). (b) RCF scan showing measurement of proton beam spot size (in green) obtained with an HC, demonstrating the focusing effect: the angular dispersion is reduced from a 20° half-angle with TNSA to 2.3° with the HC.

Fig. 4. Comparison of carbon ion spectra. (a) Spectra obtained using an HC of radius a = 0.6 mm, pitch h = 0.35 mm, length L = 5 mm and wire thickness 200 mm (dashed lines) compared with the corresponding spectra obtained in the absence of an HC (solid lines). (b) Spectra obtained using a short-circuited HC (dotted lines) compared with the corresponding spectra obtained in the absence of an HC (solid lines).

Fig. 5. Discharge current measurement in the SOPHIE simulation at the beginning of the HC as a function of time.

Fig. 6. (a) Comparison of proton spectra between experiment and simulation: the red line is the experimental spectrum for an HC of radius a = 0.6 mm, pitch h = 0.35 mm, length L = 5 mm, and wire thickness 200 mm; the black line is the experimental spectrum for TNSA without an HC; and the blue line is the spectrum simulated by SOPHIE for a current of 1.75 kA and a pulse duration of 15 ps at FWHM. (b) Comparison of carbon ion spectra between experiment and simulation: the blue line is the experimental spectrum for an HC with the same dimensions as in (a), which is comparable to the background noise shown by the green line; the black line is the experimental spectrum for TNSA without an HC; and the red line is the spectrum simulated by SOPHIE. (c) Comparison of input carbon ion spectra (black lines) and spectra simulated by SOPHIE for two future designs of HCT (not used in the present experiments), both with pitch h = 0.35 mm, length L = 15 mm, and wire thickness 200 mm: HCT No. 1 with HC radius a = 1.5 mm and tube radius b = 2 mm (solid blue and red lines) and HCT No. 2 with a = 1.8 mm and b = 2.3 mm (dotted blue and red lines).