Dynamic control of laser driven proton beams by exploiting self-generated, ultrashort electromagnetic pulses

Abstract

As part of the ultrafast charge dynamics initiated by high intensity laser irradiations of solid targets, high amplitude EM pulses propagate away from the interaction point and are transported along any stalks and wires attached to the target. Propagation of these high amplitude pulses along a thin wire connected to a laser irradiated target was diagnosed via proton radiography technique, measuring pulse duration of ∼20 ps and pulse velocity close to the speed of light. The strong electric field associated with the EM pulse can be exploited for controlling dynamically the proton beams produced from a laser-driven source. Chromatic divergence control of broadband laser driven protons (upto 75% reduction in divergence of >5 MeV protons) was obtained using a 10 TW laser by winding the supporting wire around the proton beam axis as a helical coil structure. In addition to the focussing and energy selection, the technique has the potential to postaccelerate the transiting protons by the longitudinal component of the curved electric field lines produced by the helical coil lens.

Efficient post-acceleration of protons in helical coil targets driven by sub-ps laser pulses

“…As shown in Fig. 1(f) , the experimental spectrum was well reproduced by the simulation carried out using peak charge density of ~(11.5 ± 0.5) μ C/m, which is similar to what was measured previously by probing the EM pulse using the proton radiography technique 14 , 15 . Simulations suggest that the two spectral peaks on either side of 7 MeV are produced due to the acceleration/deceleration of the leading/trailing part of the synchronised proton bunch within the coil, as previously discussed 13 .…”

“…1(e) , 10 MeV protons from the laser irradiated foil would experience the EM pulse for less than 2 mm of the coil, which, as seen from the ~2 mm coil in ref. 14 is not enough to achieve the degree of collimation observed in Fig. 1(d) .…”

“…Therefore it is reasonable to assume that the focussed beam of 10 MeV protons was produced by post-acceleration of the ~7 MeV protons during their transit through the coil, due to their prolonged exposure to the longitudinal component of the electric field produced by the EM pulse. Assuming an EM pulse of ~15 ps FWHM with a nominal peak charge density of ~10 μ C/m (as measured at the TARANIS laser under similar interaction conditions 14 ), a total charge of ~80 nC spread over 2 windings of the coil would produce a longitudinal electric field ~0.75 MV/mm, as can be estimated by using the simple charged-ring approximation discussed in ref. 13 .…”

Proton probing of laser-driven EM pulses travelling in helical coils

“…By repeating the analysis for each principal points across the coil, the temporal profile of the pulse was reconstructed, which is shown in Figure 2(c). As can be seen, the temporal profile displaying ∼5 ps rise and ∼12 ps decay is in a broad agreement with previous measurements taken under similar experimental laser conditions [17,18] .…”

“…The propagation of an electromagnetic (EM) pulse generated by intense laser interaction with a solid target was recently studied by employing a self-probing arrangement [16][17][18] . The ultra-short EM pulses with peak electric field of the order of 10 9 V m −1 , are generated following the rapid charging of the laser-irradiated target to MV potential, due to the prompt escape of the high energy (MeV) electrons produced during the interaction [2,13,19] .…”