Abstract
High-intensity ultrashort laser pulses focused on metal targets readily generate hot dense plasmas which accelerate ions efficiently and can pave way to compact table-top accelerators. Laser-driven ion acceleration studies predominantly focus on protons, which experience the maximum acceleration owing to their highest charge-to-mass ratio. The possibility of tailoring such schemes for the preferential acceleration of a particular ion species is very much desired but has hardly been explored. Here, we present an experimental demonstration of how the nanostructuring of a copper target can be optimized for enhanced carbon ion acceleration over protons or Cu-ions. Specifically, a thin (≈0.25 μm) layer of 25–30 nm diameter Cu nanoparticles, sputter-deposited on a polished Cu-substrate, enhances the carbon ion energy by about 10-fold at a laser intensity of 1.2×1018 W/cm2. However, particles smaller than 20 nm have an adverse effect on the ion acceleration. Particle-in-cell simulations provide definite pointers regarding the size of nanoparticles necessary for maximizing the ion acceleration. The inherent contrast of the laser pulse is found to play an important role in the species selective ion acceleration.
Highlights
The laser energy is transferred to the electrons and even lesser to the ions[11]
From a study of targets based on monolayers of polystyrene spheres of diameters 266, 535 and 920 nm coated on plastic substrates, the acceleration was reported to be optimal for 535 nm spheres
The question we address in this report is whether the ion acceleration with nanoparticle-coated metals is similar for all the ionic species; if not, are any specific ions preferentially accelerated to a greater degree than protons and metal ions? Our present experiments clearly show that the ion acceleration is dependent on the ion species (q/m) and an appropriately designed nanostructured coating is much more efficient in accelerating carbon or oxygen ions as compared to protons or Cu ions
Summary
Ion emission measurements were carried out from the front surface of polished Cu-targets and compared each time with those from Cu-nanoparticle coated targets to understand the systematic variation in ion generation for mean particle sizes in the 7–25 nm range, as illustrated in figure 1. It is well established that structured targets enhance the laser energy coupling to the plasma and increase the hot-electron as well as x-ray emission, several questions about the corresponding changes in the ion acceleration remain unanswered. Though nanostructured targets provide better laser energy coupling and enhanced ion acceleration, the angular divergence of the accelerated ions is broader as compared to a polished surface where it forms a relatively narrow beam[15,18]. This effect can be observed in the present study. In this context, boosting the carbon ion energy from about 90 keV to 900 keV for the same laser parameters is significant
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