Influence of target-rear-side short scale length density gradients on laser-driven proton acceleration

Abstract

The effects of a short plasma density scale length on laser-driven proton acceleration 17 from foil targets is investigated by heating and driving expansion of a large area of 18 the target rear surface. The maximum proton energy, proton flux and the divergence 19 of the proton beam are all measured to decrease with increasing extent of the plasma 20 expansion. Even for a small plasma scale length of the order of the laser wavelength (∼1 21 µm), a significant effect on the generated proton beam is evident; a substantial decrease 22 in the number of protons over a wide spectral range is measured. A combination 23 of radiation-hydrodynamic and particle-in-cell simulations provide insight into the 24 underlying physics. The results provide new understanding of the importance of even a 25 small plasma density gradient, with implications for applications that require efficient 26 laser energy conversion to ions, such as proton-driven fast-ignition of compressed fusion 27 fuel.

Automated control and optimization of laser-driven ion acceleration

“…While this is consistent with our measurements, the strong suppression of proton flux at the highest intensity may indicate that the acceleration process is further compromised at the highest laser intensities by the contrast levels of our laser, with the prepulses and amplified spontaneous emission (ASE) causing adverse pre-heating of the target. Similar disruption has previously been attributed to rear surface deformation by ASE-driven shock break-out, which can effectively steer a high energy component of the proton beam emission towards the laser axis [23,24], modifying the spectrum measured at a single angular position, or the presence of a long scale length plasma on the rear-surface which has been shown to suppress the production of ions through TNSA in experiments and simulations [51][52][53].…”

Proton imaging of high-energy-density laboratory plasmas

“…Additionally, the interaction that is being probed may also cause preplasma at the rear of the target. In either case, this preplasma at the rear surface can inhibit proton acceleration (Kaluza et al, 2004;Fuchs et al, 2007;Higginson et al, 2021). For this reason, a shield to protect the proton source foil is often used to prevent these effects (Mackinnon et al, 2006;Zylstra et al, 2012).…”

“…These include radiation pressure acceleration (RPA) (Esirkepov et al, 2004;Robinson et al, 2008) [in the hole boring (Robinson et al, 2012) and light sail (Macchi, Veghini, and Pegoraro, 2009) implementations], shock acceleration (Fiuza et al, 2012), and schemes taking place in relativistic induced transparency (RIT) regimes (Henig et al, 2009;Poole et al, 2018) such as the break-out afterburner approach (Yin et al, 2007). Hybrid regimes involving a combination of these processes have been highlighted in experiments and have recently led to record proton energies approaching 100 MeV through a combination of RPA, TNSA, and RIT acceleration (Higginson et al, 2021). Although these processes are promising in terms of energy enhancement, for instance, in view of potential medical use, they typically generate beams that do not possess the laminarity and homogeneity of TNSA beams, and their potential usefulness for proton imaging is therefore unclear at present, at least in the backlighting implementation discussed in this review.…”

Novel approach to TNSA enhancement using multi-layered targets—a numerical study

“…This process is known as shock-wave breakout (SWB) [37] and it has been shown to negatively affect ion acceleration from the rear side [38]. In fact, numerous experimental studies have examined TNSA in the presence of rear-side plasma by irradiating the back of the target using a lower intensity pulse [39][40][41][42]. The results of these studies show that a longer plasma scale length on the rear side results in lower ion cutoff energies.…”