Abstract
In this work, we present the results of an experiment aiming at proton acceleration using a focus with a homogeneous intensity distribution, called smoothed focus. To achieve this goal, we implemented a phase plate before the pre-amplifier of the Petawatt High-Energy Laser for Heavy Ion EXperiments laser facility. The phase plate was used for the first time at a high-power short-pulse laser. Demonstrating a low divergent ion beam was the main goal of this work. Numerical simulations using the particle-in-cell code Extendable PIC Open Collaboration estimated a 2–5 times reduction in the angular divergence of the proton beam using a phase plate due to a smoother sheath at the rear side of the target. However, the reduction in the angular divergence was not sensible according to the experimental data. A positive point is that the spectrum of protons that are generated with the smoothed beam is shifted toward lower energies, provided that the laser absorption is kept in check, compared to the Gaussian proton spectrum. Moreover, the number of protons that are generated with the smoothed beam is higher than the ones generated with the Gaussian beam.
Highlights
Laser-driven ion acceleration1,2 attracts much attention due to its diverse applications, such as fast ignition,3 injection into conventional accelerator structures,4 isochoric heating of samples to warmdense-matter states,5 and hadron therapy for cancer treatment.6 A key feature for some of these applications is a mono-energetic and collimated ion beam for efficient energy transfer.The most extensively studied mechanism for the laser-driven ion acceleration is Target Normal Sheath Acceleration (TNSA).2,7 According to the TNSA model, the interaction of an ultra-intense short-pulse laser with a target ionizes the target and generates energetic electrons at the front side of the target, which propagate through it and accumulate at its rear side
We investigate the proton acceleration in the TNSA regime with a smoothed focus, which can generate a uniform sheath at the rear side of the target and, probably, accelerate a more collimated ion beam
As there is no experimental evidence in the literature for Emax and Δθ of protons using a phase plate and since PIC codes cannot estimate correctly the angular divergence of the accelerated protons, we can only scale the simulation results to see any difference between the Gaussian and smoothed beams
Summary
Laser-driven ion acceleration attracts much attention due to its diverse applications, such as fast ignition, injection into conventional accelerator structures, isochoric heating of samples to warmdense-matter states, and hadron therapy for cancer treatment. A key feature for some of these applications is a mono-energetic and collimated ion beam for efficient energy transfer. Initial studies have shown that using a larger intensity distribution at the laser focus can positively influence the proton divergence.9 This can be achieved, for instance, when a very long focal length focusing element is used, but it is not experimentally feasible in many facilities due to the spatial restrictions. I = 1018 W/cm to produce a homogeneous intensity distribution at the focus, which creates a speckle-like intensity distribution over an area 1600 times the diffraction limit Following this approach, we investigate the proton acceleration in the TNSA regime with a smoothed focus, which can generate a uniform sheath at the rear side of the target and, probably, accelerate a more collimated ion beam
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