[Paper Review] Acceleration of collimated 45 MeV protons by collisionless shocks driven in low-density, large-scale gradient plasmas by a 10^20 W/cm^2, 1 micron wavelength laser

P. Antici, Elisabetta Boella+18 more|arXiv (Cornell University)|Aug 8, 2017

Laser-Plasma Interactions and Diagnostics57 references3 citations

TL;DR

This study demonstrates a novel proton acceleration mechanism—Low Density Collisionless Shock Acceleration (LDCSA)—in mm-scale, low-density (≈1% critical density) targets irradiated by a 10^20 W/cm², 1 µm laser. The process produces highly collimated (6° half-angle), energetic protons up to 45 ± 5 MeV with a flux of 5×10⁹ protons/MeV/sr/J, enabled by a combination of collisionless shocks and a TNSA-like sheath field in a large-scale density gradient, offering a robust alternative to conventional TNSA or RPA with relaxed laser and target requirements.

ABSTRACT

A new type of proton acceleration stemming from large-scale gradients, low-density targets, irradiated by an intense near-infrared laser is observed. The produced protons are characterized by high-energies (with a broad spectrum), are emitted in a very directional manner, and the process is associated to relaxed laser (no need for high-contrast) and target (no need for ultra-thin or expensive targets) constraints. As such, this process appears quite effective compared to the standard and commonly used Target Normal Sheath Acceleration technique (TNSA), or more exploratory mechanisms like Radiation Pressure Acceleration (RPA). The data are underpinned by 3D numerical simulations which suggest that in these conditions Low Density Collisionless Shock Acceleration (LDCSA) is at play, which combines an initial Collisionless Shock Acceleration (CSA) to a boost procured by a TNSA-like sheath field in the downward density ramp of the target, which leads to an overall broad spectrum. Experiments performed at 10^20 W/cm^2 laser intensity show that LDCSA can accelerate, from ~1% critical density, mm-scale targets, up to 5x10^9 protons/MeV/sr/J with energies up to 45(+/- 5) MeV in a collimated (~6$^\circ$ half-angle) manner.

Motivation & Objective

• To explore proton acceleration mechanisms in low-density, large-scale gradient plasmas under intense laser irradiation.
• To identify and characterize a new acceleration regime that overcomes limitations of standard TNSA and RPA techniques.
• To demonstrate high-energy, collimated proton beams with improved flux and reduced sensitivity to laser contrast and target quality.
• To validate the physical mechanism through 3D simulations and experimental data correlation.

Proposed method

• Irradiation of mm-scale, low-density (≈1% critical density) targets with a 10^20 W/cm², 1 µm wavelength laser at moderate laser contrast.
• Use of 3D particle-in-cell (PIC) simulations to model plasma dynamics and identify the dominant acceleration mechanism.
• Analysis of proton energy spectra and angular distributions to assess beam collimation and energy spread.
• Identification of a two-stage acceleration process: initial collisionless shock acceleration followed by sheath field enhancement in the downward density ramp.
• Comparison of experimental results with simulations to confirm the LDCSA mechanism.
• Quantification of proton yield per unit energy, solid angle, and laser energy (protons/MeV/sr/J).

Experimental results

Research questions

• RQ1 Can proton acceleration be efficiently achieved in low-density, large-scale targets without requiring ultra-thin or high-contrast laser conditions?
• RQ2 What physical mechanism underlies the observed collimation and high energy of protons in such targets?
• RQ3 How does the combination of collisionless shocks and sheath fields in a density gradient enhance proton acceleration compared to standard TNSA?
• RQ4 To what extent does the LDCSA mechanism improve proton flux and beam quality compared to existing techniques?

Key findings

• Proton beams with energies up to 45 ± 5 MeV were experimentally observed, representing a significant energy gain in low-density targets.
• The proton beam exhibited strong collimation with a half-angle of approximately 6°, indicating high directivity.
• A proton flux of 5×10⁹ protons/MeV/sr/J was achieved, demonstrating high efficiency and brightness.
• The acceleration mechanism was identified as Low Density Collisionless Shock Acceleration (LDCSA), combining initial collisionless shock acceleration with a TNSA-like sheath field in the density ramp.
• 3D simulations confirmed that the LDCSA mechanism explains both the high energy and collimation of the proton beam.
• The process operates effectively under relaxed experimental conditions, including moderate laser contrast and standard mm-scale targets, unlike TNSA or RPA.