Optimization of laser-driven proton acceleration in a near-critical-density plasma

TL;DR

This study demonstrates that reducing laser focal spot size and optimizing plasma density profiles significantly enhance proton acceleration efficiency, potentially reducing the need for high-energy lasers in medical applications.

Contribution

The paper introduces a combined approach of tight focusing and tailored plasma profiles to improve proton acceleration, supported by simulations and theoretical analysis.

Findings

Proton energy increases by 56.3% with smaller focal spots.

Optimized plasma density profiles yield an additional 61.3% energy gain.

Enhanced acceleration is due to stronger charge-separation fields from ponderomotive electrons.

Abstract

Optimizing laser and plasma parameters is crucial for enhancing accelerated proton energy in laser-driven proton acceleration with finite laser energy for applications such as cancer therapy. Tight focusing plays a significant role in improving laser-driven proton acceleration, which is generally believed as a result of the enhancement of laser intensity. However, we find that even at a fixed laser intensity, reducing the focal spot size still enhances the proton energy. Through particle-in-cell simulations and theoretical modeling, we find that at a small spot size (0.8 {\mu}m), the maximum proton energy is enhanced by 56.3% compared to that obtained at a conventional spot size (3 {\mu}m). This improvement is attributed to the dominance of ponderomotive-force-driven electrons at reduced spot sizes, which generate stronger charge-separation fields that propagate at higher velocities. Furthermore, to optimize proton acceleration, we analytically derive an ideal plasma density profile that promotes phase-stable proton acceleration, yielding an additional energy increase of 61.3% over the case of a tightly focused laser interacting with a planar target of uniform density. These findings remain robust under parameter variations, indicating that advanced focusing techniques combined with optimized plasma profiles could relax the demand for high laser energies, thereby reducing the reliance on large-scale laser facilities in medical and scientific applications.