[Paper Review] Ion acceleration by an ultrashort laser pulse interacting with a near-critical-density gas jet

M. Ehret, C. Salgado-López+29 more|arXiv (Cornell University)|Dec 17, 2020

Planetary Science and Exploration1 references4 citations

TL;DR

This study demonstrates efficient helium ion acceleration using a 30-fs, 3-J ultrashort laser pulse focused on a near-critical-density gas jet, achieving ion cut-off energies above 25 MeV and peak ion yields exceeding 10⁸ MeV⁻¹. The experiment leverages shock-induced ion acceleration in a laser-driven electrostatic shock, with stable spectra despite minor target density fluctuations due to nozzle ablation.

ABSTRACT

We demonstrate laser-driven Helium ion acceleration with cut-off energies above 25 MeV and peaked ion number above $10^8$ /MeV for 22(2) MeV projectiles from near-critical density gas jet targets. We employed shock gas jet nozzles at the high-repetition-rate (HRR) VEGA-2 laser system with 3 J in pulses of 30 fs focused down to intensities in the range between $9 imes10^{19}$ W/cm$^2$ and $1.2 imes10^{20}$ W/cm$^2$. We demonstrate acceleration spectra with minor shot-to-shot changes for small variations in the target gas density profile. Difference in gas profiles arise due to nozzles being exposed to a experimental environment, partially ablating and melting.

Motivation & Objective

• To investigate ion acceleration mechanisms in near-critical-density plasmas, a regime predicted to optimize laser-to-plasma energy coupling.
• To demonstrate high-repetition-rate, stable ion beam generation using ultrashort laser pulses on gas jet targets.
• To analyze the impact of target density profile evolution due to nozzle ablation on ion beam quality.
• To validate the role of collisionless shock acceleration (CSA) in ion energy enhancement under near-critical conditions.

Proposed method

• Employed a high-repetition-rate (HRR) VEGA-2 Ti:sapphire laser system delivering 3 J in 30-fs pulses at 0.8 µm wavelength.
• Focused laser pulses to intensities between 9×10¹⁹ and 1.2×10²⁰ W/cm² using an F/4 off-axis parabolic mirror.
• Used shock gas jet nozzles to generate transient near-critical-density plasmas (ne ≈ 1.7×10²¹ cm⁻³) via laser ionization.
• Measured ion spectra using CR-39 nuclear track detectors with controlled etching and Z-scan microscopy for track depth and diameter analysis.
• Applied particle-in-cell (PIC) simulations to model laser-plasma interaction and validate the role of charge separation and shock formation.
• Conducted strioscopic imaging and diameter evolution tracking of etch pits to infer ion energy and fluence from track morphology.

Experimental results

Research questions

• RQ1 Can ultrashort laser pulses efficiently accelerate helium ions in near-critical-density gas jets with high-repetition-rate operation?
• RQ2 How does laser-driven electrostatic shock formation contribute to ion acceleration in the near-critical regime?
• RQ3 To what extent do shot-to-shot variations in ion spectra correlate with changes in gas jet density profiles due to nozzle ablation?
• RQ4 What is the energy distribution and yield of accelerated ions, and how do they compare to theoretical predictions for collisionless shock acceleration?

Key findings

• Ion cut-off energies exceeding 25 MeV were measured, with a prominent peak at 22(2) MeV for helium ions.
• Peak ion yield reached 1.1×10⁸ MeV⁻¹, indicating high beam quality and efficient energy coupling.
• Ion spectra showed minimal shot-to-shot variation despite measurable changes in gas jet density profiles due to nozzle ablation.
• Etch pit diameter evolution and Z-scan analysis confirmed the presence of 4–40 MeV alpha particles and 56–200 MeV nitrogen ions, with dominant contributions from 4.5–6.7 MeV alphas.
• PIC simulations supported the dominance of collisionless shock acceleration (CSA) as the primary ion acceleration mechanism in the near-critical regime.
• The observed stability in ion spectra despite nozzle degradation suggests robustness of the acceleration process under real experimental conditions.