Efficient Generation of Neutrons Based on Ultrashort Laser-driven Direct Acceleration in Microwire-Array Targets

This paper demonstrates an efficient method for generating neutrons by using ultrashort laser pulses to drive direct proton acceleration within microwire-array targets, achieving a high neutron yield that could enable compact, high-repetition-rate neutron sources.

The Gist

The Tiny Wire "Super-Highway": A New Way to Make Neutron Fireworks

Imagine you are trying to launch a fleet of tiny, high-speed delivery drones (protons) across a massive, obstacle-filled field to hit a target (a converter) that will trigger a spectacular firework show (neutrons).

In the past, scientists had two main ways to do this:

1. The Heavy Artillery Approach: Use massive, expensive, building-sized machines (accelerators) that take a long time to "reload."
2. The Big Sledgehammer Approach: Use giant lasers that pack a huge punch but are so massive and slow that they can only fire once in a while.

This paper describes a clever "middle way." Instead of just blasting a flat wall with a laser, the researchers built a microscopic obstacle course that actually helps the particles speed up.

---

The Secret Ingredient: The Microwire Array

Think of a standard target as a flat, solid wall. When a laser hits it, the energy splashes everywhere, like throwing a bucket of water at a brick wall. It’s messy and inefficient.

The researchers instead used a Microwire Array (MWA). Imagine replacing that flat wall with a series of tiny, perfectly spaced parallel fences (microwires).

The Analogy: The Surfer and the Wave Channel
When the laser hits these tiny wires, it doesn't just splash; it creates "channels" of energy. Imagine a surfer in the ocean. If they are in the middle of a flat, calm sea, they can’t go very fast. But if they find a narrow, swirling channel between two reefs, the water is forced into a tight, powerful rush.

In this experiment, the laser creates "electric waves" inside the tiny gaps between the wires. The electrons get caught in these channels and are "surf-boosted" (a process called Direct Laser Acceleration) to incredible speeds. This creates a massive, focused "wind" of electricity that pushes the protons out of the target like a high-powered leaf blower.

Why Does This Matter? (The "Neutron Yield")

The goal is to create neutrons. Neutrons are like the "super-bullets" of science—they are used to peer inside materials, study fusion energy, and even take high-tech medical images.

The researchers found that by spacing these tiny wires perfectly (about 6.4 micrometers apart—that's thinner than a human hair!), they could get a massive "bang for their buck."

• The Efficiency Record: They achieved a neutron yield that is ten times higher than previous records for this type of laser.
• The "Be" Upgrade: They even used computer simulations to predict that if they swap the target material for Beryllium (Be), the neutron production would skyrocket even further.

The Big Picture: Compact and Fast

Because this method uses "ultrashort" lasers (pulses that last only quadrillionths of a second), the equipment is much smaller and more economical than the giant particle accelerators used today.

In short: By moving from a "flat wall" to a "microscopic highway," scientists have found a way to create intense, rapid-fire bursts of neutrons using smaller, smarter, and more efficient laser technology. It’s like moving from a heavy, slow-moving cannon to a high-tech, rapid-fire laser gatling gun.

Technical Summary

Technical Summary: Efficient Generation of Neutrons Based on Ultrashort Laser-driven Direct Acceleration in Microwire-Array Targets

1. Problem Statement

Traditional pulsed neutron sources, such as accelerator-based facilities, provide high peak fluxes but are limited by high energy consumption, massive infrastructure, and high economic costs. Conversely, laser-driven neutron sources offer advantages like ultrashort pulse durations ($<1$ ns), micro-scale source sizes, and ultrahigh peak fluxes. However, current laser-driven methods face a significant efficiency bottleneck:

• Long-pulse lasers (hundreds of joules) can achieve high yields but operate at low repetition rates.
• Ultrashort (femtosecond) lasers are compact and suitable for high repetition rates, but their neutron yield per joule is historically low (typically $\le 3.08 \times 10^6$ n/sr/J) because laser-ion acceleration is less efficient at moderate intensities ($\lesssim 10^{20}$ W/cm²).

2. Methodology

The researchers proposed and experimentally validated a method to enhance acceleration efficiency using Microwire-Array (MWA) targets and Direct Laser Acceleration (DLA).

• Target Design: 3D-printed MWAs consisting of ordered polymer microwires ($L_0 = 6\ \mu\text{m}$, $r_0 = 1\ \mu\text{m}$) attached to a SiN substrate. The array period ($P$) was varied to optimize the interaction.
• Laser Parameters: Used the SILEX-II facility, delivering a $1\text{ PW}$, $\sim 25\text{ fs}$ pulse at a moderate intensity of $\sim 10^{20}\text{ W/cm}^2$.
• Acceleration Mechanism (DLA): As the laser enters the MWA channels, the laser's electric field pulls overdense electron bunches from the wires. These electrons gain forward momentum via the Lorentz force and are focused by the channel geometry, creating an exceptionally intense sheath electric field behind the substrate. This field accelerates protons/ions more efficiently than the standard Target Normal Sheath Acceleration (TNSA) used with flat targets.
• Computational Approach: A self-consistent integrated simulation framework was used:
  • Radiation Hydrodynamics (RHD - FLASH): To model pre-plasma formation and target ablation.
  • Particle-in-Cell (PIC - EPOCH): To simulate laser-plasma interaction and electron/proton acceleration.
  • Monte Carlo (MC - Geant4): To model the nuclear reactions ($^7\text{Li}(p, n)^4\text{He}$ and $\text{D}(p, n)^3\text{He}$) and neutron transport.

3. Key Contributions

• Discovery of Optimal Geometry: Identified that the MWA array period ($P$) is critical. The presence of laser prepulse necessitates a larger optimal period ($P \approx 6.4\ \mu\text{m}$) compared to theoretical predictions without prepulse to account for plasma expansion.
• Mechanism Validation: Demonstrated that DLA in MWA channels significantly enhances electron temperature ($k_B T_h$) and proton cutoff energy compared to planar targets.
• Converter Optimization: Evaluated different converter materials (LiF, LiD, and Be) to maximize neutron yield and directionality.

4. Results

• Proton Acceleration: At the optimal period of $P = 6.4\ \mu\text{m}$, the proton cutoff energy and number of protons $>1\text{ MeV}$ were approximately 3 times higher than those from a standard plane target.
• Experimental Neutron Yield: Using a LiD converter, the team achieved a neutron yield of $(8.33 \pm 0.84) \times 10^6\text{ n/sr/J}$. This sets a new record for femtosecond-class lasers at moderate intensities, exceeding previous records by over an order of magnitude.
• Directionality: The use of a LiD converter enhanced forward emission due to the anisotropic nature of the $\text{D}(p, n)^3\text{He}$ reaction.
• Predictive Modeling: Simulations predicted that switching to a Be converter could push the yield to $3.67 \times 10^7\text{ n/sr/J}$, a value approaching the performance of much larger picosecond-class laser facilities.

5. Significance

This work represents a breakthrough in high-repetition-rate neutron source technology. By utilizing compact, economical, and ultrashort laser systems with MWA targets, it is now possible to achieve high neutron yields per joule. This paves the way for advanced applications requiring high-flux, short-pulse neutrons, such as:

• Neutron Resonance Imaging/Radiography.
• Fusion materials analysis and neutronics research.
• Fast neutron capture studies.