Nanowire Target Laser Absorption for Fusion

Target Geometry

Wire aspect ratio (H/D) 6.0

Packing fraction 18%

Laser Drive

Incidence angle from normal 35°

Peak intensity (10¹⁸ W/cm²) 3.0

Coupling Readouts

Absorption A

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vs. flat target

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Hot-electron temperature T_hot

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How it works

Solid, flat laser targets reflect a large share of incident light because the laser can only couple energy into the plasma over a thin resonance layer near the critical-density surface. Etching the front surface into a forest of sub-wavelength nanowires replaces that single interface with a porous, near-critical-density volume: light entering the gaps between wires scatters and re-scatters off many wire surfaces before it can escape, so a photon gets many chances to be absorbed instead of one. Real ICF/fast-ignition experiments with nanowire and nanofoam front surfaces (e.g. Purvis et al., Nature Photonics 2013) measure absorption rising from roughly 50% on a flat target to >90% on a nanostructured one, for the same drive laser.

This model combines two textbook results:

Resonance absorption (flat target, Kruer):
  τ = (k₀L)^(1/3) sinθ ,  k₀ = 2π/λ
  A_flat(τ) = 1.2 τ · exp(−2τ³/3)

Light-trapping in the wire forest (geometric multi-bounce):
  N ≈ 1 + 6 · (aspect ratio) · (fill fraction)
  A_nano = 1 − (1 − A_flat)^N

Ponderomotive hot-electron scaling (Wilks):
  T_hot [MeV] = 0.511 · ( √(1 + I_abs·λ²/1.37) − 1 )
  I_abs = I_laser · A     (I in 10¹⁸ W/cm², λ in µm)

Mode buttons — switch the target between the nanowire array and an equivalent flat surface, holding the laser parameters fixed so you can compare absorption directly.
Aspect ratio / packing fraction — taller, denser wire forests give a photon more surfaces to bounce off (larger N) before it can escape, so absorption climbs toward saturation.
Incidence angle — sets τ for the flat-target resonance-absorption baseline that both modes are built from.
Intensity — the absorbed fraction of this drive intensity sets T_hot via the relativistic ponderomotive scaling law used throughout the fast-ignition ICF literature; a higher T_hot means more energetic electrons depositing energy in the fuel.

The bright streaks are individual photon packets from the drive beam: in nanowire mode they zig-zag through the forest (multiple scattering) before being absorbed (flash) or escaping; in flat mode most either absorb in one bounce off the surface or specularly reflect away.

Nanowire array

Incoming photon Absorbed Reflected / escaped

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Nanowire Target Laser Absorption for Fusion Drives