Neutron Generation by Laser-Driven Spherically Convergent Plasma Fusion

Compression and acceleration of ions by ultrashort, ultraintense azimuthally polarized light

An efficient plasma compression scheme using azimuthally polarized light is proposed. Azimuthally polarized light possesses a donutlike intensity pattern, enabling it to compress and accelerate ions toward the optical axis across a wide range of parameters. When the light intensity reaches the relativistic regime of 10^{18}W/cm^{2}, and the plasma density is below the critical density, protons can be compressed and accelerated by the toroidal soliton formed by the light. The expansion process of the soliton can be well described by the snowplow model. Three-dimensional particle-in-cell simulations show that within the soliton regime, despite the ion density exceeding ten times the critical density, the ions' energy is insufficient for efficient neutron production. When the light intensity increases to 10^{22}W/cm^{2}, and the plasma density reaches several tens of times the critical density, deuterium ions can be compressed to thousands of times the critical density and simultaneously accelerated to the MeV level by tightly focused azimuthally polarized light during the hole-boring process. This process is far more dramatic compared to the soliton regime and can produce up to 10^{4} neutrons in a few light cycles. Moreover, in the subsequent beam-target stage, neutron yield is estimated to exceed 10^{8}. Finally, we present a comparison with the results obtained using a radially polarized light to examine the influence of light polarization.

Developing "inverted-corona" fusion targets as high-fluence neutron sources

We present experimental studies of inverted-corona targets as neutron sources at the OMEGA Laser Facility and the National Ignition Facility (NIF). Laser beams are directed onto the inner walls of a capsule via laser-entrance holes (LEHs), heating the target interior to fusion conditions. The fusion fuel is provided either as a wall liner, e.g., deuterated plastic (CD), or as a gas fill, e.g., D₂ gas. Such targets are robust to low-mode drive asymmetries, allowing for single-sided laser drive. On OMEGA, 1.8-mm-diameter targets with either a 10-μm CD liner or up to 2 atm of D₂-gas fill were driven with up to 18 kJ of laser energy in a 1-ns square pulse. Neutron yields of up to 1.5 × 10¹⁰ generally followed expected trends with fill pressure or laser energy, although the data imply some mix of the CH wall into the fusion fuel for either design. Comparable performance was observed with single-sided (1x LEH) or double-sided (2x LEH) drive. NIF experiments tested the platform at scaled up dimensions and energies, combining a 15-μm CD liner and a 3-atm D₂-gas fill in a 4.5-mm diameter target, laser-driven with up to 330 kJ. Neutron yields up to 2.6 × 10¹² were measured, exceeding the scaled yield expectation from the OMEGA data. The observed energy scaling on the NIF implies that the neutron production is gas dominated, suggesting a performance boost from using deuterium-tritium (DT) gas. We estimate that neutron yields exceeding 10¹⁴ should be readily achievable using a modest laser drive of ∼300 kJ with a DT fill.

Micro-scale fusion in dense relativistic nanowire array plasmas

Nuclear fusion is regularly created in spherical plasma compressions driven by multi-kilojoule pulses from the world's largest lasers. Here we demonstrate a dense fusion environment created by irradiating arrays of deuterated nanostructures with joule-level pulses from a compact ultrafast laser. The irradiation of ordered deuterated polyethylene nanowires arrays with femtosecond pulses of relativistic intensity creates ultra-high energy density plasmas in which deuterons (D) are accelerated up to MeV energies, efficiently driving D-D fusion reactions and ultrafast neutron bursts. We measure up to 2 × 10⁶ fusion neutrons per joule, an increase of about 500 times with respect to flat solid targets, a record yield for joule-level lasers. Moreover, in accordance with simulation predictions, we observe a rapid increase in neutron yield with laser pulse energy. The results will impact nuclear science and high energy density research and can lead to bright ultrafast quasi-monoenergetic neutron point sources for imaging and materials studies.