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We present detailed characterization of laser-driven fusion and neutron production (
$\sim {10}^5$/second) using 8 mJ, 40 fs laser pulses on a thin (<1 μm) D
${}_2$O liquid sheet employing a measurement suite. At relativistic intensity (
$\sim 5\times {10}^{18}$ W/cm
${}^2$) and high repetition rate (1 kHz), the system produces deuterium–deuterium (D-D) fusion, allowing for consistent neutron generation. Evidence of D-D fusion neutron production is verified by a measurement suite with three independent detection systems: an EJ-309 organic scintillator with pulse-shape discrimination, a 
${}^3\mathrm{He}$ proportional counter and a set of 36 bubble detectors. Time-of-flight analysis of the scintillator data shows the energy of the produced neutrons to be consistent with 2.45 MeV. Particle-in-cell simulations using the WarpX code support significant neutron production from D-D fusion events in the laser–target interaction region. This high-repetition-rate laser-driven neutron source could provide a low-cost, on-demand test bed for radiation hardening and imaging applications.
A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfvén-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base (
$P_\textrm {AWb}$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance 
$r$, which we denote by 
$\chi _{H}(r)$, and the fraction that is transferred via work, which we denote by 
$\chi _{W}(r)$. We find that 
$\chi _{W}(r_{A})$ ranges from 0.15 to 0.3, where 
$r_{A}$ is the Alfvén critical point. This value is small compared with one because the Alfvén speed 
$v_{A}$ exceeds the outflow velocity 
$U$ at 
$r < r_{A}$, so the AWs race through the plasma without doing much work. At 
$r>r_{A}$, where 
$v_{A} < U$, the AWs are in an approximate sense ‘stuck to the plasma’, which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach 
$r=r_{A}$, so the total rate at which AWs do work on the plasma at 
$r>r_{A}$ is a modest fraction of 
$P_\textrm {AWb}$. We find that heating is more effective than work at 
$r < r_{A}$, with 
$\chi _{H}(r_{A})$ ranging from 0.5 to 0.7. The reason that 
$\chi _{H} \geq 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfvén-speed scale height in RDAWT, and there are a few Alfvén-speed scale heights between the coronal base and 
$r_{A}$. A given amount of heating produces more magnetic moment in regions of weaker magnetic field. Thus, paradoxically, the average proton magnetic moment increases robustly with increasing 
$r$ at 
$r>r_{A}$, even though the total rate at which AW energy is transferred to particles at 
$r>r_{A}$ is a small fraction of 
$P_\textrm {AWb}$.
