Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism ...for the generation of sufficiently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability efficiently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.
Inertial confinement fusion seeks to create burning plasma conditions in a spherical capsule implosion, which requires efficiently absorbing the driver energy in the capsule, transferring that energy ...into kinetic energy of the imploding DT fuel and then into internal energy of the fuel at stagnation. We report new implosions conducted on the National Ignition Facility (NIF) with several improvements on recent work Phys. Rev. Lett. 120, 245003 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.245003; Phys. Rev. E 102, 023210 (2020)PRESCM2470-004510.1103/PhysRevE.102.023210: larger capsules, thicker fuel layers to mitigate fuel-ablator mix, and new symmetry control via cross-beam energy transfer; at modest velocities, these experiments achieve record values for the implosion energetics figures of merit as well as fusion yield for a NIF experiment.
Inertial confinement fusion implosions must achieve high in-flight shell velocity, sufficient energy coupling between the hot spot and imploding shell, and high areal density (ρR=∫ρdr) at stagnation. ...Asymmetries in ρR degrade the coupling of shell kinetic energy to the hot spot and reduce the confinement of that energy. We present the first evidence that nonuniformity in the ablator shell thickness (∼0.5% of the total thickness) in high-density carbon experiments is a significant cause for observed 3D ρR asymmetries at the National Ignition Facility. These shell-thickness nonuniformities have significantly impacted some recent experiments leading to ρR asymmetries on the order of ∼25% of the average ρR and hot spot velocities of ∼100 km/s. This work reveals the origin of a significant implosion performance degradation in ignition experiments and places stringent new requirements on capsule thickness metrology and symmetry.
Rayleigh-Taylor instability growth is shown to be hydrodynamically scale invariant in convergent cylindrical implosions for targets that varied in radial dimension and implosion timescale by a factor ...of 3. The targets were driven directly by laser irradiation providing a short impulse, and instability growth at an embedded aluminum interface occurs as it converges radially inward by a factor of 2.25 and decelerates on a central foam core. Late-time growth factors of 14 are observed for a single-mode m=20 azimuthal perturbation at both scales, despite the differences in laser drive conditions between the experimental facilities, consistent with predictions from radiation-hydrodynamics simulations. This platform enables detailed investigations into the limits of hydrodynamic scaling in high-energy-density systems.
For the first time, quantitative measurements of ion stopping at energies around the Bragg peak (or peak ion stopping, which occurs at an ion velocity comparable to the average thermal electron ...velocity), and its dependence on electron temperature (T(e)) and electron number density (n(e)) in the range of 0.5-4.0 keV and 3×10(22) to 3×10(23) cm(-3) have been conducted, respectively. It is experimentally demonstrated that the position and amplitude of the Bragg peak varies strongly with T(e) with n(e). The importance of including quantum diffraction is also demonstrated in the stopping-power modeling of high-energy-density plasmas.
We measured the stopping of energetic protons in an isochorically heated solid-density Be plasma with an electron temperature of ∼32 eV, corresponding to moderately coupled ...(e^{2}/a)/(k_{B}T_{e}+E_{F})∼0.3 and moderately degenerate k_{B}T_{e}/E_{F}∼2 "warm-dense matter" (WDM) conditions. We present the first high-accuracy measurements of charged-particle energy loss through dense plasma, which shows an increased loss relative to cold matter, consistent with a reduced mean ionization potential. The data agree with stopping models based on an ad hoc treatment of free and bound electrons, as well as the average-atom local-density approximation; this work is the first test of these theories in WDM plasma.
A study of the transition from collisional to collisionless plasma flows has been carried out at the National Ignition Facility using high Mach number (M>4) counterstreaming plasmas. In these ...experiments, CD-CD and CD-CH planar foils separated by 6-10 mm are irradiated with laser energies of 250 kJ per foil, generating ∼1000 km/s plasma flows. Varying the foil separation distance scales the ion density and average bulk velocity and, therefore, the ion-ion Coulomb mean free path, at the interaction region at the midplane. The characteristics of the flow interaction have been inferred from the neutrons and protons generated by deuteron-deuteron interactions and by x-ray emission from the hot, interpenetrating, and interacting plasmas. A localized burst of neutrons and bright x-ray emission near the midpoint of the counterstreaming flows was observed, suggesting strong heating and the initial stages of shock formation. As the separation of the CD-CH foils increases we observe enhanced neutron production compared to particle-in-cell simulations that include Coulomb collisions, but do not include collective collisionless plasma instabilities. The observed plasma heating and enhanced neutron production is consistent with the initial stages of collisionless shock formation, mediated by the Weibel filamentation instability.
An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly driven, β≲20 ...laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets (V_{jet}∼20V_{A}) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. The absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly driven regime.
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