Slowing of Magnetic Reconnection Concurrent with Weakening Plasma Inflows and Increasing Collisionality in Strongly Driven Laser-Plasma Experiments

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[subscript jet] ~ 20V[subscript 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.United States. Dept. of Energy (Grant DE-NA0001857)University of Rochester. Laboratory for Laser Energetics (Grant 415935-G)National Laser User’s Facility (Grant DE-NA0002035)University of Rochester. Fusion Science Center (Grant 5-24431

Measurements of Ion Stopping Around the Bragg Peak in High-Energy-Density Plasmas

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[subcontract e]) and electron number density (n[subcontract e]) in the range of 0.5–4.0 keV and 3 × 10[superscript 22] to 3 × 10[superscript 23]  cm[superscript −3] have been conducted, respectively. It is experimentally demonstrated that the position and amplitude of the Bragg peak varies strongly with T[subscript e] with n[subscript e]. The importance of including quantum diffraction is also demonstrated in the stopping-power modeling of high-energy-density plasmas.United States. Dept. of Energy (Grant DE-FG03-03SF22691)Lawrence Livermore National Laboratory (Subcontract Grant B504974)University of Rochester. Laboratory for Laser Energetics (Subcontract Grant 412160-001G

Measurement of Charged-Particle Stopping in Warm Dense Plasma

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[superscript 2]/a)/(k[subscript B]T[subscript e] + E[subscript F]) ~ 0.3] and moderately degenerate [k[subscript B]T[subscript e]/E[subscript 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.United States. Dept. of Energy (Grant DE-NA0001857)United States. Dept. of Energy (Grant DE-FC52-08NA28752)Lawrence Livermore National Laboratory (Grant B597367)University of Rochester. Laboratory for Laser Energetics (Grant 415935-G)University of Rochester. Fusion Science Center (Grant 524431)National Laser User’s Facility (Grant DE-NA0002035)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374

Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions

Anomalous reduction of the fusion yields by 50% and anomalous scaling of the burn-averaged ion temperatures with the ion-species fraction has been observed for the first time in D[superscript 3]He-filled shock-driven inertial confinement fusion implosions. Two ion kinetic mechanisms are used to explain the anomalous observations: thermal decoupling of the D and [superscript 3]He populations and diffusive species separation. The observed insensitivity of ion temperature to a varying deuterium fraction is shown to be a signature of ion thermal decoupling in shock-heated plasmas. The burn-averaged deuterium fraction calculated from the experimental data demonstrates a reduction in the average core deuterium density, as predicted by simulations that use a diffusion model. Accounting for each of these effects in simulations reproduces the observed yield trends.United States. National Nuclear Security Administration (Grant DE-NA0001857)University of Rochester. Fusion Science Center (Grant 415023-G)National Laser User’s Facility (Grant DE-NA0002035)University of Rochester. Laboratory for Laser Energetics (Grant 415935-G)Lawrence Livermore National Laboratory (Grant B600100

Observation of a Reflected Shock in an Indirectly Driven Spherical Implosion at the National Ignition Facility

A 200  μm radius hot spot at more than 2 keV temperature, 1  g/cm[superscript 3] density has been achieved on the National Ignition Facility using a near vacuum hohlraum. The implosion exhibits ideal one-dimensional behavior and 99% laser-to-hohlraum coupling. The low opacity of the remaining shell at bang time allows for a measurement of the x-ray emission of the reflected central shock in a deuterium plasma. Comparison with 1D hydrodynamic simulations puts constraints on electron-ion collisions and heat conduction. Results are consistent with classical (Spitzer-Harm) heat flux.United States. Dept. of Energy (Contract DE-AC52-07NA27344)Brookhaven National Laboratory (Laboratory Directed Research and Development Grant 11-ERD-050

Observation of strong electromagnetic fields around laser-entrance holes of ignition-scale hohlraums in inertial-confinement fusion experiments at the National Ignition Facility

Energy spectra and spectrally resolved one-dimensional fluence images of self-emitted charged-fusion products (14.7 MeV D[superscript 3]He protons) are routinely measured from indirectly driven inertial-confinement fusion (ICF) experiments utilizing ignition-scaled hohlraums at the National Ignition Facility (NIF). A striking and consistent feature of these images is that the fluence of protons leaving the ICF target in the direction of the hohlraum's laser entrance holes (LEHs) is very nonuniform spatially, in contrast to the very uniform fluence of protons leaving through the hohlraum equator. In addition, the measured nonuniformities are unpredictable, and vary greatly from shot to shot. These observations were made separately at the times of shock flash and of compression burn, indicating that the asymmetry persists even at ~0.5–2.5 ns after the laser has turned off. These phenomena have also been observed in experiments on the OMEGA laser facility with energy-scaled hohlraums, suggesting that the underlying physics is similar. Comprehensive data sets provide compelling evidence that the nonuniformities result from proton deflections due to strong spontaneous electromagnetic fields around the hohlraum LEHs. Although it has not yet been possible to uniquely determine whether the fields are magnetic (B) or electric (E), preliminary analysis indicates that the strength is ~1 MG if B fields or ~10[superscript 9] V cm[superscript −1] if E fields. These measurements provide important physics insight into the ongoing ignition experiments at the NIF. Understanding the generation, evolution, interaction and dissipation of the self-generated fields may help to answer many physics questions, such as why the electron temperatures measured in the LEH region are anomalously large, and may help to validate hydrodynamic models of plasma dynamics prior to plasma stagnation in the center of the hohlraum.United States. Dept. of Energy (DE-FG52-07 NA280 59)United States. Dept. of Energy (DE-FG03-03SF22691)Lawrence Livermore National Laboratory (B543881)Lawrence Livermore National Laboratory (LD RD-08-ER-062)University of Rochester. Fusion Science Center (412761-G)General Atomics (DE-AC52-06NA 27279)Stewardship Science Graduate Fellowship (DE-FC52-08NA28752

Using Inertial Fusion Implosions to Measure the T + 3He Fusion Cross Section at Nucleosynthesis-Relevant Energies

Light nuclei were created during big-bang nucleosynthesis (BBN). Standard BBN theory, using rates inferred from accelerator-beam data, cannot explain high levels of [superscript 6]Li in low-metallicity stars. Using high-energy-density plasmas we measure the T([superscript 3]He,γ)[superscript 6]Li reaction rate, a candidate for anomalously high [superscript 6]Li production; we find that the rate is too low to explain the observations, and different than values used in common BBN models. This is the first data directly relevant to BBN, and also the first use of laboratory plasmas, at comparable conditions to astrophysical systems, to address a problem in nuclear astrophysics.United States. Department of Energy (DE-NA0001857)United States. Department of Energy (DE-FC52-08NA28752)United States. Department of Energy (DEFG02-88ER40387)United States. Department of Energy (DE-NA0001837)United States. Department of Energy (DE-AC52- 06NA25396)Lawrence Livermore National Laboratory (B597367)Lawrence Livermore National Laboratory (415935- G)University of Rochester. Fusion Science Center (524431)National Laser User’s Facility (DE-NA0002035)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374)Los Alamos National Laboratory. Laboratory Directed Research and Development Program (20150717PRD2

Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility

An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures T[subscript ion] are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD T[subscript ion] are observed and the difference is seen to increase with increasing apparent DT T[subscript ion]. The line-of-sight rms variations of both DD and DT T[subscript ion] are small, ∼ 150 eV, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed T[subscript ion]. Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT T[subscript ion] greater than the DD T[subscript ion], but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results.Lawrence Livermore National Laboratory (Contract No. DE-AC52- 07NA27344

Experimental Evidence of a Variant Neutron Spectrum from the T(t,2n)α Reaction at Center-of-Mass Energies in the Range of 16–50 keV

Full calculations of six-nucleon reactions with a three-body final state have been elusive and a long-standing issue. We present neutron spectra from the T(t,2n)α (TT) reaction measured in inertial confinement fusion experiments at the OMEGA laser facility at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (E[subscript c.m.]) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two E[subscript c.m.], with the ⁵He ground state resonant peak at 8.6 MeV being significantly stronger at the higher than at the lower energy. The data provide the first conclusive evidence of a variant TT-neutron spectrum in this E[subscript c.m.] range. In contrast to earlier available data, this indicates a reaction mechanism that must involve resonances and/or higher angular momenta than L=0. This finding provides an important experimental constraint on theoretical efforts that explore this and complementary six-nucleon systems, such as the solar ³He(³He,2p)α reaction