On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Abstract. Spatial variation in snow depth is a main driver of heterogeneity in discontinuous permafrost landscapes, exerting a strong control on thermal and hydrological processes, vegetation dynamics, and carbon cycling. Topography and vegetation are understood to play an important role in driving variation in snow depth, but complex morphology often impedes efforts to disentangle these drivers. Maps of ground, vegetation and snow surface elevation were collected using an Unmanned Aerial Vehicle (UAV) over multiple years across a watershed on the Seward Peninsula in Alaska. Here, we quantify drivers of snow depth variation using the inferred maps of snow depth during peak snow accumulation in 2019 and 2022 and collocated ground surface elevation and vegetation height. A novel approach to extract microtopographic information from complex landscape morphologies is used to classify different features (e.g. drainage paths, risers and terraces, thermokarst patterned ground) and characterize their relationships with snow depth variation. A simple model developed using topographic information alone is shown to correlate strongly with local snow depth variation where vegetation height is low. We build a machine learning model to quantify snow trapping by shrub canopies in the watershed and show that snow trapping can be characterized by an exponential function of canopy height above snow (RMSE = 0.12 m, R2 = 0.5). Finally, we demonstrate that relationships between microtopography, vegetation height, and snow depth hold in years of deep and shallow snowpack. These results can be applied to improve representation of heterogeneity and vegetation-snow feedbacks in Earth System Models and to increase the spatial resolution of pan-arctic estimates of snow depth.

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. As recently reported, a burning plasma state, where the alpha-heating in the plasma is the primary source of heating, was achieved in laboratory experiments. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin ``burn propagation'' into surrounding cold fuel, enabling the possibility of high energy gain. While ``scientific breakeven'' (i.e. unity target gain) has not yet been achieved, this talk reports the first controlled fusion experiment on the National Ignition Facility to produce capsule gain greater than unity (here 5.8) and reach ignition by many different formulations of the Lawson criterion. In the talk, we will discuss some key basic physics inertial confinement fusion (ICF) principles behind the burning plasma and ignition results as well as discuss future challenges.

Understanding the structure and function of vasculature in the brain requires us to monitor distributed hemodynamics at high spatial and temporal resolution in three-dimensional (3D) volumes in vivo. Currently, a volumetric vasculature imaging method with sub-capillary spatial resolution and blood flow-resolving speed is lacking. Here, using two-photon laser scanning microscopy (TPLSM) with an axially extended Bessel focus, we capture volumetric hemodynamics in the awake mouse brain at a spatiotemporal resolution sufficient for measuring capillary size and blood flow. With Bessel TPLSM, the fluorescence signal of a vessel becomes proportional to its size, which enables convenient intensity-based analysis of vessel dilation and constriction dynamics in large volumes. We observe entrainment of vasodilation and vasoconstriction with pupil diameter and measure 3D blood flow at 99 volumes/second. Demonstrating high-throughput monitoring of hemodynamics in the awake brain, we expect Bessel TPLSM to make broad impacts on neurovasculature research.

The elastic properties of the cubic (c-) phase GaN confined in a nanoscale one-dimensional (1D) v-grooved 'Si(001) substrate are investigated. Along a similar to 900 nm-wide v-groove formed with two facing Si(111)-type facets, submicrometer-wide c-GaN is achieved by the hexagonal- to c-phase (h-c) transition from the h-GaN which plays the role of an interlayer in its epitaxy on Si. The resulting nonplanar stack of c-GaN/h-GaN on Si has complicated stress distribution. This work focuses on the elastic properties the c-GaN, which are critically affected by its low dimensionality, and presents experimental evidence for it with an analytical stress modeling. A reciprocal lattice map reveals that the c- GaN in each groove consists of several micrometer-long single crystals which are microscopically tilted from each other in their serial coalescence, as its unit structures. The corresponding micrometer-scale lateral correlation length, d(c), results from the h-c transition that is interrupted by the groove imperfections generated in fabrication and the stress fluctuation in the misoriented h-GaN interlayer. The modeling suggests that d(c) is long enough to induce the tensile stress, dominating with the longitudinal strain parallel to the groove which is , similar to 2.5x the transverse strain, and the c-GaN can be regarded as a serial array of a quasi-1D unit structures which retain such anisotropic stress resulting from their geometrical shape. The Poisson ratio of the c-GaN in < 110 > is similar to 0.21, close to 0.26 from a theoretical prediction. The variation of the c-phase bandgap under the given tensile stress is addressed.

The incorporation of In on the non-polar, piezoelectric-free (001) facet of cubic (c-) GaN epitaxially grown over a Si(001) substrate by metal-organic vapor phase epitaxy is reported. Relying on a hexagonal (h-) to c-phase transformation during epitaxy on an 800 nm-wide, Si(111)-faceted v-groove patterned into the substrate, the GaN epilayer at cross sectional view retains a triangular c-phase inside a chevron-shaped h-phase that results in a top surface bounded by a (001) facet parallel to Si(001) at the center and (11¯01) facets at both edges. A stack of five, ∼3 nm-thick, InxGa1−xN/GaN quantum wells (QWs) was deposited on the double-phased top surface. The c-phase region up to the QWs keeps extremely small misfit (∼0.002) to the fully relaxed h-GaN underneath it and is in tensile stress implying undefected by the h-c phase interface. The In incorporation on a strained non-polar (001) of c-GaN is comparable with that on totally relaxed semi-polar (11¯01) of h-GaN without noticeable adatom migration across the phase boundary, and sufficient to provide the room-temperature green emission at 496 nm from the c-InxGa1−xN/GaN QWs on Si(001) in photoluminescence.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.