Michael Rosenberg

Irradiating a small capsule containing deuterium and tritium fuel directly with intense laser light causes it to implode, which creates a plasma hot enough to initiate fusion reactions between the fuel nuclei. Here we report on such laser direct-drive experiments and observe that the fusion reactions produce more energy than the amount of energy in the central so-called hot-spot plasma. This condition is identified as having a hot-spot fuel gain greater than unity. A hot-spot fuel gain of around four was previously accomplished at the National Ignition Facility in indirect-drive inertial confinement fusion experiments where the capsule is irradiated by X-rays. In that case, up to 1.9 MJ of laser energy was used, but in contrast, our experiments on the OMEGA laser system require as little as 28 kJ. As the hot-spot fuel gain is predicted to grow with laser energy and target size, our work establishes the direct-drive approach to …

Magnetic reconnection is a ubiquitous and fundamental process in plasmas by which magnetic fields change their topology and release magnetic energy. Despite decades of research, the physics governing the reconnection process in many parameter regimes remains controversial. Contemporary reconnection theories predict that long, narrow current sheets are susceptible to the tearing instability and split into isolated magnetic islands (or plasmoids), resulting in an enhanced reconnection rate. While several experimental observations of plasmoids in the regime of low-to-intermediate β (where β is the ratio of plasma thermal pressure to magnetic pressure) have been made, there is a relative lack of experimental evidence for plasmoids in the high-β reconnection environments which are typical in many space and astrophysical contexts. Here, we report strong experimental evidence for plasmoid formation in laser …

We present results from X-ray imaging of high-aspect-ratio magnetic reconnection experiments driven at the National Ignition Facility. Two parallel, self-magnetized, elongated laser-driven plumes are produced by tiling 40 laser beams. A magnetic reconnection layer is formed by the collision of the plumes. A gated X-ray framing pinhole camera with micro-channel plate (MCP) detector produces multiple images through various filters of the formation and evolution of both the plumes and current sheet. As the diagnostic integrates plasma self-emission along the line of sight, 2-dimensional electron temperature maps are constructed by taking the ratio of intensity of these images obtained with different filters. The plumes have a characteristic temperature eV at 2 ns after the initial laser irradiation and exhibit a slow cooling up to 4 ns. The reconnection layer forms at 3 ns with a temperature eV as the result of the collision of the plumes. The error bars of the plumes and current sheet temperatures separate at ns, showing the heating of the current sheet from colder inflows. Using a semi-analytical model, we find that the observed heating of the current sheet is consistent with being produced by electron-ion drag, rather than the conversion of magnetic to kinetic energy.

Focussing laser light onto the surface of a small target filled with deuterium and tritium implodes it and leads to the creation of a hot and dense plasma, in which thermonuclear fusion reactions occur. In order for the plasma to become self-sustaining, the heating of the plasma must be dominated by the energy provided by the fusion reactions—a condition known as a burning plasma. A metric for this is the generalized Lawson parameter, where values above around 0.8 imply a burning plasma. Here, we report on hydro-equivalent scaling of experimental results on the OMEGA laser system and show that these have achieved core conditions that reach a burning plasma when the central part of the plasma, the hotspot, is scaled in size by at least a factor of 3.9 ± 0.10, which would require a driver laser energy of at least 1.7 ± 0.13 MJ. In addition, we hydro-equivalently scale the results to the 2.15 MJ of laser …

Precise modeling of shocks in inertial confinement fusion implosions is critical for obtaining the desired compression in experiments. Shock velocities and postshock conditions are determined by laser-energy deposition, heat conduction, and equations of state. This paper describes experiments at the National Ignition Facility (NIF)[EM Campbell and WJ Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)] where multiple shocks are launched into a cone-in-shell target made of polystyrene, using laser-pulse shapes with two or three pickets and varying on-target intensities. Shocks are diagnosed using the velocity interferometric system for any reflector (VISAR) diagnostic [PM Celliers et al., Rev. Sci. Instrum. 75, 4916 (2004)]. Simulated and inferred shock velocities agree well for the range of intensities studied in this work. These directly-driven shock-timing experiments on the NIF provide a good measure of early …

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)]. 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.

Magnetic fields generated from a laser-foil interaction are measured with high fidelity using a proton radiography scheme with in situ x-ray fiducials. In contrast to prior findings under similar experimental conditions, this technique reveals the self-generated, Biermann-battery fields extend beyond the edge of the expanding plasma plume to a radius of over 3.5 mm by t=+1.4 ns, a result not captured in state-of-the-art magneto-hydrodynamics simulations. An analysis of two mono-energetic proton populations confirms that proton deflection is dominated by magnetic fields far from the interaction (>2 mm) and electric fields are insignificant. Comparisons to prior work suggest a new physics mechanism for the magnetic field generation and transport in laser-solid interactions.

Energy coupling is a critical determinant of implosion performance. In this talk, results from experiments that systematically study laser drive during different times of the laser pulse on the NIF are presented. Shock radiography using solid spheres is used to infer early-time coupling during the foot and rise to the main pulse. Since shocks decouple from the laser drive shortly after peak intensity, studying shock trajectories isolates the effect of laser drive during this time. Simulations that include the effects of non-local transport and CBET reproduce these trajectories very well for varying on-target intensities, though some uncertainties remain. Implosions are sensitive to coupling throughout the drive. Self-emission and backlit implosion trajectories will be presented for varying on-target intensities. Implications for direct-drive implosion performance will be discussed.

Hot electron preheat and its mitigation using mid-Z dopants has been quantified in ignition-scale polar direct-drive (PDD) implosion on the National Ignition Facility (NIF). Hard x-ray emission from buried Ge-doped layers was measured in NIF implosions of 2.3 mm CH shells at 730 and 850 kJ (intensities 1 to 1.25 x10 15 W/cm 2), some of which contained Si dopant in the outer portion of the ablator. The Si-doped ablators were found to reduce stimulated Raman scattering (SRS) and consequently the hot electron energy deposition in the unablated shell by 30%, close to levels (~ 0.2% of laser energy) considered tolerable for direct drive. The extension of these experiments to ignition-relevant 3.0-mm, 1.3-MJ CH implosions and the extrapolation of these results to cryogenic DT designs will be discussed. These results provide a promising hot-electron preheat mitigation strategy that can expand the operable design …

Advancements in experimental techniques and data-driven modeling have resulted in considerable progress in DT-layered implosion experiments on the OMEGA laser, bringing the possibility of thermonuclear ignition in direct-drive configurations with megajoule-class lasers closer to reality. Statistical modeling is applied to every cryogenic implosion to infer various degradation mechanisms such as preheat, engineering features, and vapor pressure. Dedicated focused-physics implosions were performed, scanning a single parameter to experimentally observe and quantify the impact of each of these challenges on energy coupling and hydrodynamic stability. These studies include subscale implosions with bilayer targets where it was shown that Si dopants increase the threshold for the two-plasmon-decay instability, enabling greater intensities to be delivered on target without significant fast-electron preheat …

Experiments have been conducted on the OMEGA EP laser facility to study the effect of density scale length and overlapping beam geometry on laser-plasma instabilities near and below the quarter-critical density. Experiments were conducted in both planar geometry (density scale length L n∼ 190 to 300 μm) and spherical geometry (L n∼ 150 μm) with up to four overlapping beams and were designed to have overlapped intensities and density scale lengths comparable to OMEGA spherical experiments, but with many fewer beams. In comparison with previous experiments on OMEGA and National Ignition Facility, it is confirmed that shorter density scale lengths favor the two-plasmon decay (TPD) instability, while longer density scale lengths favor stimulated Raman scattering (SRS). In addition, for experiments at the same scale length and overlapped laser intensity, higher single-beam intensities favor SRS …

Statistical modeling of direct-drive cryogenic implosions on OMEGA has led to substantial improvements in nuclear yield and metrics related to alpha-heating. 1, 2 Techniques like these depend on the quality and range of data, and can be used to motivate experiments that refine further analyses. We report on studies in beam to target radius (R b/R t), in-flight aspect ratio (IFAR), and target radius (R t) that reduce sources of uncertainty to enhance understanding. A similar approach might be used at facilities capable of high shot rates. New regression formulae are derived with respect to hydrodynamic seeds, growth, and effects, and degradations to yield and areal density, providing new perspectives on design. Optimization and extrapolation of these data motivate the development of future targets and laser systems, and suggest Lawson parameters>~ 1 are possible at incident energies at/below 2.6 MJ …

Hydrodynamic simulations with high-Z ablators indicate a uniform increase absorption that would lead to higher implosion velocities in laser-direct-drive (LDD) inertial confinement fusion (ICF) experiments. An increase in implosion velocity achieved exclusively by replacing the ablator (without having to intentionally adjust the IFAR or adiabat) could lead to higher primary nuclear yields in cryogenic implosions at the expense of additional radiation preheat by introducing a high-Z material. One candidate that has been studied recently is a pure Silicon 2-μm thick ablator where simulations do exhibit more laser energy absorption without an increase on hot-electron production. Preliminary experimental results will be shown that an increase in laser absorption and decrease in hot-electron production has been observed generating a higher yield-over-clean (YOC) as compared to tradition plastic CH ablators with a small …

• Hydrodynamic scaling underpins the extrapolation of direct-drive implosion performance from OMEGA to NIF, but not all aspects of physics scale (eg hot electron preheat)

Ablation of heterogeneous material is not well understood and will play an important role in the implosion of targets such as the Polar Direct Drive-Wetted Foam concept 1 that employ advanced fabrication techniques. To this end, we are designing an experiment in a simplified planar configuration on the OMEGA platform which will drive a shock through the target by ablating a heterogeneous medium of two-photon-polymerization (2PP) 3D-printed lattice and liquid deuterium, where shock propagation speed and planarity of the shock structure relative to lattice morphology will be measured. To make sure the bulk of the ablation occurs in the heterogenous medium, we explore several options for front window material and thickness to minimize burnoff time, thereby maximizing ablation of the desired heterogenous material beneath. Results of initial 1D and 2D simulations of shock propagation will be presented. We …

We present results from high-aspect ratio magnetic reconnection experiments driven at the National Ignition Facility. In these experiments, 40 beams are tiled to produce two elongated laser-driven plumes which are self-magnetized by the Biermann battery effect. As these plumes expand, they collide and drive magnetic reconnection. A gated X-ray framing camera with micro-channel plate (MCP) detector produces 16 filtered images of the formation and evolution of both the plumes and current sheet up to 8 ns after the lasers fire. The 2-dimensional electron temperature is estimated by taking the ratio of intensity of these images obtained with different filters, a technique developed for the sub-keV temperatures expected in these experiments. The data show that the plasma plumes are initially several hundreds of eV hot and collide at~ 2 ns after initial laser irradiation. As the current sheet forms, we observe that it …

Recent spherical-target laser–plasma interaction experiments, performed on the OMEGA EP laser, have been analyzed for stimulated Raman scattering (SRS). This has been motivated by results obtained on the National Ignition Facility (NIF) that have demonstrated the importance of SRS, and in particular SRS side scatter, for directly driven inertial confinement fusion (ICF) conditions [Rosenberg et al. Phys. Rev. Lett. 120, 055001 (2018); Michel et al. Phys. Rev. E 99, 033203 (2019)]. The analysis, based on a generalized ray tracing approach, is described and is shown to explain the observed scattered light spectra: it identifies SRS convective scattering, from portions of each incident beam where the scattered electromagnetic wave is generated in the direction parallel to contours of constant density, as the dominant contribution. This result is novel, as SRS is mostly associated with plasmas of higher electron …

A new, efficient, algorithmic approach to create illumination configurations for laser driven high energy density physics experiments is proposed. The method is applied to a polar direct drive solid target experiment at the National Ignition Facility (NIF), where it is simulated to create more than x2 higher peak pressure and x1.4 higher density by maintaining better shock uniformity. The analysis is focused on projecting shocks into solid targets at the NIF, but with minor adaptations the method could be applied to implosions, other target geometries and other facilities.

We investigate the feasibility to achieve high nuclear yields using a liquid DT-wetted layer capsule directly driven by the National Ignition Facility's (NIF's) current laser capabilities. The capsule is composed of a thin plastic shell used to enclose a thick annular 3D-printed matrix layer that contains the liquid DT fuel. Comparisons across several simulation codes indicate that a high level of laser absorption can occur that drives a central gas pocket convergence of 15 enabling higher levels of gain and the potential to robustly ignite (using the current laser energy available at NIF). High laser absorption is consistent with previous polar direct drive (PDD) MJ-class NIF experiments where> 95% capsule absorption of the laser drive energy was achieved using a 5 mm diameter plastic capsule. The results of simulations using the HYDRA radiation-hydrodynamics code will be shown to elucidate the laser driven 3D …

In recent years, the application of statistical modeling has enabled development of accurate predictive models of fusion yield in direct-drive inertial confinement fusion experiments on OMEGA. Through the choice of separable basis functions and a careful selection of parameters guided by physical considerations, the statistical model has been used to quantify the effect of all the major sources of fusion yield degradation on OMEGA. The strongest dependencies include the age of the deuterium tritium fuel from filling to shot time, the asymmetry of inferred ion temperatures as a proxy of the L= 1 mode, the ratio of the laser beam radius to target radius as a proxy of the illumination nonuniformity of overlapping beams, and parameters related to the hydrodynamic stability such as the shell adiabat, in-flight aspect ratio and convergence ratio. Here, we present statistical models of the areal density, ion temperature and …