Session NM10: Mini-Conference: Progress in Making IFE-based Concepts a Reality II

Experimental demonstration of ozone gratings created by interfering ultraviolet lasers

The final optics in a laser-driven inertial fusion energy (IFE) plant would face significant neutron, x-ray, and debris fluxes from the target for extended periods of continuous operation. Manufacturing optics that can properly operate while enduring this harsh environment is a non-trivial challenge. To address this issue, we can create optics in gas by using interfering ultraviolet lasers to induce substantial density modulations in an ozone-oxygen mixture via periodically heating the gas in space. The spatially modulated gas will then act as a volume diffraction grating. These transient optics are debris-resistant and feature much higher damage thresholds than traditional solid-state optics, providing a promising method towards efficiently manipulating high-energy laser beams. In this work, we created an ozone grating experimentally using the 4th harmonic of an Nd:YAG laser. We demonstrated the efficient diffraction of a nanosecond probe beam by a gas grating and characterized several key properties of the system, including its lifetime and the impact of energy deposition on the temporal evolution of the gas response. The experimental results support a theoretical model of the process that includes the heat deposition mechanism, gas dynamics, and the resultant optical response, suggesting parameters that allow the efficient creation of gas gratings.   

Nonlinear Modeling of Photochemically-Induced Gaseous Optical elements

A proof-of-principle experiment [1] recently demonstrated dielectric mirrors made out of neutral gas, operating at fluences above 1.5 kJ/cm2 at 10 Hz repetition rate with diffraction efficiencies above 95%. By increasing the damage threshold by two or three order of magnitude compared to solid elements, such gaseous optics have a transformative potential for high-power laser applications such as Inertial Fusion Energy (IFE). Their operation relies on the modulated energy deposition of a low-energy “imprint” beam (such as a pair of overlapping beams) via absorption by a dopant element in the gas (e.g., ozone, for UV imprint beams). The resulting gas heating can initiate an acoustic/entropy wave which modulates the gas density and hence its refractive index, turning the gas into a grating or other diffractive optics elements. Here, we present results from a comprehensive modeling suite that includes: i) the chemistry of UV absorption by ozone and gas heating from the subsequent chemical reactions; ii) the nonlinear hydrodynamic response of the gas from a 1D hydrodynamic code resolving Euler equations; iii) a 3D Fresnel diffraction code to calculate the diffraction of an external, high-power laser off the resulting index modulation. For small perturbations, the simulations show an excellent agreement with linear theory [2]. For stronger perturbations, nonlinear effects arise due to the depletion of ozone and nonlinear wave excitation. We will present comparisons with recent experiments at Stanford University, and discuss future directions and applications of such gas-optics.

[1] Y. Michine and H. Yoneda, Commun. Phys. 3, 24 (2020).

[2] P.Michel et al., Submitted to Phys. Rev. Applied (2024).   

Development of a Cross-Beam Energy Transfer–Mitigation Demonstration Platform

The efficient and uniform absorption of laser energy is essential for successful laser-direct-drive inertial confinement fusion implosions. Cross-beam energy transfer (CBET) is a major challenge to this goal. CBET significantly reduces the total laser absorption by tens of percent and increases the nonuniformity of the absorption above the 1% rms required for successful implosions. Modeling has shown that adding large bandwidth to the lasers will significantly mitigate the effects of CBET. Absorption fractions and uniformities near no-CBET levels can be achieved with large bandwidth. To this end, the large-bandwidth FLUX laser is being deployed as a proof-of-concept laser on the OMEGA Laser System. Demonstrating CBET mitigation during a multibeam implosion, however, is not straightforward since a single large-bandwidth beam will not significantly affect the overall implosion performance. To measure a noticeable CBET mitigation effect, we have developed a beamlet diagnostic that images unabsorbed implosion light. A beamlet of light from each OMEGA beam is captured as a discreet spot in the image. The intensities of a few of the beamlets will be strongly dependent on CBET with the FLUX laser, as determined by the beam/diagnostic geometry. With the FLUX laser and the beamlet diagnostic, the mitigation of CBET using bandwidth will be demonstrated.   

Radiography Studies of Isolated Feature Growth on Laser-Driven Capsules and Foils

Calculations have motivated consideration of target features as a source of hydrodynamic instability in direct-drive implosions.  We report results of radiography of surface features on imploding spheres and planar targets.   The initial experiments were done with capsules selected to have (exceptional) domes features intrinsic to their fabrication within the field of view of the radiography.  Although the domes were predicted to generate highly visible signatures, the observed contrast was weak and suggested that the growth was reduced.   Two primary hypotheses under consideration are (i) sensitivity of the dome growth to 3D deviations from assumption of 2D axi-symmetry in calculation, and (ii) target heating by fast electrons generated by laser-plasma instability.   We will also discuss more recent planar experiments in which we are using 3-D printed foils, which provide reproducible and varied features, to test these hypotheses.      

Utilizing Two Photon Polymerization 3D Printing for Producing ICF Targets and Studying Imprint Mitigation

Recent advances in Two Photon Polymerization (2PP) technology have enabled the ability to design and print targets approaching the scale and precision required for inertial confinement fusion (ICF) experiments. We present the iterative process of producing complex planar targets with low density foam structures and translating these towards spherical targets to be shot on the OMEGA and EP lasers at the Laboratory for Laser Energetics. The foam structure was developed for imprint mitigation, relevant to direct-drive ICF. Several iterations of foam structure led to the development of full, single-print spherical and planar targets with foam and solid density material. Measurements utilizing x-ray backlit radiography conclusively show the foam significantly mitigated laser imprint for targets of similar mass. Previous 2PP efforts have encountered difficulties with stitching print elements together to make larger targets while maintaining high resolution. We present target development work that has been done to address and fix the source of these issues.   

Wetted foam target production for Inertial Fusion Energy*

Production of wetted foam capsules for inertial fusion energy (IFE) is priority research opportunity listed in the DOE Basic Research Needs for IFE workshop report¹. A wetted foam target typically consists of or includes a spherical polymer shell with a layer of low-density polymer foam just inside the shell. The foam layer would be used to wick in and create a layer of liquid DT filling the foam.

We are developing spherical foam shells with density ≤50mg/cm³. Two different approaches to the manufacture of these shells are being pursued: microencapsulation and additive manufacturing via two photon polymerization (2PP). We report on our progress towards making the low-density foam shells by these two approaches.

Initial work to prepare dicyclopentadiene (DCPD) foam for micro-encapsulation will be discussed. Methods for deterministic symmetry control in micro-encapsulated capsules/foam shells will also be outlined.

*Portions of the presented work were performed under General Atomics Internal R&D funding and portions receive funding from the DOE FES IFE-STAR hubs STARFIRE and RISE.   

Inertial Fusion Energy Target Designs using Low Velocity, High Mass Targets and Advanced Ignition Schemes

Focused Energy is pursuing inertial fusion energy using advanced ignition schemes --proton fast ignition and shock ignition -- using laser direct drive to drive a low velocity (~ 200 km/s), high mass (~ 2.5 mg DT) implosion.   The low implosion velocity has several advantages when compared to the high implosion velocities needed for central hotspot ignition:  the thick fuel layer results in a lower in-flight aspect ratio (IFAR), which is a measure of the hydrodynamic stability of the implosion to break up in-flight, increased robustness to low mode asymmetry, and higher target gain.  In addition, the lower velocity can result in reduced laser intensity and LPI risk.   The trade-off is that we need an alternate method to ignite these designs – we are considering proton fast ignition as well as shock ignition schemes.    In this talk, we will describe our current designs as well as the trade-offs that will feed into our integrated systems model, which will ultimately determine the best combination of target, laser, final optics, and reactor chamber design for an inertial fusion power plant.   

Integrated Model and Digital Twin Development at Focused Energy

In the development of a commercially viable, scientifically feasible inertial fusion energy (IFE) reactor concept everything is a tradeoff: current and near-future technological capabilities, uncertainties in the science, and constraints on the final cost of electricity result in a challenging optimization problem that considers all components of a fusion power plant. Tackling this problem requires an integrated approach that properly captures all design parameters and can support design optimization studies, set R&D priorities, and ultimately act as a true digital twin of demonstration facilities. 

Focused Energy is rapidly developing an integrated simulation model that informs inertial fusion power plant (IF-PP) design and R&D efforts. The model is being used to inform the design of our conceptual reactor and to calculate the overall cost of an IF-PP. We will combine robust software, multiple levels of simulation and machine learning, multi-objective optimization, and uncertainty quantification to accelerate Focused Energy’s development of an IF- PP design.

In this talk we will describe the software considerations and initial design of the Focused Energy integrated model. We will discuss key decision points in our preconceptual IF-PP design and the numerical approach being used to inform those decisions. Finally, we will present current and future efforts in machine learning to accelerate simulations and to include the results of various high-fidelity Multiphysics simulation studies.   

MEC and MEC-U: Discovery science and technology and workforce development for fusion energy

The MEC-U project will deliver a new open-access experimental facility on an LCLS XFEL hard X-ray beamline. Its major scope elements include: a high-intensity, high-rep-rate laser producing 1 petawatt (PW) peak power at 10 Hz; a high-rep-rate shock compression laser at 10 Hz; a kilojoule shock compression laser; and a flexible target chamber enabling a broad range of experimental configurations exploiting the diagnostic power of LCLS. The facility will enable a broad range of experiments to address priority HED and fusion plasma science with unmatched precision and will be built to support future upgrades to the laser systems.

While conceived as a facility for general HEDLP plasma research, MEC–U can address a range of priority research needs for the developing IFE program.  For example, in experiments to improve understanding of how the flaws and impurities within IFE targets will impact the driver coupling efficiency, the fuel compressibility, and material performance limits at extreme conditions. While several US facilities can reach the relevant conditions, only MEC-U is designed to reach them with a diagnostic resolving power sufficient to discover and constrain the underlying physics controlling plasma performance.  Further, MEC-U will contribute to the development of the required high repetition rate lasers and target delivery systems necessary for IFE, through the development of experiments at up to 10 Hz, with associated developments in AI/ML and operation under harsh environments.