The advancement of next-generation microelectronics relies on the development of efficient light sources for extreme ultraviolet (EUV) and beyond-EUV (BEUV) lithography [1]. This work details a computational investigation into optimizing the conversion efficiency (CE) of laserproduced plasma (LPP) sources using tin (Sn) for 13.5-nm EUV emission and gadolinium (Gd) for 6.7-nm BEUV emission. We utilize the 1D radiation-hydrodynamics code HELIOS-CR [2] to perform extensive parameter studies, varying laser intensities from $5 \times 10^{10}$ to $1.5 \times 10^{12} \mathrm{W}/\text{cm}^{2}$ and target thicknesses from 100 nm to 10 microns.

We performed experiments to investigate the response of tamped ablators to a range of direct drive laser pulses generated at the Jupiter Laser Facility (Janus), spanning intensity $\left(10^{9}-10^{12} ~\mathrm{W} / \text{cm}^{2}\right)$, pulse duration (350 ps - 10 ns), wavelength (fundamental and $2^{\text {nd }}$ harmonic), ablator material (Al and Ti) and tamper material (sapphire, fused silica, LiF, and bare metal). For diagnostics, we used line VISAR to obtain pressure/density, and SOP/ultrafast interferometry for ablation plume thermal emission and density, respectively. More than 130 shots were performed spanning the parameter space. This work aims to elucidate mechanisms responsible for enhancement of pressure with tamped ablation and the limitations of pressure enhancement as a function of intensity, pulse duration and wavelength. For tamped ablation, sub-nanosecond pulses at $10^{11} ~\mathrm{W} / \text{cm}^{2}$ are known to obtain pressures well into the 10s of GPa, whereas longer pulses (in the 10 ns range) typically obtain <10 GPa pressure due to nonlinear absorption in the tamper material. The plasma plume diagnostics (SOP and interferometry) of this experiment obtain information about energy deposited in the tamper, which limits pressure in the ablator. A schematic of the experiment and some results are shown in Fig 1. We observe a correlation between absorption in the tamper and reduction of pressure at the ablator and good agreement between simulations of plume dynamics and observed plasma density. Based on the quality of the data and the strength of correlations, these experiments suggest a useful relationship between the upstream plasma properties and the loss of pressure at the ablation plane.

Extreme ultraviolet (EUV) light in commercial lithography systems is generated through a twostep process in which high-power laser pulses irradiate small, mass-limited tin droplets. The masslimited nature of these tin droplets offers several advantages, including optimizing the shape and density of the target formed during the pre-pulse stage to maximize the conversion efficiency (CE) of laser energy into 13.5 nm EUV emission while minimizing debris. Mass-limited targets can also be produced through alternative approaches, such as ultrathin foils, deposited thin films, lowconcentration targets, liquid spray systems, and others. Developing mass-limited targets is an important consideration for next-generation beyond EUV (BEUV) lithography light sources.

Extreme ultraviolet (EUV) lithography uses CO2 laser-produced Sn plasmas emitting at 13.5 nm as a light source. Improving EUV conversion efficiency (CE) has been a major focus over the last decade, involving careful optimization of target design, geometry, and laser parameters. Meanwhile, emerging demands for beyond EUV (BEUV) light sources for next-generation lithography have led to consideration of several targets that provide strong emissions in the shorter wavelength spectral regime $(\leq 7 ~\text{nm})$. One potential target for a BEUV light source is Gd, which emits a strong unresolved transition array (UTA) band around 6.7 nm. Developing BEUV LPP light sources presents many challenges, including achieving efficient laser coupling to produce high CE within a narrow spectral band, addressing opacity issues, and mitigating debris. Previous studies have shown promising results when picosecond-duration pulses were used as the main pulse, yielding higher attainable CEs than nanosecond and femtosecond pulses while also reducing ion kinetic energy and thus potentially reducing mirror debris [1], [2].

This work investigates extreme ultraviolet (EUV) emission at 13.5 nm from laser produced tin plasmas generated using a mass limited laser blow off (LBO) plume.

Generation of strong pressures via laser impulse is of significant interest for national security and inertial confinement fusion applications [1]. One effective approach to increasing pressure in lasermatter interactions is through plasma confinement using optically transparent tamper materials positioned on the laser incident side of the ablator. By confining the laser-generated plasma at the tamper-ablator interface, tampers enable higher pressures, temperatures, and shock velocities when compared to untamped targets [2]–[4].

The warm dense matter (WDM) is an exotic state of matter encountered in inertial confinement implosions for fusion energy, as well as the interiors of giant planets like Jupiter, brown dwarfs, the atmospheres of white dwarfs, neutron star crusts, and newly discovered exo-planets. One efficient way to create WDM is to use protons accelerated by a high-intensity short-pulse laser to isochorically heat dense samples to WDM states. Despite its importance, direct temperature measurements within WDM targets are scarce. This study utilizes an intense proton beam generated by the kilojoule EP laser further focused and guided by a curved foil and cone structure to efficiently heat a thin copper sample. A high-resolution streaked spectrometer tuned to copper Kα fluorescence lines provided bulk temperature measurements every ~2 ps, revealing temperatures exceeding 100 eV in under 50 ps. Particle-in-cell simulations of proton transport and energy deposition closely matched the observed heating dynamics, including transverse temperature gradients revealed by the broadening of Kα lines.

Due to the difficulties associated with experimental measurements of laser-produced plasma (LPP) properties during the earliest stages of plasma evolution, radiation hydrodynamic codes are often used. However, although these codes have been extensively validated in the higher intensity regimes, validation at low to moderate intensities has been limited. In this study, the spatio-temporal electron density evolution in an LPP generated at moderate laser intensities and at various laser wavelengths was validated against the FLASH code for times up to 20 ns. The LPP was generated by focusing the fundamental and various harmonics radiation (1064, 532, and 266 nm) from a 6 ns full width half maximum Nd:YAG laser, at a laser intensity of 10 GW cm−2, onto a copper target. The spatio-temporal density evolution of the expanding plasma was analyzed using Nomarski interferometry. Experimental measurements were found to be consistent with FLASH simulations, and the dependence of electron densities on wavelength was found to be in agreement with analytical models, varying as ne∝λ−0.7. However, slight differences were noted in the widths and shape functions of the experimental and simulated electron density profiles.

This study investigates the expansion dynamics of femtosecond laser-induced plasmas, emphasizing the impact of plasma thermophysical properties and ambient gas composition. Through shadowgraphy experiments and multiphase computational fluid dynamics (CFD) simulations, the influence of parameters such as heat capacity, molecular weight, and thermal conductivity on plume morphology, shockwave evolution, and energy dissipation mechanisms is examined. A mixture multiphase model is implemented to capture the interaction between the plasma and the surrounding gas. Simulation results reveal that plasma expansion is strongly inertia-driven. Results show that differences in plasma properties and ambient conditions affect the shape and temperature distribution of the expanding plume. The early-stage dynamics are primarily dictated by pressure forces, whereas thermal and viscous effects play a growing role in the plume's behavior during later stages of expansion. The CFD findings show the necessity of accurate initial condition characterization, including crater geometry and plasma pressure and temperature, for reliable modeling of plasma evolution in laser ablation processes.

High-resolution spectroscopy of laser ablation plumes or laser-produced plasmas (LPPs) is valuable for various applications, including measuring crowded spectral features, determining spectroscopic constants, characterizing plasmas, and performing isotopic analysis. However, various line broadening mechanisms in plasmas limit its use for several applications. Here, we present the use of saturated absorption spectroscopy (SAS) for Doppler-free linewidth analysis in a uranium LPP. We demonstrate that SAS is effective in obtaining Doppler-free Lamb dip profiles with linewidths on the order of <= 20 MHz. The effects of saturation on the Doppler-broadened absorption and Lamb dip profiles at different pressures and probe intensities are quantified, and their impact on measurements of plasma properties is discussed. Finally, we show that the linewidths of the Lamb dips can be used for quantitative estimates of pressure and natural broadening in the plasma.

The effect of laser pulse duration on energy coupling into a planar silicon target is investigated in experiments at the OMEGA-EP facility by varying the laser pulse length τ—spanning 3 orders of magnitude from 100 ps to 10 ns—while maintaining a constant peak laser intensity, I0=5×1014 W/cm2. In theoretical models, the ablation pressure primarily scales for a given material with laser intensity and wavelength, which are all fixed variables here, allowing us to explore the specific role of laser pulse duration. Two-dimensional radiation-hydrodynamics simulations benchmarked with optical probing of the expanding plasma show that the pulse duration is critical for the ablation pressure to reach a steady state. Moreover, the pulse duration impacts shock decay and multiple wave effects, which strongly dictate the evolving shock profile that propagates within the laser-shocked target as ultimately measured by rear-surface diagnostics. The shock velocities inferred from the theoretical model, after considering shock decay, impedance matching, and shock Hugoniot, are found to be in good agreement with velocimetry measurements. However, discrepancies are observed with simulations for the shorter (0.1 ns) and longer (10 ns) pulse durations, which are respectively attributed to unaccounted contributions of kinetic absorption mechanisms and instabilities in simulations. Published by the American Physical Society 2024

The effect of laser pulse duration on energy coupling into a planar silicon target is investigated in experiments at the OMEGA-EP facility by varying the laser pulse length τ—spanning 3 orders of magnitude from 100 ps to 10 ns—while maintaining a constant peak laser intensity, I_{0}=5×10^{14} W/cm^{2}. In theoretical models, the ablation pressure primarily scales for a given material with laser intensity and wavelength, which are all fixed variables here, allowing us to explore the specific role of laser pulse duration. Two-dimensional radiation-hydrodynamics simulations benchmarked with optical probing of the expanding plasma show that the pulse duration is critical for the ablation pressure to reach a steady state. Moreover, the pulse duration impacts shock decay and multiple wave effects, which strongly dictate the evolving shock profile that propagates within the laser-shocked target as ultimately measured by rear-surface diagnostics. The shock velocities inferred from the theoretical model, after considering shock decay, impedance matching, and shock Hugoniot, are found to be in good agreement with velocimetry measurements. However, discrepancies are observed with simulations for the shorter (0.1 ns) and longer (10 ns) pulse durations, which are respectively attributed to unaccounted contributions of kinetic absorption mechanisms and instabilities in simulations.

Efficient generation of 13.5 nm light with increased conversion efficiency and output power is important for Extreme Ultraviolet (EUV) lithography applications. In this study, we present a computational investigation of plasma dynamics and EUV generation from laser-driven plasma, with specific focus on the influence of magnetic fields, ranging up to 50 T. Simulations show that the plasma expansion is restricted based on the direction and strength of the magnetic field, resulting in an anisotropic plasma confinement, which in turn allows for radiation escape with a reduced loss. Moreover, angle-dependent measurements show an increase in in-band EUV (2% bandwidth around 13.5 nm) yield, reaching a peak enhancement of up to 40% when a magnetic field is applied, particularly when it is oriented perpendicular to the laser axis. The ability to control plasma dynamics by magnetic field offers exciting prospects for optimizing EUV radiation sources.

We present measurements of magnetic fields generated in laser-driven coil targets irradiated by laser pulses of nanosecond duration, 1.053 μm wavelength, 500 J energy, and ∼1015 W/cm2 intensity, at the LULI2000 facility. Using two perpendicular probing axes, proton deflectometry is used to characterize the coil current and static charge at different times. Results reveal various deflection features that can be unambiguously linked to a looping quasi-steady current of well-understood polarity or to a static charging of the coil surface. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped circuit model, supporting the quasi-steady assessment of the discharges. Peak magnetic fields of ∼50 T at the center of 500-μm-diameter coils, obtained at the moderate laser intensity, open up the use of such laser-driven coil targets at facilities worldwide to study numerous phenomena in magnetized high-energy-density plasmas, and its potential applications.

Bright sources of mega-electron volt (MeV) x-rays have many unique applications, including nuclear physics, radiation oncology, and imaging high areal density systems. High intensity lasers (> 10(18) W cm(-2)) incident on mm-thick metal targets can deliver MeV x-rays via the bremsstrahlung process, providing sources with ultrashort duration ( similar to ps) and small source size (similar to 100 mu m). Here, we report on a reproducible regime of laser-driven MeV x-ray sources, where the x-ray dose can be further increased by 60% by coating the metal target with micrometers of plastic. High fidelity numerical simulations indicate that the interaction is a result of relativistic transparency in the preplasma. Though relativistic transparency is present in both cases, the greater sound speed and smaller ion inertia of the plastic target allow the laser to more deeply penetrate and couple more efficiently to electrons. Radiography with this system demonstrates a resolving power < 300 mu m, important for imaging applications.

Magnetized liner inertial fusion (MagLIF) is an attractive concept for producing thermonuclear fusion reactions. The MagLIF platform involves the operation of Helmholtz coils to apply a 15 Tesla axial magnetic field to the load region, where a cylindrical, fuel-filled metal liner is imploded by a 20 MA current pulse. The fringe field from these coils extends into the transmission line that delivers the current to the target. We investigated the extent to which this applied field disturbs the nominal power flow within that transmission line. A simplified model of the geometry shows that adding the applied magnetic field results in magnetic field lines that connect the cathode to the anode, suggesting electrons may not be magnetically insulated in this region. Particle-in-cell simulations indicated the addition of the applied magnetic field would not significantly impact the current delivery to the load. Velocimetry was used to experimentally assess the current delivery with and without the applied magnetic field. We find no measurable effects of the applied field on current delivery in the configuration investigated in this study.

This study compares the role of laser wavelength on the early time electron density evolution of the plasma following laser ablation using Nomarski interferometry. The laser-produced plasma was generated by focusing fundamental (1064 nm), second (532 nm), and fourth (266 nm) harmonic radiation from a 6 ns Nd:YAG laser at a laser intensity of 10 GW/cm(2) onto a copper target placed in vacuum. The dependence of plasma properties such as the electron density distribution, plume velocity, and plume morphology on the laser wavelength are discussed.

The laser ablation and subsequent shock generation in solid targets plays an important role in a variety of research topics from equation of state models for materials to inertial confinement fusion. One of the long-standing issues is the knowledge of ablation depth in the picosecond time regime. We report on a direct technique for determining the ablation depth in aluminum using x-ray diffraction data from Linac Coherent Light Source at the Stanford Linear Accelerator Center. This technique gives a direct measurement of the shock wave propagation in the bulk target, enabling an ability to discern early timescale physics from late timescale effects not available in postmortem analysis. We find that the ablation depths only vary by 0.2 mu m across three orders of magnitude of laser intensity, while the pressure increased by a factor of 10 following a square root dependence on laser pulse energy. We further observe that the ablation depth in this intensity range ( 1011-1013 W/cm2 in intensity, corresponding to 0.8-80 J/cm2 in fluence) cannot be modeled by a universal scaling law, given the complexity of the mechanisms governing laser ablation in this intensity regime.

Measurement of the temporal evolution of laser-produced plasma temperature is very important for many of its applications, and several plasma diagnostic tools are routinely used by researchers. However, it is very challenging to measure the properties of the plasma at the early and late times of its evolution using a single diagnostic tool. In this study, we combined emission and laser absorption spectroscopy to compare the excitation temperatures of a laser-produced uranium plasma system. Several U I transitions in the near-infrared spectral range (775–800 nm) were considered, and the Boltzmann plot method was used to measure the excitation temperatures using both emission and absorption spectroscopy. Emission spectroscopy provided early-time temperature measurements of the plasma up to times 2–20 µs, while absorption spectroscopy provided temperature measurements at late times of plasma evolution (for times 5–80 µs). The emission and absorbance of U I transitions were found to follow the Boltzmann distribution, indicating the plasma is likely in the state of local thermodynamic equilibrium even at late times of its lifetime. The emission and absorption-based time-resolved excitation temperatures demonstrated good agreement at earlier times (≤15 µs) in the overlapped temporal region, while a deviation in the measured values was seen at times (≥15 µs), and potential reasons for such a disagreement are discussed.