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Metal foils with precise thicknesses are required for use as laser targets in high energy density physics experiments. To achieve the micrometer scale tolerances accompanying these applications, the authors have developed a laser ablation technique utilizing burst pulses from a femtosecond laser operated in concert with a high-speed galvo scanning system. For authors’ application, an optimum laser processing format consisted of 250 fs laser pulses arranged in pairs, with the pulse separation in each pair equal to 400 ps and high galvo scanning speeds (∼2000 mm/s). These parameters enable the authors to thin and shape metals (depleted uranium, beryllium, tantalum, and iron) and remove unwanted variations in foil thickness over meaningful foil sizes. Double femtosecond pulses enable ablation that mitigates some undesirable pre-existing surface structures such as residual scalloping and scratches and lead to a surface roughness near 300 nm. Precise ablation rates were correlated to laser processing area and foil thickness. Finally, ablation rates were correlated to bulk surface temperature by tracking workpiece heating during ablation. These techniques yield the ability to apply highly controlled thickness maps with micrometer accuracy and tens of nanometer precisions. This control has demonstrated the ability to level foils with micrometer scale thickness deformities and create various programmed patterns that are difficult or out of reach of traditional machining methods.

Complex and precise research targets are required for the inertial confinement fusion (ICF) experiments conducted at the National Ignition Facility. During the National Ignition Campaign (NIC) the ...

Microfabrication techniques are critical for the rapid prototyping and development of applications for cutting edge materials. Recently diamond has gained considerable interest for quantum photonic, biosensing, inertial confinement fusion and magnetometer applications. In this article, ultra-high aspect ratio milling of diamond micropores by photon, ion and electron based methods is reported. A multiphoton absorption laser ablation approach is revealed to rapidly produce sub-10 μm diameter micropores in diamond with an aspect ratio of 14:1 and a tapered profile at the surface interface. Chemically active, oxygen focused ion beam milling produces high-aspect ratio pores in diamond with an aspect ratio of 65:1 and minimal tapering over the length of the pore, overcoming the physical interaction volume limitations imposed in conventional gallium based focused ion beam milling and laser ablation methods. Oxygen-mediated electron beam induced etching is revealed to negate the limitations imposed by physical sputtering mechanisms utilized in focused ion beam milling via the direct initiation of chemical reactions at the receding surface, producing aspect ratios on the order of 200:1. Numerical simulations reveal the physical basis for the superior aspect-ratio pore milling of the oxygen focused ion beam milling and electron beam induced etching methods. Our results demonstrate direct-write methods for the fabrication of ultra-high aspect micropores in diamond and provide insight into the underlying mechanisms of these physical processes. The three methods demonstrated here can be interchanged for applications based on the desired characteristic aspect ratio and process throughput.

X-ray penumbral imaging has been successfully fielded on a variety of inertial confinement fusion (ICF) capsule implosion experiments on the National Ignition Facility (NIF). We have demonstrated sub-5 μm resolution imaging of stagnated plasma cores (hot spots) at x-ray energies from 6 to 30 keV. These measurements are crucial for improving our understanding of the hot deuterium-tritium fuel assembly, which can be affected by various mechanisms, including complex 3-D perturbations caused by the support tent, fill tube or capsule surface roughness. Here we present the progress on several approaches to improve x-ray penumbral imaging experiments on the NIF. We will discuss experimental setups that include penumbral imaging from multiple lines-of-sight, target mounted penumbral apertures and variably filtered penumbral images. Such setups will improve the signal-to-noise ratio and the spatial imaging resolution, with the goal of enabling spatially resolved measurements of the hot spot electron temperature and material mix in ICF implosions.

Here we present data from copper foil X-pinch experiments conducted on the 250kA, 150ns rise-time GenASIS driver at UCSD. Previous laser-cut foil experiments on this driver showed that foil X-pinches produced micron-order spot sizes and nanosecond-order single emission pulses1. For the current experiments, three different designs of foils were cut using two different laser-cutting platforms at the General Atomics Laser Micro-Machining (LMM) Center. From these targets we present a dataset including absolutely calibrated flux and both L-shell and K-shell spectroscopy comparing the parameters of foils of different designs to one another and to Cu wire X-pinches of comparable cross-point mass. The foil X-pinches show improved reliability over the wire X-pinches in emission timing, pulse width, source location, single-source production.

Nanosecond (ns) laser ablation can provide a competitive solution for silicon micromachining in many applications. However, most of the previous studies focus on ns lasers at visible or ultraviolet (UV) wavelengths. The research is very limited for ns lasers at infrared (e.g., 1064 nm) wavelengths (which often have the advantage of much lower cost per unit average output power), and the research is even less if the ns laser also has a long pulse duration on the order of ∼100 ns. In this paper, time-resolved observation using an ICCD (intensified charge-coupled device) camera has been performed to understand the physical mechanism of silicon ablation by 200-ns and 1064-nm laser pulses. This kind of work has been rarely reported in the literature. The research shows that for the studied conditions, material removal in laser silicon ablation is realized through surface vaporization followed by liquid ejection that occurs at a delay time of around 200–300 ns. The propagation speed is on the order of ∼1000 m/s for laser-induced plasma (ionized vapor) front, while it is on the order of ∼100 m/s or smaller for the front of ejected liquid. It has also been found that the liquid ejection is very unlikely due to phase explosion, and its exact underlying physical mechanism requires further investigations.

The current capsule target design for the first ignition experiments at the NIF Facility beginning in 2009 will be a copper-doped beryllium capsule, roughly 2 mm in diameter with 160-µm walls. The capsule will have a 75-µm layer of solid deuterium-tritium on the inside surface, and the capsule will be powered by X-rays generated from a gold/uranium cocktail hohlraum. The design specifications are extremely rigorous, particularly with respect to interfaces, which must be very smooth to inhibit Rayleigh-Taylor instability growth. This paper outlines the current design, and focuses on the challenges and advances in capsule fabrication and characterization; hohlraum fabrication, and deuterium-tritium layering and characterization.

Quantitative data analyses of laser-cut Cu foil X-pinch experiments on the 150 ns quarter-period, ∼250 kA GenASIS driver are presented. Three different foil designs are tested to determine the effects of initial structure on pinch outcome. Foil X-pinch data are also presented alongside the results from wire X-pinches with comparable mass. The X-ray flux and temporal profile of the emission from foil X-pinches differed significantly from that of wire X-pinches, with all emission from the foil X-pinches confined to a ∼3 ns period as opposed to the delayed, long-lasting electron beam emission common in wire X-pinches. Spectroscopic data show K-shell as well as significant L-shell emission from both foil and wire X-pinches. Fits to synthetic spectra using the SCRAM code suggest that pinching foil X's produced a ∼1 keV, ne ≥ 1023 cm−3 plasma. The spectral data combined with the improved reliability of the source timing, flux, and location indicate that foil X-pinches generate a reproducible, K-shell point-proj...

We report on the development of a suite of novel techniques to measure important characteristics in intense ultrashort laser solid target experiments such as critical surface displacement, ablation depth, and plasma characteristics. Measurement of these important characteristics on an ultrafast ({approx}50 fs) time scale is important in understanding the primary event mechanisms in laser ablation of metal targets. Unlike traditional methods that infer these characteristics from spectral power shifts, phase shifts in frequency domain interferometry (FDI) or laser breakthrough studies of multiple shots on bulk materials, these techniques directly measure these characteristics from a single ultrafast heating pulse. These techniques are based on absolute displacement interferometry and nanotopographic applications of wavefront sensors. By applying all these femtosecond time-resolved techniques to a range of materials (Al, Au, and Au on plastic) over a range of pulse energies (10{sup 11} to 10{sup 16} W/cm{sup 2}) and pulse durations (50 to 700 fs), greater insight into the ablation mechanism and its pulse parameter dependencies can be determined. Comparison of these results with hydrocode software programs also reveals the applicability of hydrocode models.

We report on results from experiments using laser-cut X shaped foils (or foil X-pinches) driven by the 250 kA, 150 ns rise-time GenASIS Linear Transformer Driver. Optical probing of the laser-cut foil X-pinches shows considerable dynamic and qualitative similarity with the more traditional wire X-pinches. Results demonstrate excellent quality X-ray emission flux, timing, and source size. The foil X-pinches offer advantages such as improved, reproducible cross-point symmetry, and the opportunity for rep-rated operation, which is not possible with conventional X-pinches using wires.

Most of the previous studies on nanosecond (ns) laser-induced plasma typically use relatively short ns laser pulses (pulse duration less than ∼30 to 50 ns). In this paper, relatively long ns laser pulses with 200 ns duration have been used, and the produced plasma during metal ablation in air at atmospheric pressure has been studied through time-resolved observation using an intensified charge-coupled device camera. Due to the much longer ns laser pulse duration, the plasma radiation intensity distribution and the plasma front propagation have different physical features from those produced by much shorter ns laser pulses. In particular, it has been observed that during the laser pulse the plasma has two high-radiation-intensity regions: one is located right above the target surface while the other is behind the expanding plasma front. The former region will disappear once the laser pulse completes. This interesting physical phenomenon has been rarely reported, and requires further experimental and modeli...

This paper studies the double-pulse effect in high-intensity (≥∼GW/cm2) nanosecond (ns) laser ablation, which refers to the significant material removal rate enhancement for ablation by two ns laser pulses (often separated by a delay time of ∼10 to 100 ns). The early-stage interaction of the second laser pulse with the plasma plume created by the first pulse is very important for understanding the physical mechanism of the double pulse effect. However, the plasma properties in the early stage (during a laser pulse or within 20 to 30 ns after the completion of the pulse) are very difficult to measure experimentally. In this letter, a physics-based predictive model is used as the investigation tool, which was previously verified based on experiments on plasma properties in the late stage, which are relatively easy to measure. The study shows that the second laser pulse does not directly strike the target condensed phase. Instead, it mainly interacts with the plasma plume created by the first laser pulse, he...

Several nanosecond 0.53-μm laser pulses separated by several tens of nanoseconds have been shown to significantly enhance (three to ten times) material removal rates while minimizing redeposition and heat-affected zones. Economic, high-quality, high-aspect ratio holes (>10:1) in metals are produced as a result. A phenomenological model whereby the second laser pulse interacts with the ejecta produced by the first laser pulse and in close proximity to the material surface is consistent with the observations. Incident laser wavelengths of 1.05 and 0.35 μm also benefit from this pulse format.

We report on the development of a novel technique to measure the critical surface displacement in intense, ultrashort, laser‐solid target experiments. Determination of the critical surface position is important for understanding near solid density plasma dynamics and transport from warm dense matter systems, and for diagnosing short scale length plasma expansion and hydrodynamic surface motion from short pulse, laser‐heated, solid targets. Instead of inferring critical surface motion from spectral power shifts using a time‐delayed probe pulse or from phase shifts using ultrafast pump‐probe frequency domain interferometry (FDI), this technique directly measures surface displacement using a single ultrafast laser heating pulse. Our technique is based on an application of a Michelson Stellar interferometer to microscopic rather than stellar scales, and we report plasma scale length motion as small as 10 nm. We will present results for motion of plasmas generated from several target materials (Au, Al, Au on C...