Andrew Forsman

Double-pulse machining as a technique for the enhancement of material removal rates in laser machining of metals

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.

Time-resolved observation of the plasma induced by laser metal ablation in air at atmospheric pressure

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...

Study of laser-plasma interaction using a physics-based model for understanding the physical mechanism of double-pulse effect in nanosecond laser ablation

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...

X-ray penumbral imaging diagnostic developments at the National Ignition Facility

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.

Comparative analysis of X-ray emission and dynamics of Cu foil and wire X-pinches

Summary form only given. 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. L and K-shell spectra are compared to simulated spectra to estimate the plasma temperature, density, and ionization level. We apply these values, along with source-size measurements and flux data to kinetic z-pinch theories including runaway electrons and anomalous resistivity2 to attempt to explain the emission characteristics from foil X-pinches and their differences from wire X-pinches.

Interferometric diagnostic suite for ultrafast laser ablation of metals

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 (~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 (1011 to 1016 W/cm2) 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.