Characterization of laser-induced plasmas as a complement to high-explosive large-scale detonations

Clare Kimblin,Gene A Capelle,Rusty Trainham,Xianglei Mao,Richard E Russo

doi:10.1063/1.4999793

Abstract

Experimental investigations into the characteristics of laser-induced plasmas indicate that LIBS provides a relatively inexpensive and easily replicable laboratory technique to isolate and measure reactions germane to understanding aspects of high-explosive detonations under controlled conditions. Spectral signatures and derived physical parameters following laser ablation of aluminum, graphite and laser-sparked air are examined as they relate to those observed following detonation of high explosives and as they relate to shocked air. Laser-induced breakdown spectroscopy (LIBS) reliably correlates reactions involving atomic Al and aluminum monoxide (AlO) with respect to both emission spectra and temperatures, as compared to small- and large-scale high-explosive detonations. Atomic Al and AlO resulting from laser ablation and a cited small-scale study, decay within ∼10-5 s, roughly 100 times faster than the Al and AlO decay rates (∼10-3 s) observed following the large-scale detonation of an Al-encased explosive. Temperatures and species produced in laser-sparked air are compared to those produced with laser ablated graphite in air. With graphite present, CN is dominant relative to N2+. In studies where the height of the ablating laser’s focus was altered relative to the surface of the graphite substrate, CN concentration was found to decrease with laser focus below the graphite surface, indicating that laser intensity is a critical factor in the production of CN, via reactive nitrogen.

Highlights

Large-scale high explosive (HE) field tests, conducted at the Nevada National Security Site and other locations, provide data for optical[1] and radio frequency[2,3] (RF) models of detonation environment emissions
To demonstrate concordance and differences between laser-induced plasmas and explosions generated by HE charges, Al species produced by laser ablation of aluminum in our lab (Fig. 2a) were compared to the decay of those produced by the detonation of 20 grams of aluminized explosive (PBXN-113) in an explosive test chamber (Fig. 2b).[6]
In spite of the different methods used for their generation, there are considerable similarities in the decay rate of neutral, atomic aluminum (Al-I: 394.40 nm, 396.15 nm, 3s24s 2S → 3s23p 2P0 transition) and the growth and decay of aluminum monoxide AlO

Summary

INTRODUCTION

Air ionized by a focused laser was studied ( in Section III.B) because significant air ionization results from high thermal temperatures (T ∼11,000 K) generated by HE detonation product shock waves.[30] In this work and others’[16,31] species produced by laser-sparked air (N2+, N2, N-I, N-II, O-I, O-II) correspond to a number of those predicted by models of high temperature air (1000 K < T < 24,000 K).[32] CN is a major spectral component observed in air plasma created with an inductively coupled torch[33] and in laser-sparked air plasma, as reported by Harilal et al.[16] and . Aiding test and evaluation of HE field diagnostics that are better able to detect species and provide temperatures relevant to optical and RF simulations, (2) informing field collection parameters and analysis methods, and (3) providing empirical data of interest to modelers, are discussed

EXPERIMENTAL DETAILS

Al species observed and temporal dependence

Temperature derivations from AlO emission

CONCLUSIONS