Approaches to Simulating the Prompt Electromagnetic Pulse

LLNL is developing a suite of modern tools for simulating the generation and propagation of the prompt (E1) electromagnetic pulse (EMP). These include the 3-D EMPulse code, based on PIC methods with a Cartesian grid in the laboratory frame [1], and a companion 3-D approach which builds on the methods used in Longmire's fast-running CHAP code [2]. In CHAP, and in our own CHAP-lite [3], 1-D spherical symmetry is assumed, and the calculation takes advantage of a separation of scales. The independent coordinates are (r,$\tau )$, where r is the distance from the source and $\tau \quad =$ t-r/c; the pulse varies slowly with r at fixed $\tau $, so a coarse radial grid can be used. We seek similar efficiencies in 3-D, incorporating non-spherically-symmetric physics via a vector spherical harmonic decomposition. For each (l,m) harmonic, the radial equation is similar to that in CHAP-lite. We describe this approach, along with other aspects of our project. [1] B. I. Cohen, et. al., \textit{this Conference}. [2] C. L. Longmire, \textit{IEEE Trans. Electromagnetic Compatibility} \textbf{20} no. 1, 3 (1978). [3] W. A. Farmer, et al., \textit{IEEE Trans. Nuclear Science} \textbf{63}, 1259 (2016).