Hydrodynamic disassembly and expansion of electron-beam-heated warm dense copper

Abstract

Cu foils, $200\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ in thickness, were heated in two stages by a $\ensuremath{\sim}100$-ns-long monoenergetic electron bunch at 19.8 MeV and a current of 1.7 kA ($8.5\ifmmode\times\else\texttimes\fi{}{10}^{14} {e}^{\ensuremath{-}}$) in a 2-mm-spot to ${T}_{e}\ensuremath{\sim}1$ eV. After 45 ns of isochoric heating, the pressure in the foil builds up to $g20$ GPa (200 kbar), it begins to hydrodynamically disassemble, and a velocity spread is measured. Near the end of the electron pulse, the 1550 nm probe is cut off or absorbed. Photonic Doppler velocimetry measurements were made to quantify the expansion velocity, hydrodynamic disassembly time, and pressure of the foil prior to cutoff. Measurements indicate foil motion begins the instant electrons pass through the foil and continues until the particle velocity approaches the ambient sound velocity of Cu and the bulk density exceeds the critical density of the probe. Once the density of the plasma drops below the critical threshold and begins reflecting again, an expansion velocity of the classical plasma is also measured, similar to the point-source solution.