Shock-driven ejecta

Ejecta are jets of liquid metal that are formed when a strong shock impinges on a thin metallic sheet. The passage of the shock melts the metal, while depositing vorticity at the surface. If there are any perturbations at the metal’s free surface due to scratches or roughness effects, they will grow in the form of liquid sheets called ejecta, and under the influence of the deposited vorticity. Eventually, as the liquid sheets stretch they will breakup due to surface tension effects, first into longitudinal ligaments and then into spherical droplets. In some cases, the sheets, ligaments and droplets will also chemically react with the surrounding gas forming a product. The details of how the sheets breakup is poorly understood, and is an active area of research in our group.

Ejecta have several applications, which range from the microscale to astrophysical examples which span several lightyears. The formation of ejecta at plasma interfaces is of important in nuclear fusion, specifically Inertial Confinement Fusion (ICF). In ICF, ejecta can form at nanoscales between the fuel and surrounding plasma, and result in degrading the yield performance. Thus, controlling the formation and development of ejecta will be important in such applications. Similarly, ejecta play a significant role in supernovae explosions at the scale of parsecs, as dense matter that constitutes the core of a white dwarf is ejected and mixed into the interstellar medium by an explosive blastwave. Ejecta can also occur at lower energy densities, in applications such as inject printing. Similarly, space satellites can be exposed to impingement by micrometeors (or other space debris) which can result in ejecta formation which can harm the spacecraft.

At CHAMP, we use research simulation codes such as FLASH and our in-house code IMPACT to better understand how ejecta are formed, how they transport in the gas, and their eventual breakup. Based on our simulations, we are able to develop low-order models for such physical phenomena, which can then be used in system-level simulations using engineering codes.

Figure: Images showing the development at different times of an ejected jet from a solid substrate (shown in red) driven by a shock. The shock passes through the material at t = 0, while the liquid metal jet penetrates through the ambient gas (shown here in blue). Simulations were performed using the FLASH code.

Related Publications:

  1. F.J. Cherne, J.E. Hammerberg, et al., “On shock-driven jetting of liquid from non-sinusoidal surfaces into a vaccuum”, J. Appl. Phys., 118, 185901 (2015).
  2. J.E. Hammerberg et al., “A source model for ejecta”, J. Dyn. Beh. Matls., 3, 316-320, (2017).
  3. V. Karkhanis, P. Ramaprabhu et al., “A numerical study of bubble and spike velocities in shock-driven liquid metals”, J. Appl. Phys., 123, 025902 (2018).
  4. V. Karkhanis, P. Ramaprabhu et al., “Ejecta production from second shock: Numerical simulations and experiments”, J. Dyn. Beh. Matls., 3, 265-279, (2017).
  5. V. Karkhanis and P. Ramaprabhu, “Ejecta velocities in twice-shocked liquid metals under extreme conditions: A hydrodynamic approach”, Matter and Radiation at Extremes, 4, 044402 (2019).