  Pulsed plasmas in liquids exhibit complex interaction between three phases of
matter (liquids, gas, plasmas) and are currently used in a wide range of
applications across several fields, however significant knowledge gaps in our
understanding of plasma initiation in liquids hinder additional application and
control; this area of research currently lacks a comprehensive predictive
model. To aid progress in this area experimentally, here we present the
first-known ultrafast (50 ps) X-ray images of pulsed plasma initiation
processes in water (+25 kV, 10 ns, 5 mJ), courtesy of the X-ray imaging
techniques available at Argonne National Laboratory's Advanced Photon Source
(APS), with supporting nanosecond optical imaging and a computational X-ray
diffraction model. These results clearly resolve narrow (~10 micron)
low-density plasma channels during initiation timescales typically obscured by
optical emission (<100 ns), a well-known and difficult problem to plasma
experiments without access to state-of-the-art X-ray sources such as the APS
synchrotron. Images presented in this work speak to several of the prevailing
plasma initiation hypotheses, supporting electrostriction and bubble
deformation as dominant initiation phenomena. We also demonstrate the plasma
setup used in this work as a cheap ($<$US\$100k), compact, and repeatable
benchmark imaging target (29.1 km/s, 1 TW/cm$^2$) useful for the development of
next-generation ultrafast imaging of high-energy-density physics (HEDP), as
well as easier integration of HEDP research into synchrotron-enabled
facilities.
