Measurement of the Principal Quasi-Isentrope of Lead to $\sim$3Mbar using the ``Z'' Machine

We have measured the principal quasi-isentrope of pure lead to $\sim$3 Mbar, using magnetically-driven ramp compression on SNL's ``Z'' machine. Multiple point-VISARs were used to measure the surface velocities of the compressed samples, and iterative Lagrangian analysis was used to find the wave speed as a function of ramp velocity to an accuracy of $<$2\%. This was then integrated to longitudinal stress as a function of volume on the quasi-isentrope. The experiment used a stripline configuration with samples arranged in pairs at each of 4 vertical positions on opposite drive panels: three of the four pair positions held two lead samples of different thicknesses, while the fourth consisted of one lead sample and a bare panel for drive measurement. The thicker samples of the 3 pairs experienced weak shocks at low stress so their quasi-isentrope data is unreliable there. The single-sample data was good at low stress but affected at high stress by either effects of closure of the stripline gap and / or reflections of the compression pulse from the drive-panel rear surfaces. Data from both methods overlapped at intermediate stresses so have been combined to give the required quasi-isentrope data.