Shock temperatures of preheated MgO

Shock temperature measurements via optical pyrometry are being conducted along $<$100$>$ single-crystal MgO preheated before compression to 1923 K. Planar shocks were generated by impacting hot Mo(driver plate)-MgO targets with Mo flyers launched by the Caltech two-stage light-gas gun up to 6.5 km/s. At this velocity, calculated Hugoniot pressure in MgO is 172 GPa and the 4-channel brightness temperature is 5700 K. Extrapolating previous Hugoniot EOS centered at 298 K to present preheated states predict Hugoniot temperature of 6800 K for the solid. The $\sim $1000 K temperature deficit suggests MgO melting above 155 GPa and 5200 K. This is consistent with the relatively flat melting curve extrapolated from diamond anvil cell results (Zerr {\&} Boehler) and some molecular dynamics (MD) studies (Strachan et al) but not with the steeper melting curves from most MD studies (Cohen {\&} Weitz, Belonoshko). Features of preheated shock temperature experiments, pyrometer calibration and multichannel radiance data processing are discussed.