Probing ionization of proton-heated matter with X-ray Thomson scattering

We have demonstrated for the first time the effect of band structure on ionization state in the warm, dense matter regime. We compare the ionization state of two materials, B and BN, which have different room temperature band gaps. We show that in dense plasma conditions at temperatures of 13-18 eV, the band-gap of BN persists and lowers the ionization state to 0.5, compared to 2.5 in B. The experiment was performed on LLNL's Titan laser. The ultra-intense laser beam was split into two beams: one generated a proton beam while the other created a K-alpha x-ray probe. The proton beam isochorically heated the target foil, creating a solid-density plasma. Forward scattered x-rays were spectrally dispersed, providing an accurate measurement of the ionization and temperature from the position of the plasmon feature and the ratio of up- to down-shifted plasmon signals. *This work was performed under the auspices of the U.S. DOE by the LLNL, through the Institute for Laser Science and Applications, under contract DE-AC52-07NA27344. The authors also acknowledge support from LDRD Grant No. 08-LW-004 and the NNSA SSGF program.