Design of a diagnostic technique for timing the 4$^{th}$ shock on NIF

Ignition capsule implosions planned for the National Ignition Facility (NIF) require a pulse shape with a carefully designed series of steps, which launch a series of shocks through the ablator and DT ice shell. The relative timing of these shocks is critical for maintaining the DT fuel on a low adiabat. The current NIF specification requires that the timing of the first three shocks be determined to an accuracy of +/- 50ps and the 4$^{th}$ shock be tuned to an accuracy of +/- 100ps. To meet these requirements, experiments are being planned to measure the shock timing on NIF. A reliable technique (VISAR) has been demonstrated [1] for timing the first three shocks. At the hohlraum conditions present during transit of the 4$^{th}$ shock, however, the required VISAR window is expected to go opaque. A new technique using the Streaked Optical Pyrometer (SOP) to measure shock breakout through a diagnostic ``inner'' shell is proposed. The accuracy of this technique is presented. \newline [1] D. H. Munro \textit{et al}., \textit{Phys. Plasmas} \textbf{8}(5), 1552 (2001).