Diagnosing ablator ρR and ρR asymmetries in capsule implosions using charged-particle spectrometry at the National Ignition Facility

By fielding several compact proton spectrometers at various locations around an ignition-capsule implosion at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004)], rho R and rho R asymmetries of the ablator for a failed implosion can be obtained through absolute measurements of knock-on proton (KO-P) spectra. For ignition capsules with a Cu-doped beryllium (Be) ablator, 50:50 mixture of deuterium-tritium (DT) fuel and similar to 1% residual hydrogen (H) by atom, failed implosions can be diagnosed for neutron yields ranging from similar to 10(11) to similar to 6x10(15) and local rho Rs up to similar to 240 mg/cm(2). For capsules with an ablator of Ge-doped CH, which contains a large amounts of H, failed implosions can be diagnosed for neutron yields ranging from similar to 10(10) to similar to 6x10(15) and local rho Rs up to similar to 200 mg/cm(2). Prior to the first ignition experiments, capsules with a Cu-doped Be ablator (or Ge-doped CH ablator), more deuterium-lean fuel mixture and H-dopant levels up to 25% in the fuel will be imploded to primarily reduce the neutron yield. The HDT-filled Be-capsule implosion, which can be diagnosed for neutron yields ranging from similar to 5x10(9) to similar to 6x10(15) and local rho Rs up to similar to 240 mg/cm(2), is more suitable to diagnose using KO-Ps as the signal-to-background ratio is significantly higher than for an ignition-capsule implosion. In addition, analysis of CH-ablator data obtained from analogous OMEGA [T. R. Boehly, D. L. Brown, R. S. Craxton , Opt. Commun. 133, 495 (1997)] experiments indicate that the shape of the KO-P spectrum is affected mainly by the ablator rho R. Other effects such as ablator-density-profile variations, time evolution of the ablator rho R, fuel-ablator mix and electron temperature variations typically predicted for the ablator play minor roles.