Proton core imaging of the nuclear burn in inertial confinement fusion implosions

A proton emission imaging system has been developed and used extensively to measure the nuclear burn regions in the cores of inertial confinement fusion implosions. Three imaging cameras, mounted to the 60-beam OMEGA laser facility [T. R. Boehly etal, Opt. Commun. 133, 495 (1997)], use the penetrating 14.7 MeV protons produced from D ³He fusion reactions to produce emission images of the nuclear burn spatial distribution. The technique relies on penumbral imaging, with different reconstruction algorithms for extracting the burn distributions of symmetric and asymmetric implosions. The hardware and design considerations required for the imaging cameras are described. Experimental data, analysis, and error analysis are presented for a representative symmetric implosion of a fuel capsule with a 17-μm-thick plastic shell and 18 atm D ³He gas fill. The radial burn profile was found to have characteristic radius R_burn, which we define as the radius containing half the D ³He reactions, of 32±2 μm (burn radii measured for other capsule types range from 20 to 80 μm). Potential sources of error due to proton trajectory changes from interactions with electric fields and scattering in capsule and camera hardware are estimated with simple analytic and Monte Carlo calculations; they are predicted to be small compared with statistical errors. Experimental tests were performed to look for any inconsistencies between results from different cameras and different imaging geometries, or- evidence of error due to ambient electric or magnetic fields, and none were found.