Neutron backscatter edges – a novel diagnostic for Inertial Confinement Fusion

Observations during the stagnation of ICF implosions are primarily focused towards diagnosing the hotspot, with the cold dense fuel shell properties largely unmeasured. Efficient energy conversion between the kinetic energy of the shell and the internal energy of the hotspot is a key requirement for ignition. Residual kinetic and excess internal energy in the shell are energy sinks which will prevent ignition. Direct measurement of the energy content of the shell is therefore critically important to the understanding of implosion performance.

The scattered neutron spectrum emitted from an ICF implosion contains a wealth of information about the dense fuel. Neutrons which undergo a single 180° scattering event from D and T ions produce sharp edges in the spectrum. The spectral shapes of the backscatter edges are especially sensitive to the ion velocity distribution. Similar to the DT primary spectrum, thermal and non-thermal broadening and fluid velocity Doppler shifts govern the spectral shape of the backscatter edge. Thus, the backscatter edges provide a unique diagnostic feature to measure the hydrodynamic conditions of the dense fuel at stagnation.

We have developed a model to fully describe the spectral shape of the backscatter edges. The model has been used to fit experimental nT backscatter edge data from cyrogenic implosions performed on the OMEGA 60 laser. This has allowed inference of the bulk flow velocity and total velocity variance of the scattering triton population, providing the first direct measurement of dense fuel conditions. The deviation in experimental measurements from 1D predictions are correlated with hydrodynamic measures of shell stability. Separation of thermal and non-thermal effects can be achieved through simultaneous measurement of the nD and nT edges. In combination with measurements of the hotspot conditions, this will enable a more complete description of the stagnating capsule and provide insights into current implosion performance.