Inferring fuel areal density from secondary neutron yields in laser-driven magnetized liner inertial fusion

Abstract

A technique to infer the areal density ρR of compressed deuterium (D) in cylindrical implosions from the ratio of secondary D–T (deuterium–tritium) neutrons to primary D–D neutrons is described and evaluated. For ρR to be proportional to the ratio of D–T to D–D yield, the increase in the D–T fusion cross-section with collisional slowing down of the tritium must be small, requiring ρR≪15T keV3/2 mg/cm2, where TkeV is the electron temperature in keV. The technique is applied to the results from laser-driven magnetized liner inertial fusion (MagLIF) targets on OMEGA, where ρR is certainly less than 4 mg/cm2. OMEGA MagLIF targets do not achieve a sufficiently high, radially integrated, axial magnetic field BR to confine the tritium, as occurs in Z MagLIF targets, because they are ∼10× smaller in radius. The inferred areal densities show that fuel convergence is reduced by preheating, by an applied axial magnetic field, and by increasing the initial fuel density, which are key features of the MagLIF scheme. The results are compared with 1-D and 2-D magnetohydrodynamic simulations for nominal laser and target parameters, which predict areal densities 2× to 3× higher than the measurements.