Producing a burning plasma via inertial confinement fusion implosions within a shaped I-Raum radiation cavity *

A burning deuterium and tritium (DT) plasma, in which the heating from fusion produced alpha particles exceeds the compressional work done on the reacting plasma, has been achieved for the first time via the inertial confinement approach at the National Ignition Facility.  One of the major obstacles to achieving a burning plasma is maintaining the implosion symmetry of larger capsule targets. In this work, the symmetry was controlled using a shaped radiation cavity (i.e., hohlraum), known as an I-Raum [1], and by using cross beam energy transfer. The I-Raum reduces the radiation drive asymmetry produced by the 3 incident laser cones by adding recessed cavities to the traditional cylindrical hohlraum at the location of the two outer laser cones.  This offset the bubble of absorbing plasma created at these locations and enables the central inner laser cone to propagate to the waist of the I-Raum allowing improved radiation drive symmetry over the timescale required to implode a 1.1X larger scale capsule.

Here experimental results showing how increasing the scale of the capsule, while maintaining drive symmetry, implosion velocity, and the radiation cavity temperature of previous high performing experiments, allows for burning plasmas to be produced.  In this manner, a DT plasma with burn weighted ion temperature of > 5keV, an inferred pressure of >300 Gbar and an areal density of 0.35 g/cm², that produced 158 kJ of fusion produced neutrons and alpha particles has been created. The sensitivity of the stagnation conditions to the input design parameters and degradations, particularly low mode drive asymmetry will be examined, and the results compared to expectations from analytic theory and detailed radiation hydrodynamic calculations. 

[1] H.F. Robey, et al., Phys. Plasmas 25, 012711 (2018)