Sensitivity of ICF ignition conditions to non-Maxwellian DT fusion reactivity

Abstract

The hotspot ignition conditions in ICF are determined by considering the power balance between fusion energy deposition and energy loss terms. Uncertainty in any of these terms has potential to modify the ignition conditions, changing the optimum ignition capsule design. This paper considers the impact of changes to the DT fusion reaction rate due to non-thermal ion energy distributions. The DT fusion reactivity has been evaluated for a class of non-Maxwellian distributions representing a perturbation to the tail of a thermal distribution. The resulting reactivity has been used to determine hotspot ignition conditions as a function of the characteristic parameter of the modified distribution. The hotspot ignition conditions in ICF can be determined by considering the power balance between fusion alpha particle energy deposition, hydrodynamic work and energy loss via radiation and electron thermal conduction (1). The resulting conditions may be sensitive to uncertainties in any of the key plasma physics models or assumptions. This paper considers the impact of changes to the DT fusion reaction rate due to the presence of non-thermal ion energy distributions. The reactivity,� v� , for thermonuclear fusion reactions is conventionally derived by averaging cross- section data over a Maxwell-Boltzmann energy distribution. In many situations, such as when plasmas are strongly shocked or rapidly heated by energetic particles, reacting ions may be better characterised by a non-Maxwellian distribution. Since the self-relaxation time is longer for ions at higher energies (2), such effects typically produce an enhancement or depletion within the tail of the distribution. At temperatures where the reactivity is dominated by the contribution of particles in the tail, even small changes to the distribution may have a potentially significant impact on the fusion reaction rate. The DT fusion reactivity has been evaluated for a class of non-Maxwellian distributions, the q-Gaussian distribution (3), which represent a perturbation to the tail of a thermal distribution. The resulting reactivity has been used to calculate a revised ideal ignition temperature (4) and to determine hotspot ignition conditions as a function of the characteristic parameter of the distribution. This initial assessment does not consider in detail the physical development of non-Maxwellian distributions. If conditions are found to show significant sensitivity, then the generation of distributions may be explored in future work via kinetic simulation.