Central Density and Low-Mode Perturbation Control of Inertial Confinement Fusion Dynamic-Shell Targets

Abstract

The dynamic-shell target is a new class of design for inertial confinement fusion (ICF). These targets address some of the target fabrication challenges prevalent in current ICF targets and take advantage of advances in manufacturing technologies. This study first examines how the dynamic-shell design can be used to control the density of the central region and therefore convergence ratio, thus expanding the design space for ICF. Additionally, the concern of low-mode perturbation growth is considered. A new class of high-performing beam configurations, based on icosahedral polyhedra and charged-particle simulations is proposed. These configurations achieve low levels of irradiation nonuniformity through selection of beam shapes that suppress the dominant symmetrical mode.

Optimization of irradiation configuration using spherical t-designs for laser-direct-drive inertial confinement fusion

“…Also, σ ℓ of low ℓ modes decrease only by a factor of few between charged particle configurations with 96 beams (M96) and 240 beams (M240) (compare blue and green lines in figure 6(a)), which explains why the total σ rms decays slowly with N for the charged particle configurations (figure 5(a)). Previous studies [7,8] also reported slow decay of σ rms with N for charged particle configurations. As a note, the above contrast between charged particle and t-design configurations is similar to that between basic methods of numerical integration (such as trapezoidal or Simpson) and the Gaussian quadratures, [16] which show much faster convergence with the number of integration points compared to the basic methods.…”

“…Advantages of this method include a symmetric intensity distribution with minimized nonuniformity for a few select values of N. A disadvantage is the lack of a systematic extension to an arbitrary N. The other method is the charged-particle method, which uses a system of N particles constrained to a sphere that repel each other with a Coulomb force (or another distance-dependent force). The beam configurations are chosen to correspond to particle configurations that minimize the potential energy [6][7][8]. The advantage of the method is its simplicity in obtaining beam configurations for arbitrary N. The disadvantage is a slow decay of nonuniformity with the number of beams as, e.g.…”

“…The advantage of the method is its simplicity in obtaining beam configurations for arbitrary N. The disadvantage is a slow decay of nonuniformity with the number of beams as, e.g. 1/ √ N in [8]. Neither of the above methods offer a way of finding beam configurations that simultaneously eliminate spherical harmonic modes below a certain number, which has been recognized as an important strategy in designing the irradiation system [5].…”

“…Here ⟨I⟩ = ´I(θ, ϕ)dΩ/4π. For a system of N beams that have identical intensity profiles azimuthally symmetric around each beam axis with each beam axis passing through the center of the sphere, σ rms can be written as [5,8,12]…”

“…where angle θ is measured from the beam axis, I 0 is the incident intensity on the beam axis, η is the absorption fraction of the incident intensity, and c and n are parameters corresponding to the 1/e radius and order of the super-Gaussian incidentintensity profile. In equation (2) G ℓ is the geometrical factor [5,8] which can be written as…”