A. R. Christopherson

Theory of hydro-equivalent ignition for inertial fusion and its applications to OMEGA and the National Ignition Facilitya)

The theory of ignition for inertial confinement fusion capsules [R. Betti et al., Phys. Plasmas 17, 058102 (2010)] is used to assess the performance requirements for cryogenic implosion experiments on the Omega Laser Facility. The theory of hydrodynamic similarity is developed in both one and two dimensions and tested using multimode hydrodynamic simulations with the hydrocode DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] of hydro-equivalent implosions (implosions with the same implosion velocity, adiabat, and laser intensity). The theory is used to scale the performance of direct-drive OMEGA implosions to the National Ignition Facility (NIF) energy scales and determine the requirements for demonstrating hydro-equivalent ignition on OMEGA. Hydro-equivalent ignition on OMEGA is represented by a cryogenic implosion that would scale to ignition on the NIF at 1.8 MJ of laser energy symmetrically illuminating the target. It is found that a reasonable combination of neutron yield and areal density...

A comprehensive alpha-heating model for inertial confinement fusion

A comprehensive model is developed to study alpha-heating in inertially confined plasmas. It describes the time evolution of a central low-density hot spot confined by a compressible shell, heated by fusion alphas, and cooled by radiation and thermal losses. The model includes the deceleration, stagnation, and burn phases of inertial confinement fusion implosions, and is valid for sub-ignited targets with ≤10× amplification of the fusion yield from alpha-heating. The results of radiation-hydrodynamic simulations are used to derive realistic initial conditions and dimensionless parameters for the model. It is found that most of the alpha energy (∼90%) produced before bang time is deposited within the hot spot mass, while a small fraction (∼10%) drives mass ablation off the inner shell surface and its energy is recycled back into the hot spot. Of the bremsstrahlung radiation emission, ∼40% is deposited in the hot spot, ∼40% is recycled back in the hot spot by ablation off the shell, and ∼20% leaves the hot ...

Effects of residual kinetic energy on yield degradation and ion temperature asymmetries in inertial confinement fusion implosions

The study of Rayleigh–Taylor instability in the deceleration phase of inertial confinement fusion implosions is carried out using the three-dimensional (3-D) radiation-hydrodynamic Eulerian parallel code DEC3D. We show that the yield-over-clean is a strong function of the residual kinetic energy (RKE) for low modes. Our analytical models indicate that the behavior of larger hot-spot volumes observed in low modes and the consequential pressure degradation can be explained in terms of increasing the RKE. These results are derived using a simple adiabatic implosion model of the deceleration phase as well as through an extensive set of 3-D single-mode simulations using the code DEC3D. The effect of the bulk velocity broadening on ion temperature asymmetries is analyzed for different mode numbers l=1–12. The jet observed in low mode l=1 is shown to cause the largest ion temperature variation in the mode spectrum. The vortices of high modes within the cold bubbles are shown to cause lower ion temperature variat...

Electron Shock Ignition of Inertial Fusion Targets

It is shown that inertial confinement fusion targets designed with low implosion velocities can be shock-ignited using laser-plasma interaction generated hot electrons (hot-es) to obtain high energy gains. These designs are robust to multimode asymmetries and are predicted to ignite even for significantly distorted implosions. Electron shock ignition requires tens of kilojoules of hot-es which can be produced only at a large laser facility like the National Ignition Facility, with the laser-to-hot-e conversion efficiency greater than 10% at laser intensities ∼10^{16}  W/cm^{2}.

Alpha Heating and Burning Plasmas in Inertial Confinement Fusion

Assessing the degree to which fusion alpha particles contribute to the fusion yield is essential to understanding the onset of the thermal runaway process of thermonuclear ignition. It is shown that in inertial confinement fusion, the yield enhancement due to alpha particle heating (before ignition occurs) depends on the generalized Lawson parameter that can be inferred from experimental observables. A universal curve valid for arbitrary laser-fusion targets shows the yield amplification due to alpha heating for a given value of the Lawson parameter. The same theory is used to determine the onset of the burning plasma regime when the alpha heating exceeds the compression work. This result can be used to assess the performance of current ignition experiments at the National Ignition Facility.

Analysis of trends in experimental observables: Reconstruction of the implosion dynamics and implications for fusion yield extrapolation for direct-drive cryogenic targets on OMEGA

This paper describes a technique for identifying trends in performance degradation for inertial confinement fusion implosion experiments. It is based on reconstruction of the implosion core with a combination of low- and mid-mode asymmetries. This technique was applied to an ensemble of hydro-equivalent deuterium–tritium implosions on OMEGA which achieved inferred hot-spot pressures ≈56 ± 7 Gbar [Regan et al., Phys. Rev. Lett. 117, 025001 (2016)]. All the experimental observables pertaining to the core could be reconstructed simultaneously with the same combination of low and mid-modes. This suggests that in addition to low modes, which can cause a degradation of the stagnation pressure, mid-modes are present which reduce the size of the neutron and x-ray producing volume. The systematic analysis shows that asymmetries can cause an overestimation of the total areal density in these implosions. It is also found that an improvement in implosion symmetry resulting from correction of either the systematic mid or low modes would result in an increase in the hot-spot pressure from 56 Gbar to ≈ 80 Gbar and could produce a burning plasma when the implosion core is extrapolated to an equivalent 1.9 MJ symmetric direct illumination [Bose et al., Phys. Rev. E 94, 011201(R) (2016)].This paper describes a technique for identifying trends in performance degradation for inertial confinement fusion implosion experiments. It is based on reconstruction of the implosion core with a combination of low- and mid-mode asymmetries. This technique was applied to an ensemble of hydro-equivalent deuterium–tritium implosions on OMEGA which achieved inferred hot-spot pressures ≈56 ± 7 Gbar [Regan et al., Phys. Rev. Lett. 117, 025001 (2016)]. All the experimental observables pertaining to the core could be reconstructed simultaneously with the same combination of low and mid-modes. This suggests that in addition to low modes, which can cause a degradation of the stagnation pressure, mid-modes are present which reduce the size of the neutron and x-ray producing volume. The systematic analysis shows that asymmetries can cause an overestimation of the total areal density in these implosions. It is also found that an improvement in implosion symmetry resulting from correction of either the systematic mid...

Theory of alpha heating in inertial fusion: Alpha-heating metrics and the onset of the burning-plasma regime

A detailed and comprehensive 1-dimensional theory of alpha-heating metrics is developed to determine the onset of burning plasma regimes in inertial fusion implosions. The analysis uses an analytic model of the deceleration, stagnation, and burn phases of inertial confinement fusion implosions combined with the results from a database of radiation-hydrodynamic simulations. The onset of the burning-plasma regime occurs when the alpha-heating rate in the hot spot exceeds the compression power input and is represented by the parameter Qα=1/2 α energy/PdV work. A second burning plasma regime is also identified, where the alpha-heating rate exceeds the compression input to the entire stagnated plasma, including the hot spot and confining shell, and is represented by Qαtot. It is shown that progress towards the burning-plasma regime is correlated with the yield enhancement caused by alpha-heating but is more accurately related to the fractional alpha energy fα=1/2 α energy/hot-spot energy. In the analysis presented here, we develop a method to infer these intermediate metrics from experiments and show that the alpha power produced in National Ignition Facility High-Foot implosions is approximately 50% of the external input power delivered to the hot spot and 25% of the total external power (from compression) delivered to the stagnated core.A detailed and comprehensive 1-dimensional theory of alpha-heating metrics is developed to determine the onset of burning plasma regimes in inertial fusion implosions. The analysis uses an analytic model of the deceleration, stagnation, and burn phases of inertial confinement fusion implosions combined with the results from a database of radiation-hydrodynamic simulations. The onset of the burning-plasma regime occurs when the alpha-heating rate in the hot spot exceeds the compression power input and is represented by the parameter Qα=1/2 α energy/PdV work. A second burning plasma regime is also identified, where the alpha-heating rate exceeds the compression input to the entire stagnated plasma, including the hot spot and confining shell, and is represented by Qαtot. It is shown that progress towards the burning-plasma regime is correlated with the yield enhancement caused by alpha-heating but is more accurately related to the fractional alpha energy fα=1/2 α energy/hot-spot energy. In the analysis pr...

Enhanced hot-electron production and strong-shock generation in hydrogen-rich ablators for shock ignition

Experiments were performed with CH, Be, C, and SiO2 ablators interacting with high-intensity UV laser radiation (5 × 1015 W/cm2, λ = 351 nm) to determine the optimum material for hot-electron production and strong-shock generation. Significantly more hot electrons are produced in CH (up to ∼13% instantaneous conversion efficiency), while the amount is a factor of ∼2 to 3 lower in the other ablators. A larger hot-electron fraction is correlated with a higher effective ablation pressure. The higher conversion efficiency in CH is attributed to stronger damping of ion-acoustic waves because of the presence of light H ions.