High-mode Perturbations Research Articles

Measurement of early time outer laser beam reflection inside a cylindrical hohlraum at the National Ignition Facility

In a National Ignition Facility (NIF) cylindrical hohlraum, any specular reflection (“glint”) off the wall from the outer cone (incident angle of 50° and 44° relative to the surface of the wall) laser beam will irradiate the capsule poles. If the glint power is sufficiently large during the picket (early time) of the laser pulse, it may seed high-mode perturbations on the capsule surface that can grow during the implosion. To quantify the glint power on the capsule during the picket by the outer beams, we performed dedicated experiments on NIF using a flat witness foil as a surrogate for the capsule in a half-hohlraum target. We found that the measured glint power is approximately comparable to simulations using a nominal electron conduction flux limiter of f = 0.15, and over an order of magnitude lower than that predicted using f = 0.03 in the wall. Based on our current understanding, we conclude that the glint from the outer beams plays an insignificant role in capsule drive asymmetry.

Biermann battery magnetic fields in ICF capsules: Total magnetic flux generation

This paper focuses on the process of magnetic flux generation in inertial confinement fusion implosions. Hot-spots are shown to be dominated by fields generated during stagnation when the temperature and density gradients are largest. A scaling of hot-spot magnetic flux is derived and compared with simulations, revealing that perturbations with both larger amplitudes and higher mode numbers generate more magnetic flux. The model allows for greater understanding of which target designs will be susceptible to magnetohydrodynamic effects. For example, the model can be used to ascertain the time when most magnetic flux is generated. If generation is weighted more toward early times, then more high-mode magnetic field loops will be present. A hot-spot with no high-mode perturbations at time of peak neutron production can still contain significant magnetic flux on those scales. By assuming that magnetic flux is deposited at the hot-spot edge by Nernst advection, the model can be used to post-process radiation-hydrodynamics data to estimate magnetic field strengths and magnetizations.

Spin depolarization induced by self-generated magnetic fields during cylindrical implosions

Spin-polarized fuels are promising for inertial confinement fusion due to the enhanced fusion cross section. One significant concern of spin-polarized inertial confinement fusion is whether the nuclei polarization could survive in the implosions and contribute to ignitions. Here we present numerical simulation methods and results of spin dynamics of polarized deuterium-tritium fuels in strong self-generated magnetic fields during the implosions of dense cylindrical shells. The magnetic field generation and evolution is modeled with generalized Ohm's laws combined with hydrodynamic equations. The spin dynamics is investigated with a particle-tracking method, by solving the spin precession equations of tracked particles. Rayleigh-Taylor instabilities and Richtmyer-Meshkov instabilities are found to be the main cause of depolarization. Hydrodynamic instabilities lead to depolarization of nuclei near the hot-spot shell interface, and an asymmetric shock front leads to depolarization of nuclei inside a hot spot. Deuterium polarization is more stable than tritium polarization due to its smaller gyromagnetic ratio. Low-mode perturbations can lead to higher depolarization inside a hot spot than high-mode perturbations. In the multimode simulations, the modes around 16-32 are significant for hot-spot depolarization.

Review of hydrodynamic instability experiments in inertially confined fusion implosions on National Ignition Facility

Hydrodynamic instabilities are a major factor in degradation of inertial confinement fusion (ICF) implosions. In the highest performing implosions on National Ignition Facility, yield amplification (YA) due to alpha particle heating approached ∼3, while YA of ∼15–30 is needed for ignition. Understanding and mitigation of the instabilities are critical to achieving ignition. This article reviews several experimental platforms that have been developed to directly measure these instabilities in all phases of ICF implosions. Measurements of ripple-shock propagation at OMEGA laser has provided results on initial seeds for the instabilities in three ablators—plastic (CH), beryllium, and high-density carbon. At the ablation front, instability growth of pre-imposed modulations was measured in the linear regime using the hydrodynamic growth radiography platform. This platform was extended for modulation growth of ‘native roughness’ modulations and engineering features (fill tubes and capsule support membranes or ‘tents’). Several new experimental platforms have or are being developed to measure instability growth at the ablator–ice interface. In the deceleration phase of implosions, complementary ‘self-emission’ and ‘self-backlighting’ platforms were developed to measure low-mode asymmetries and high-mode perturbations near peak compression.

Development of new platforms for hydrodynamic instability and asymmetry measurements in deceleration phase of indirectly driven implosions on NIF

Hydrodynamic instabilities and asymmetries are a major obstacle in the quest to achieve ignition at the National Ignition Facility (NIF) as they cause pre-existing capsule perturbations to grow and ultimately quench the fusion burn in experiments. This paper reviews the development of two new experimental techniques to measure high-mode instabilities and low-mode asymmetries in the deceleration phase of indirect drive inertial confinement fusion implosions. In the first innovative technique, self-emission from the hot spot was enhanced with an argon dopant to “self-backlight” the shell in-flight, imaging the perturbations in the shell near peak velocity. Experiments with pre-imposed two-dimensional perturbations showed hydrodynamic instability growth of up to 7000× in areal density. These experiments discovered unexpected three-dimensional structures originating from the capsule support structures. These new 3-D structures became one of the primary concerns for the indirect drive ICF program that requires their origin to be understood and their impact mitigated. In a second complementary technique, the inner surface of the decelerating shell was visualized in implosions using x-ray emission of a high-Z dopant added to the inner surface of the capsule. With this technique, low mode asymmetry and high mode perturbations, including perturbations seeded by the gas fill tube and capsule support structure, were quantified near peak compression. Using this doping method, the role of perturbations and radiative losses from high atomic number materials on neutron yield was quantified.

Visualizing deceleration-phase instabilities in inertial confinement fusion implosions using an “enhanced self-emission” technique at the National Ignition Facility

High-mode perturbations and low-mode asymmetries were measured in the deceleration phase of indirectly driven, deuterium gas filled inertial confinement fusion capsule implosions at convergence ratios of 10 to 15, using a new “enhanced emission” technique at the National Ignition Facility [E. M. Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. In these experiments, a high spatial resolution Kirkpatrick-Baez microscope was used to image the x-ray emission from the inner surface of a high-density-carbon capsule's shell. The use of a high atomic number dopant in the shell enabled time-resolved observations of shell perturbations penetrating into the hot spot. This allowed the effects of the perturbations and asymmetries on degrading neutron yield to be directly measured. In particular, mix induced radiation losses of ∼400 J from the hot spot resulted in a neutron yield reduction of a factor of ∼2. In a subsequent experiment with a significantly increased level of short-mode initial perturbations, shown through the enhanced imaging technique to be highly organized radially, the neutron yield dropped an additional factor of ∼2.

Comparison of plastic, high density carbon, and beryllium as indirect drive NIF ablators

Detailed radiation hydrodynamic simulations calibrated to experimental data have been used to compare the relative strengths and weaknesses of three candidate indirect drive ablator materials now tested at the NIF: plastic, high density carbon or diamond, and beryllium. We apply a common simulation methodology to several currently fielded ablator platforms to benchmark the model and extrapolate designs to the full NIF envelope to compare on a more equal footing. This paper focuses on modeling of the hohlraum energetics which accurately reproduced measured changes in symmetry when changes to the hohlraum environment were made within a given platform. Calculations suggest that all three ablator materials can achieve a symmetric implosion at a capsule outer radius of ∼1100 μm, a laser energy of 1.8 MJ, and a DT ice mass of 185 μg. However, there is more uncertainty in the symmetry predictions for the plastic and beryllium designs. Scaled diamond designs had the most calculated margin for achieving symmetry and the highest fuel absorbed energy at the same scale compared to plastic or beryllium. A comparison of the relative hydrodynamic stability was made using ultra-high resolution capsule simulations and the two dimensional radiation fluxes described in this work [Clark et al., Phys. Plasmas 25, 032703 (2018)]. These simulations, which include low and high mode perturbations, suggest that diamond is currently the most promising for achieving higher yields in the near future followed by plastic, and more data are required to understand beryllium.

Weakly nonlinear incompressible Rayleigh-Taylor instability in spherical and planar geometries

The relationship between the weakly nonlinear (WN) solutions of the Rayleigh-Taylor instability in spherical geometry [Zhang et al., Phys. Plasmas 24, 062703 (2017)] and those in planar geometry [Wang et al., Phys. Plasmas 19, 112706 (2012)] is analyzed. In the high-mode perturbation limit (Pn(cos θ), n≫1), it is found that at the equator, the contributions of mode P2n along with its neighboring modes, mode P3n along with its neighboring modes, and mode Pn at the third order along with its neighboring modes are equal to those of the second harmonic, the third harmonic, and the third-order feedback to the fundamental mode, respectively, in the planar case with a perturbation of the same wave vector and amplitude as those at the equator. The trends of WN results in spherical geometry towards the corresponding planar counterparts are found, and the convergence behaviors of the neighboring modes of Pn, P2n, and P3n are analyzed. Moreover, the spectra generated from the high-mode perturbations in the WN regime are provided. For low-mode perturbations, it is found that the fundamental modes saturate at larger amplitudes than the planar result. The geometry effect makes the bubbles at or near the equator grow faster than the bubbles in planar geometry in the WN regime.

Mix and hydrodynamic instabilities on NIF

Several new platforms have been developed to experimentally measure hydrodynamic instabilities in all phases of indirect-drive, inertial confinement fusion implosions on National Ignition Facility. At the ablation front, instability growth of pre-imposed modulations was measured with a face-on, x-ray radiography platform in the linear regime using the Hydrodynamic Growth Radiography (HGR) platform. Modulation growth of "native roughness" modulations and engineering features (fill tubes and capsule support membranes) were measured in conditions relevant to layered DT implosions. A new experimental platform was developed to measure instability growth at the ablator-ice interface. In the deceleration phase of implosions, several experimental platforms were developed to measure both low-mode asymmetries and high-mode perturbations near peak compression with x-ray and nuclear techniques. In one innovative technique, the self-emission from the hot spot was enhanced with argon dopant to "self-backlight" the shell in-flight. To stabilize instability growth, new "adiabat-shaping" techniques were developed using the HGR platform and applied in layered DT implosions.

Effects of mode coupling between low-mode radiation flux asymmetry and intermediate-mode ablator roughness on ignition capsule implosions

The low-mode shell asymmetry and high-mode hot spot mixing appear to be the main reasons for the performance degradation of the National Ignition Facility (NIF) implosion experiments. The effects of the mode coupling between low-mode P2 radiation flux asymmetry and intermediate-mode L = 24 capsule roughness on the implosion performance of ignition capsule are investigated by two-dimensional radiation hydrodynamic simulations. It is shown that the amplitudes of new modes generated by the mode coupling are in good agreement with the second-order mode coupling equation during the acceleration phase. The later flow field not only shows large areal density P2 asymmetry in the main fuel, but also generates large-amplitude spikes and bubbles. In the deceleration phase, the increasing mode coupling generates more new modes, and the perturbation spectrum on the hot spot boundary is mainly from the strong mode interactions rather than the initial perturbation conditions. The combination of the low-mode and high-mode perturbations breaks up the capsule shell, resulting in a significant reduction of the hot spot temperature and implosion performance.

Dependence of Ablative Rayleigh—Taylor Instability on High-Z Dopant Concentration

We conducted two-dimensional simulations of inertial confinement fusion targets to evaluate effects of high-Z doping on implosion hydrodynamics. It was found that an ablation structure drastically changes with concentration of dopant material. We also confirmed that even a lightly-doped target can suppress Rayleigh-Taylor instability on short wavelength, while a long-wavelength perturbation is difficult to be suppressed with any dopant concentration. The high-Z doping is thus only effective for a spherical implosion with high-mode perturbations.

Linear and non-linear amplification of high-mode perturbations at the ablation front in HiPER targets

The linear and non-linear sensitivity of the 180 kJ baseline HiPER target to high-mode perturbations, i.e. surface roughness, is addressed using two-dimensional simulations and a complementary analysis by linear and non-linear ablative Rayleigh–Taylor models. Simulations provide an assessment of an early non-linear stage leading to a significant deformation of the ablation surface for modes of maximum linear growth factor. A design using a picket prepulse evidences an improvement in the target stability inducing a delay of the non-linear behavior. Perturbation evolution and shape, evidenced by simulations of the non-linear stage, are analyzed with existing self-consistent non-linear theory.

Direct-drive laser target designs for sub-megajoule energies

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength to lower the laser energy required for substantial gain (>30×) to sub-MJ level. These low laser-energy pellets are useful in systems that could form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [S. P. Obenschain et al., Phys. Plasmas 13, 056320 (2006)]. The short wavelength laser should allow higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities to be achieved while keeping the low aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500kJ. A target design is presented with various laser pulse shapes and both 1D and 2D simulation results are shown. The sensitivity of these targets to both low-mode and high-mode perturbations is examined. The analysis and simulations in this paper indicate that significant gain (G=57) can be achieved for these targets even in the presence of hydrodynamic instabilities.

Update on NIF Indirect Drive Ignition Target Fabrication Specifications

Indirect drive ignition target simulations are described as they are used to determine target fabrication specifications. Simulations are being used to explore options for making the targets more robust, and to develop more detailed understanding of the performance of a few point designs. The current array of targets is described. A new target is described with radially dependent Cu dopant in Be. This target has significantly looser specifications for high-mode perturbations than previous targets. Current estimates of size limitations for fill tubes, holes, and isolated defect are discussed. Recent 3D simulations are described.