Developing one dimensional implosions for inertial confinement fusion science
High Power Laser Science and Engineering 4, e 44 (2016)
For indirect drive inertial confinement fusion (ICF), a spherical capsule filled with deuterium-tritium (DT) fuel is placed in the center of a cylindrical high-z container called a hohlraum. High power lasers entering the hohlraum from both ends impinge on the walls where the laser energy is converted to x-rays to form a small x-ray oven. The x-rays absorbed by the capsule ablate the outer surface and act as a rocket force to implode the capsule to ~1/30th of its original radius.
Since compressing the capsule is inherently unstable, any imperfections in the x-ray drive uniformity, the capsule manufacturing, or the target assembly lead to implosions that are aspherical or shell breaking-up that due to hydrodynamic instability. Thus, driving spherical implosions to such high convergences to create conditions for thermo-nuclear ignition is quite challenging. It is analogous to trying to squeeze a balloon filled with water spherically. As one tries to squeeze the balloon, the balloon will be out of the spherical shape and expand through the gaps in your fingers just as instabilities grow as the fuel filled ICF pellet is compressed. The smaller the object is compressed, the stronger these imperfections grow. However, it is impossible to create the perfect system free of any perturbations, since for instance the capsule has to be mounted in the hohlraum by some means, the real question for ICF is how close to ideal the implosions need to be for success.
Another aspect to this challenge is that the simulations are not capable of calculating all of the experiments with enough fidelity to capture all perturbations given the current computing capabilities.
Fig. 1 Plots of the hohlraum cross sections showing the laser ray traces with the hohlraum density for a (a) 2200 and (b) 1290 mm outer diameter beryllium capsule. The yellow regions correspond to densities greater than the 1/4 critical density for 351 nm light.
Initial ICF experiments on the NIF went directly to the challenging regime necessary for ignition and the experimental performance was well below expectation. Gains in performance had been made in subsequent experiments by trading-off ignition conditions to relax constraints on the implosions for improved performance. However, these experiments still showed that implosion asymmetry and hydrodynamic instabilities continued to have a major impact on performance.
Based on previous results, J. L. Kline et al. are taking several experimental approaches in which they move away from ignition to produce implosions that perform as predicted by simulations, i.e. nearly one dimensionally. One dimensionality means the radial position of the shell and DT fuel can be described by a single radial value rather than three dimensional parameters, i.e. neither azimuthal nor poloidal dependence is taken into consideration.
The work examines ways to create more “1D” like implosions so the effects of various parameters on the ability to achieve ignition are studied. To improve symmetry control, the researchers have moved to large case-to-capsule ratios defined as the radius of the hohlraum to the radius of the capsule. This helps the laser beams deposit their energy where they are pointed on the hohlraum wall, which is not occurring in the ignition hohlraums. The figure shows calculated plasma density maps of a hohlraum with a 1100 μm and a 600 μm radius capsule with the yellow regions representing densities above the cut-off density for the laser light. In the case of the larger capsule, a large fraction of the laser rays are blocked from the wall, while for the smaller capsule most of the laser rays make it to the wall. The cost is that the smaller capsule cannot absorb enough energy to achieve ignition.
Another campaign aims to decrease the impact of the drive and capsule imperfections by using liquid layer targets. Standard ICF targets use an ice layer frozen near the triple point of 19.6 K. Liquid DT layers can be fielded between 20 -26 K and as the temperature changes the vapor pressure varies, and thus the initial mass of gas in the central cavity, also changes. As the mass is increased, the convergence ratio of the implosion decreases to ~12, which is a factor of 3 less than ice layers. By varying the field temperature, the convergence ratio changes without changing any other experimental parameters, so the impact of the experimental imperfections which is varying with convergence can be assessed.
The final approach is double shells. For double shell targets, an outer shell is accelerated by the x-ray drive and collides with an inner shell filled with the DT fuel. The outer shell acts as a buffer to mitigate non-uniformities in the drive. In addition, the convergence of the inner shell is only ~10 for ignition, much less than those of other approaches. Each of the changes relaxed the tolerances on the drive. The challenge with double shell is the target fabrication and at this time the transfer of the shape of the outer shell to the inner shell is not known for large targets.
What each of these approaches has in common is that the primary goal is developing implosions that are understandable and predictable. From there, explicit effects can be predicted and systematic changes can be exploited in order to further our understanding of ignition science.