Theory for Laser Plasma (TLP)

Summary

The TLP group’s research includes laser fusion research as well as high-energy-density physics research of ion particles accelerated to relativistic speeds, relativistic Coulomb explosions, and other phenomena resulting from mutual interactions between matter and ultra-high-intensity, ultra-short pulse lasers. The group’s research also encompasses applications such as cancer treatment, fuel cell development, radioactive waste disposal, and laser fusion based on this high-energy-density physics research. The group strives to use theory- and simulation-based methods to perform integrated research that extends from fundamentals to applications for a wide range of physical phenomena, and to systematize fundamental theory.

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Research topics

1.Relativistic Coulomb explosion and neutron source development

When ultra-high-intensity lasers are used to irradiate nanoclusters measuring several hundred nanometers (roughly several hundred times narrower than a human hair, or the length of several thousand atoms lined up end to end), electrons and ions are produced and scattered in every direction. These accelerated high-speed ions (e.g., protons) can be used to generate neutrons. These protons and neutrons could be used for various applications, including cancer treatment, fusion energy development, landmine detection, fuel cell development, and nuclear waste disposal.

2.Laser fusion irradiation arrangement optimization using self-organizing algorithms

High-density compression is essential for inertial confinement fusion. In order to achieve it, the fuel needs to be irradiated and compressed as uniformly as possible. One of the most critical tasks of irradiation system design is the optimization of the irradiation arrangement to produce the most uniform radiation from a limited number of laser (X-ray) sources. Conventional designs have been optimized purely based on regular polyhedrals or geometrical designs based on them. For example, the OMEGA Laser located in the U.S. at the University of Rochester has 60 beams, but it has not always been clear whether its beam arrangement is optimal.

Recently, the TLP group has developed a new optimization algorithm based on a self-organizing design. The principles of this design are extremely simple; N point charges are scattered across a spherical surface and then allowed to move freely based on Coulomb repulsions. The final arrangement they reach is the most stable arrangement.

3.Impact ignition fusion

In the Spring of 2004, the TLP group proposed a third ignition method for laser fusion and named it “impact ignition.” The method consists of (i) a collision of an ignition fuel shell arranged within a suspended cone with the compressed main fuel at an ultra-high speed of over 1000 km/s, (ii) the generation of a hot spot from the direct conversion of kinetic energy into thermal energy by the shock wave compression process, and (iii) high-efficiency fusion combustion. This ignition method and target structure are of a completely new and unique design, not proposed by any other research institutions. Impact ignition has the following attractive advantages for future fusion reactors.
(a) Simple physics (fundamentally based on fluid physics alone)
(b) Possibility of high energy acquisition design
(c) Possibility of low-cost, compact reactor design
The key physical issues are (1) whether Rayleigh-Taylor fluid instability can be controlled while accelerating the target to an unprecedented speed of 1000 km/s while maintaining g/cm³-order density, and (2) whether there is a sufficiently high energy transfer rate from the incident laser to the fuel core. With regard to (1), the latest preliminary studies have achieved speeds of 1000 km/s, which is almost three times the previous record speed, drawing international attention to this research.

4.Self-similar solutions in non-linear plasma dynamics

Many self-similar phenomena can be observed in nature. Even if the term “self-similarity” does not immediately ring a bell, one can get a rough idea of what it entails when one considers fractals, which have become a sensation in recent years. The structures of crystals, veins, coasts, turbulence, and space are often used as examples when discussing self-similarity. In other words, for static systems, it refers to similar repeated patterns of different sizes, and for dynamic systems, it refers to similar patterns being maintained amidst constantly changing physical phenomena.

Laser-induced ablation acceleration physics also exhibit self-similarity. Over time, the mass of the target decreases and the speed increases. The target thickness also progressively decreases until ultimately, the target is completely consumed. However, simulations have confirmed that even as this change over time occurs, spatial profiles remain self-similar. Furthermore, the acceleration movement of slab (spherical shell) targets is non-steady as indicated above. Until now, most theoretical models of laser ablation have assumed steady states. This makes integration relatively simple and produces plausible spatial profiles, making this approach highly valuable. However, when the resulting differential equation system is numerically integrated, a certain point (singularity) is always reached, stopping the calculation process. In other words, the conventional steady state analysis model does not possess a real-life form. By contrast, we considered the system to be a non-steady one from the start; we kept the time-dependent elements of the partial differential equations and discovered a self-similar solution that develops over time without any contradictions. We are currently using this analytical solution to determine the accurate time evolution of Rayleigh-Taylor instability. In addition, we have also discovered new self-similar solutions, such as the time-dependent self-similar solution of the opposing forces of self-gravity and energy dissipation during the generation of stars, and the self-similar expansion of finite mass plasma as electron and ion fluids, without hypothesizing charge neutrality as is conventionally done.

Other research themes

• Proton beam generation using carbon nanotube accelerators
• High density compression using highly multidimensional targets
• New high-speed ignition target design
• Electron acceleration using relativistic lasers
• Shock wave dynamics of supernova explosions
• Laser ion acceleration
• Polar-Direct-Drive irradiation design
• Laser anomalous absorption using vacuum heating

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