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Fast Heating of Ultrahigh-density Plasma as a Step towards Laser Fusion Ignition

 



 

Introduction

Controlled thermonuclear fusion—long regarded as one of the ideal energy sources—has remained out of our grasp even as concern over global warming and other environmental problems rises ever higher and as fossil fuel reserves ever dwindle. Ultra-intense laser technologies are now exploring one of the most attractive fast-track paths to the energy products in laser fusion research - the advanced fast ignition concept [1-3]. Laser driven implosions [4] of spherical shells have achieved up to ×1000 solid density [5]. These densities are large enough to enable controlled fusion burning but to achieve energy gain a small volume (the spark) in the compressed fuel must be heated to temperatures >10 keV (~108 K). In the conventional approach [6], the spark is produced by accurately timed shock waves4, but this requires both precise implosion symmetry and very large drive energy. In principle, these requirements can be significantly relaxed by performing the compression and fast heating separately; however, this ‘fast ignitor’ approach also suffers drawbacks, such as propagation losses and deflection of the ultra-intense laser pulse by the plasma surrounding the compressed fuel. Here we have demonstrated a new compression geometry that eliminates these problems; we combine production of compressed matter in a laser-driven implosion with picosecond-fast heating by a laser pulse timed to coincide with the peak compression. Our approach therefore permits efficient compression and heating to be carried out simultaneously, providing one possibility for efficient fusion energy production.

Fast Ignition Concept

Advances in laser technology were making development of a new, more compact ignition technology a real possibility. Laser energy can be concentrated in one-trillionth of a second, which has made it possible to develop ultra-intense lasers that have a peak intensity several hundred times the global output of electricity. The pressure generated by this laser light reaches approximately a hundred billion atmospheres, meaning that scientists can create on Earth conditions similar to the gravitational acceleration that occurs near a black hole. Such ultra-intense short pulse laser can inject the required energy to the high-density imploded core plasmas within the core disassembling time (less than a few times 10-11 second). The laser energy is coupled to the highly compressed plasma via relativistic electrons efficiently generated with a conversion efficiency of about 30-40% [7] when such ultra-intense laser interacts with a high-density plasma. Enormous electromagnetic fields of the laser accelerate the high-density relativistic electrons and immediately heat up the compressed fuel to ignition condition Figure 1 shows how compressed fusion fuel is controlled from outside and then ignited with an ultra-intense laser. The conventional method for igniting fusion fuel can be compared to how a diesel engine works, where a piston compresses fuel until it spontaneously ignites, whereas the new ignition method works like a gasoline engine, where ignition and combustion of the compressed fuel is controlled by the spark plug.

 

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