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Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition

Abstract

Modern high-power lasers can generate extreme states of matter that are relevant to astrophysics1, equation-of-state studies2 and fusion energy research3,4. Laser-driven implosions of spherical polymer shells have, for example, achieved an increase in density of 1,000 times relative to the solid state5. These densities are large enough to enable controlled fusion, but to achieve energy gain a small volume of compressed fuel (known as the ‘spark’) must be heated to temperatures of about 108 K (corresponding to thermal energies in excess of 10 keV). In the conventional approach to controlled fusion, the spark is both produced and heated by accurately timed shock waves4, but this process requires both precise implosion symmetry and a very large drive energy. In principle, these requirements can be significantly relaxed by performing the compression and fast heating separately6,7,8,9,10; however, this ‘fast ignitor’ approach7 also suffers drawbacks, such as propagation losses and deflection of the ultra-intense laser pulse by the plasma surrounding the compressed fuel. Here we employ 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 a route to efficient fusion energy production.

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Figure 1: Ultraviolet images showing the heating of solid targets by relativistic electrons, and a sketch of the set-up used to obtain the images.
Figure 2: The implosion target for efficient heating of the highly compressed plasma, an X-ray image of the implosion, and the density profile of the plasma.
Figure 3: Time-integrated X-ray image of the short-pulse laser heating, and time-resolved X-ray images of the highly compressed plasma heated by the short-pulse laser.
Figure 4: Neutron spectrum from the highly dense plasma heated by the short-pulse laser at the time of maximum compression.

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Acknowledgements

We thank the mm-Wave Technology Centre at the Rutherford Appleton Laboratory, and the target fabrication, laser operation and data acquisition groups at ILE Osaka University. This work was supported by the Japan Society for the Promotion of Science, and the UK Royal Society.

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Correspondence to R. Kodama.

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Kodama, R., Norreys, P., Mima, K. et al. Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition. Nature 412, 798–802 (2001). https://doi.org/10.1038/35090525

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