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 astrophysics, equation-of-state studies and fusion energy research. Laser-driven implosions of spherical polymer shells have, for example, achieved an increase in density of 1,000 times relative to the solid state. 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 waves, 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 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 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.

Evidence of anomalous resistivity for hot electron propagation through a dense fusion core in fast ignition experiments

“…The FI scheme has been demonstrated using a guiding gold cone at the Gekko XII and PW laser facility, Osaka University as reported by Kodama et al [2,3]. The compression of a spherical shell with a gold cone is achieved with nine beams of GXII laser with a total energy of 2.5 kJ of the wavelength λ = 532 nm in a pulse duration of 1.2 ns.…”

“…The profile of imploded core plasma is estimated from measurement with an x-ray backlight method. A high dense core with a density (radius) of 50-100 g cm −3 (15-25 µm) is created at the front of the cone by high energy, nanosecond implosion laser pulses [2,3]. A PW class ultra-intense laser (UIL) pulse (λ = 1 µm) [11] is injected into the cone with an energy of 200-300 J in a pulse duration of 0.5 ps.…”

“…The displacement current contribution is ignored under the EMHD approximation. This relates the electron velocity to the magnetic field directly though Ampere's law in equation (2). The collisional friction between electron and ions has been retained through the resistivity coefficient η.…”

“…The attractiveness of the FI scheme in terms of (a) the separation of the compression and ignition steps and (b) the relaxation of the symmetry otherwise required in the implosion process has stimulated investigations by laboratories around the world. In 2001, the first support for its feasibility was offered in experiments where neutron emission was seen to be enhanced by a factor of 1000 when a deuterated-polystyrene (CD) pellet compressed to 55 g cm −3 density was ignited by a few MeV and a few tens of MA of hot electron current generated by a petawatt laser pulse [2,3]. Hopes thus engendered of the feasibility of FI, however, continue to encounter skepticism regarding the appropriate transport of MA electron currents through the tens of microns distance of the compressed core at number densities as high as 10 25 cm −3 .…”

Time-resolved compression of a capsule with a cone to high density for fast-ignition laser fusion

“…Early experiments studying the FI concept 10,11 reported a coupling efficiency of 15-30% of the short-pulse laser energy into the compressed plasma. Those experiments were limited by several factors, including insufficient drive-laser energy (2.5 kJ), no pulse-shaping capability and a longer drive-laser wavelength (532 nm instead of 351 nm)-unfavourable factors for achieving high compression.…”