7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Recent progress of experimental studies on fast-ignition inertial fusion energy
S. FUJIOKA1, R. TAKIZAWA1, J. Y. DUN1, T. MAEKAWA1, T. TSUIDO1, Y. KARAKI1, H.
MATSUBARA1, R. YAMADA1, A. YOGO1, N. IWATA1, A. MORACE1, Y. ARIKAWA1, T.
SANO1, H. NAGATOMO1, H. SAWADA2, A. IWAMOTO3, T. JOHZAKI4,Y. SENTOKU1, R. KODAMA1
1
Institute of Laser Engineering, Osaka University
2.
Department of Physics, University of Nevada, Reno3.
National Institute for Fusion Science, National Institute for Natural Sciences
4.
Graduate School of Advanced Science and Engineering, Hiroshima University
fujioka.shinsuke.ile@osaka-u.ac.jp
The ignition of inertial confinement fusion wasdemonstrated at the National Ignition Facility in theUnited States in December 2022. This historical shot
utilizes the indirect-drive method and the central-ignition
scheme. The energy gain must exceed 100 to realize
inertial fusion energy (IFE). The indirect-drive method
has a low energy utilization efficiency because the laser isconverted once into thermal X-rays. The fuel geometry iscomplex because a hohlraum is required in addition to afusion fuel. The direct-drive method, which has arelatively simple fuel geometry and high energy
utilization efficiency, is more suitable for IFE. In thisstudy, we conducted the research aiming at the realizationof IFE using the direct-drive method and fast-ignition
scheme. The fast-ignition scheme can have a higher
heating efficiency of fusion fuel than the central-ignition
scheme , and the fast-ignition may be able to form a hot
spark more efficiently than the central-ignition scheme.
One of the most serious obstacles of the centralignition is hydrodynamic instability. The general fusion
fuel is a spherical shell containing a thin deuterium-tritium
ice layer on the inner surface of a thin hollow capsulecalled an ablator. The cavity of the spherical shell is filledwith deuterium-tritium gas at saturated vapor pressure.
The tiny holes and irregularities on the surface and insideof the spherical shell are instability seeds, and thespherical shell is greatly deformed as it is compressed.
Since the hot spark, which is the starting point of fusion
ignition, is formed by adiabatic compression of thedeuterium-tritium gas, the deformation of the sphericalshell makes the formation of the hot spark inefficient. In
the fast-ignition, a portion of the high-density fusion fuelproduced by implosion is heated by a short and intenselaser pulse injected from outside the plasma to create thehot spark.
Interactions between the picosecond PW laser pulse
and a plasma generate an electron beam with relativisticenergy, called a relativistic electron beam (REB). This
REB heats the high-density plasma in the REB-based fastignition scheme. The advantage of the REB-based scheme
is the high energy conversion efficiency from laser to REB(>30%). On the other hand, the average REB energy must
be below 3 MeV to heat locally and efficiently a tiny
volume of the high-density plasma, and REB has a large
divergence angle (> 90 deg.) [1]. We are conducting thefast-ignition experiments using GEKKO-XII, ananosecond TW laser system, to generate a high-density
plasma by an implosion, and LFEX laser, a picosecond
PW laser system, to heat the high-density plasmainstantaneously. We have introduced plasma mirrors to theLFEX [2] to significantly improve the pulse contrast for
heating the plasma by low-energy REB and shortening thedistance between the high-density plasma and theinteraction region so that the heat can diffusively
propagate from the directly heated interaction region to
the high-density region[3]. In addition, externallyapplying a laser-generated magnetic field improves thedirectionality of REBs, and the heating efficiency bycollisions between REBs and ions (drag heating) is
successfully enhanced [4].
The heating efficiency
depends on the plasma density. We have proposed a solidball fuel as a target for avoiding the breakup of theimploding plasma due to hydrodynamic instability [5]. We
have also conducted experiments using a solid ball target
to produce high-density plasma with a three-step pulse.
We are also developing a liquid deuterium-containing
solid ball for the world's first fast ignition experiment withdeuterium plasma.
[1] S. Fujioka et al., Heating Efficiency Evaluation with
Mimicking Plasma Conditions of Integrated Fast-Ignition
Experiment, Phys. Rev. E 91, 063102 (2015).
[2] Y. Arikawa et al., Ultrahigh-Contrast Kilojoule-ClassPetawatt LFEX Laser Using a Plasma Mirror, Appl. Opt.
55, 6850 (2016).
[3] K. Matsuo et al., Petapascal Pressure Driven by FastIsochoric Heating with a Multipicosecond Intense Laser
Pulse, Phys. Rev. Lett. 124, 035001 (2020).
[4] S. Sakata et al., Magnetized Fast Isochoric Laser
Heating for Efficient Creation of Ultra-High-EnergyDensity States, Nat. Commun. 9, 3937 (2018).
[5] H. Sawada et al., Development of 4.5 KeV
Monochromatic X-Ray Radiography Using the HighEnergy, Picosecond LFEX Laser, J. Phys. Conf. Ser. 717,
7 (2016).