Fast heating of fuel assembled in a spherical deuterated polystyrene shell target by counter-irradiating tailored laser pulses delivered by a HAMA 1 Hz ICF driver
Fast heating is a method of heating an assembled high-density plasma into a hot state by irradiating it with short-duration (sub-picosecond), high-intensity ($>$ $10^{18}$ W ${\rm cm}^{-2}$ ) laser pulses before the plasma expands and dissolves hydrodynamically. In this paper, we present detailed experimental results of fast heating fuel assembled in a spherical deuterated polystyrene shell target of 500 ${\mu}$ m diameter and 7 μm thickness with counterbeam illumination by using a HAMA 1 Hz, 5.9 J inertial confinement fusion laser driver with pulse tailoring. These tailored pulses contain three pulses in sequence: a 'foot' pulse of 2.4 J/25 ns, a 'spike' pulse of 0.5 J/300 ps and a 'heater' pulse of 0.4 J/110 fs; these pulses are designed to assemble the fuel and heat it. By varying the energy of the foot pulse, we find that fast heating the fuel is achieved only if the fuel is weakly ablated by the foot pulse and then shock-assembled by the spike pulse into the target centre so that the heater pulse can access the fuel with a focal intensity greater than $10^{18}$ W ${\rm cm}^{-2}$ . Without a foot pulse, the heater pulse contributes to assembling the fuel. For higher foot-pulse energies, the heater pulse drives a hydrodynamic motion with speeds of the order 107 cm ${\rm s}^{-1}$ with intensities of the order $10^{17}$ W ${\rm cm}^{-2}$ , resulting in re-assembling and additional heating of the pre-assembled fuel. Once a shock-assembled core is achieved at the target centre, we succeed qualitatively in fast heating the core for shots in sequence with variations of laser energy within 18%. The coupling efficiency from the heating laser to the core is inferred to be $(10 \pm 2)$ % in total: $(8 \pm 1.6)$ % for the ionized bulk electrons and $(2 \pm 0.4)$ % for the bulk ions. The fusion neutron spectrum detected on the laser axis exhibits peaks at 1.0 MeV, 1.7 MeV and 3.8 MeV. These peaks are attributed to the $C(d, n){\hspace{0pt}}^{13}$ N and $d(d, n){\hspace{0pt}}^3$ He reactions induced by counterpropagating fast deuterons accelerated by the photon pressure of the heating pulses.