First Measurements of Deuterium-Tritium and Deuterium-Deuterium Fusion Reaction Yields in Ignition-Scalable Direct-Drive Implosions

Abstract

The deuterium-tritium (D-T) and deuterium-deuterium neutron yield ratio in cryogenic inertial confinement fusion (ICF) experiments is used to examine multifluid effects, traditionally not included in ICF modeling. This ratio has been measured for ignition-scalable direct-drive cryogenic DT implosions at the Omega Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using a high-dynamic-range neutron time-of-flight spectrometer. The experimentally inferred yield ratio is consistent with both the calculated values of the nuclear reaction rates and the measured preshot target-fuel composition. These observations indicate that the physical mechanisms that have been proposed to alter the fuel composition, such as species separation of the hydrogen isotopes [D. T. Casey et al., Phys. Rev. Lett. 108, 075002 (2012)], are not significant during the period of peak neutron production in ignition-scalable cryogenic direct-drive DT implosions. DOI: 10.1103/PhysRevLett.118.095002 In direct-drive inertial confinement fusion (ICF) ignition designs, a cryogenic deuterium-tritium (DT) shell surrounding a vapor and encased in a thin-CH or thin-CD ablator (<10 μm) is symmetrically heated with nominally identical laser beams. In most designs, laser ablation launches one or multiple shocks through the converging shell and into the vapor region. The shock-transit stage of the implosion is followed by a deceleration phase, where the kinetic energy of the converging shell is converted to the internal energy of the hot spot. Thermonuclear fusion reactions are initiated in both the shock phase and the compression phase once sufficiently high temperatures and densities are reached [1]. To achieve conditions relevant for ignition implosion designs, the hot-spot size must exceed the mean free path of fusing ions. This requirement is essential to maximize the energy deposition of the alpha particle in the hot spot.Previous experiments on OMEGA [2] have reported anomalous Y DT =Y DD values (different by as much as a factor of 4) with the measured preshot fuel composition and experimentally inferred ion temperatures in roomtemperature implosions [3]. Several studies suggest that species separation of the hydrogen isotope resulting from multifluid effects [4,5] is likely responsible for the observed discrepancies in the yield ratios. This class of implosions, for example, exploding pushers that use thin-glass (∼3-μm SiO 2 ) or thin-CH (<16 μm) shells are, however, characterized by fusion reactions that occur predominantly during the shock phase at very high temperatures (≥10 keV) and relatively low densities (≤10 mg=cm 3 ). The mean free path for 90°deflection is given by λ ii ∼ T 2 i =Z 2 i Z 2 ρ [6] for ions of charge Z i , average ion temperature T i , ion charge Z, and density ρ. Conditions during the shock phase result in large mean-free-path lengths of the ions relative to the size of the fusing-plasma region (see Table I). These conditions are also typical of ignition-scalable direct-drive cryogenic implosions [7] during ...

Spectral composition of thermonuclear particle and recoil nuclear emissions from laser fusion targets intended for modern ignition experiments

“…The nonlocal transport of charged thermonuclear particles and recoil nuclei is the most important kinetic effect relating to combustion wave propagation [8,43,44]. In particular, the results of [43] convincingly testify to the need for the most accurate description of α-particle transport in the calculation of ICF target burning.…”

The National Direct-Drive Inertial Confinement Fusion Program

“…more than three diagnostic lines of sight (LOS)) are being developed to study multidimensional effects on the hot-spot at stagnation [9][10][11][12]. X-ray [9] and nuclear [10,11,32] diagnostics are being developed to study hot-spot formation and stagnation. 3D gated x-ray imaging of the hot-spot formation and disassembly with less than 5 µm spatial, ~10 ps temporal resolution is being developed [9].…”

“…3D gated x-ray imaging of the hot-spot formation and disassembly with less than 5 µm spatial, ~10 ps temporal resolution is being developed [9]. The neutron spectrum will be diagnosed with the neutron time of flight detectors along multiple LOS to infer the hot-spot ion temperature, the compressed areal density of the DT shell, spatial variations in these quantities, the fusion yields, and evidence of residual flows in the hot-spot [10,11,32]. Developing brighter x-ray sources and higher spatial resolution x-ray imagers is underway to probe the compressed fuel layer using 1.86 keV x-ray backlighting at stagnation [62].…”

“…The National Direct-Drive Program includes OMEGA and NIF experiments that study direct-drive target-physics, as shown in figure 1. Laser coupling, preheat, imprint and hydrodynamically scaled implosions from ignition designs for the NIF are investigated on OMEGA [19,[25][26][27][28][29][30][31][32]. The NIF LDD target has a diameter that is about four times the size of an OMEGA target and is driven with laser energy that is approximately 70× larger than OMEGA.…”

A simple analytical model of laser direct-drive thin shell target implosion

“…The implosion is that the high-intensity laser directly interacts with a thin shell, such as a plastic or glass wall, to generate a strong shock wave, heat and compress the fuel, and produce thermonuclear neutrons when the strong shocks collide at the target center. Such a platform has the advantages of high neutron yield, ultrashort fusion time, micro fusion zone, isotropic and monoenergetic neutron, and is usually used to develop the nuclear diagnostic system, [1,2] study the neutron irradiation effect, [3] measure the nuclear reaction [4,5] and cross section, [6] probe the fluid kinetic effects, [7][8][9][10] et al The glass target implosion is the simplest and earliest experiment on the laser facility, and demonstrates that thermonuclear neutrons resulting from laser-induced microshell implosion at KMS. [11] In order to understand the functional relationships between the various parameters of experiments and increase experiential neutron yield, some analytical models for long wavelength laser, such as 1.05 µm, have been proposed.…”