First implosion experiments with cryogenic thermonuclear fuel on the National Ignition Facility

Abstract

Non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion. These experiments use cryogenic fuel ice layers, consisting of mixtures of tritium and deuterium with large amounts of hydrogen to control the neutron yield and to allow fielding of an extensive suite of optical, x-ray and nuclear diagnostics. The thermonuclear fuel layer is contained in a spherical plastic capsule that is fielded in the center of a cylindrical gold hohlraum. Heating the hohlraum with 1.3 MJ of energy delivered by 192 laser beams produces a soft x-ray drive spectrum with a radiation temperature of 300 eV. The radiation field produces an ablation pressure of 100 Mbar which compresses the capsule to a spherical dense fuel shell that contains a hot plasma core 80 µm in diameter. The implosion core is observed with x-ray imaging diagnostics that provide size, shape, the absolute x-ray emission along with bangtime and hot plasma lifetime. Nuclear measurements provide the 14.1 MeV neutron yield from fusion of deuterium and tritium nuclei along with down-scattered neutrons at energies of 10-12 MeV due to energy loss by scattering in the dense fuel that surrounds the central hot-spot plasma. Neutron time-of-flight spectra allow the inference of the ion temperature while gamma-ray measurements provide the duration of nuclear activity. The fusion yield from deuterium-tritium reactions scales with ion temperature, which is in agreement with modeling over more than one order of magnitude to a neutron yield in excess of 10 14 neutrons, indicating large confinement parameters on these first experiments.

Dielectric function of dense plasmas, their stopping power, and sum rules

“…Modeling of the dielectric function (DF) (k,ω) or the inverse dielectric function (IDF) −1 (k,ω) of strongly coupled Coulomb systems is actively discussed in the literature, in particular, because the corresponding loss function L(k,ω) = − Im −1 (k,ω)/ω 0, (1) which is an even function of the real frequency ω, determines the polarization stopping power of such systems [15]. The nonnegativity of the loss function stems from a similar property of − Im −1 (k,ω) for positive frequencies, which in turn follows from the fluctuation-dissipation theorem since the (chargecharge) dynamic structure factor S cc (k,ω) is non-negative by definition L(k,ω) = πβφ(k)b(β ω)S cc (k,ω).…”

“…III A in more detail, we wish to determine whether the sum rules (other than the f -sum rule) are satisfied by these models. The convergent fourth-order sum rule, which is the fourth frequency power moment of the loss function (1) C 4 (k), includes the correlation contributions and thus its fulfillment is presumably crucial for strongly coupled systems such as those of inertial fusion [1]. Note also that the long-wavelength limiting form of C 4 (k) related to the target electron-ion correlations modifies the well-known Bethe-Larkin logarithmic asymptotic form of the polarization stopping power of heavy ions in strongly coupled two-component plasmas [29] and we believe that if this fourth-moment sum rule does not hold for a certain DF model, the predictions of this model on the plasma stopping power should differ from those based on the MM and eventually from the real experimental data.…”

“…The extension of numerous models for the description of the coupled or nonideal plasma dynamic properties onto the density-temperature domain characteristic of inertial fusion bound experiments [1] is a hot problem nowadays. In particular, the diagnostic methods applied in these experimental studies require a reliable method of reconstruction of the ion-ion dynamic structure factor and the method of moments [2][3][4][5][6][7][8] has demonstrated its advantages here with respect to other approaches recently [9].…”

Soft x-ray images of the laser entrance hole of ignition hohlraums

“…The capsule is filled with helium gas doped with deuterium and tritium. 3,4 Ninety-six 351 nm laser beams, 64 in outer cones (50 • , 44.5 • ) and 32 in inner cones (30 • , 23.5 • ) enter the laser entrance holes (LEH) at each end of the hohlraum with 20 ns temporally shaped pulses with total energies ranging from 1.2 to 1.65 MJ. The laser energy is converted into x-ray energy via the inverse bremsstrahlung interaction with the gas fill and the high-Z hohlraum walls.…”

“…The x-rays heat and ablate the outer shell of the capsule, thereby compressing the inner core. [2][3][4] It is important to understand the coupling of this x-radiation drive to the capsule. The first a) Contributed paper, published as part of the Proceedings of the 19th Topical Conference on High-Temperature Plasma Diagnostics, Monterey, California, May 2012. b) Author to whom correspondence should be addressed.…”