Abstract
The Pillars of Creation, one of the most recognized objects in the sky, are believed to be associated with the formation of young stars. However, so far, the formation and maintenance ...mechanism of the pillars are still not fully understood due to the complexity of the nonlinear radiation magnetohydrodynamics (RMHD). Here, assuming laboratory laser-driven conditions, we studied the self-consistent dynamics of pillar structures in magnetic fields by means of two-dimensional and three-dimensional (3D) RMHD simulations, and the results support our proposed experimental scheme. We find that only when the magnetic pressure and ablation pressure are comparable, the magnetic field can significantly alter the plasma hydrodynamics. For medium-magnetized cases (
β
initial
≈ 3.5), the initial magnetic fields undergo compression and amplification. This amplification results in the magnetic pressure inside the pillar becoming large enough to support the sides of the pillar against radial collapse due to pressure from the surrounding hot plasma. This effect is particularly pronounced for the parallel component (
B
y
), which is consistent with observational results. In contrast, a strong perpendicular (
B
x
,
B
z
) magnetic field (
β
initial
< 1) almost retains its initial distribution and significantly suppresses the expansion of blown-off gas plasma, leading to the inability to form pillar-like structures. The 3D simulations suggest that the bending at the head of “Column I” in the Pillars of Creation may be due to nonparallel magnetic fields. After similarity scaling transformation, our results can be applied to explain the formation and maintenance mechanism of the pillars, and can also provide useful information for future experimental designs.
In inertial confinement fusion (ICF), overlapping of laser beams is common. Owing to the effective high laser intensity of the overlapped beams, the collective mode of stimulated Brillouin scattering ...(SBS) with a shared scattered light wave is potentially important. In this work, an exact analytic solution for the convective gain coefficient of the collective SBS modes with shared scattered wave is presented for two overlapped beams based on a linear kinetic model. The effects of the crossing angle, polarization states, and finite beam overlapping volume of the two laser beams on the shared light modes are analyzed for cases with zero and nonzero wavelength difference between the two beams. It is found that all these factors have a significant influence on the shared light modes of SBS. Furthermore, the out-of-plane modes, in which the wavevectors of daughter waves lie in different planes from the two overlapped beams, are found to be important for certain polarization states and especially for obtuse crossing angles. In particular, adjusting the polarization directions of the two beams to be orthogonal to each other or tuning the wavelength difference to a sufficiently large value (of the order of nanometers) are found to be effective methods to suppress the shared light modes of SBS. This work will be helpful for comprehending and suppressing collective SBS with shared scattered waves in ICF experiments.
We present the first integrated implosion experiments which were carried out on the SG-III Laser facility in 2015. Vacuum hohlraums and squared laser pulses were used in the experiments. The purpose ...of the experiments was to demonstrate the one-dimensional implosion performance at low-convergence ratios. To characterize the implosion performance, the hohlraum energetics, the implosion dynamics and the nuclear products were measured. In particular, the radiation flux was diagnosed through laser entrance hole at different angles by an array of flat response detectors, and the energy and the spectrum of the back-scattered lasers were monitored for 8 laser beams which correspond to 2 beams in each cone at the lower hemisphere. Moreover, the hotspot shape was imaged using KB microscopes of high spatial resolution simultaneously from the equator and polar views. Taking advantage of the high resolution, some detailed structures in the hotspots were noticed. For the best implosion performances, the neutron yields over 1D calculation reached 50% at the convergence-ratios of 15.
The persistence of spin-polarized fuels is a crucial problem for polarized magnetic and inertial confinement fusion (ICF). The depolarizations of polarized deuterium-tritium (DT) fuels in indirectly ...driven ICF implosions are investigated with three-dimensional spin transport hydrodynamics simulations. The spin transport equations for deuterons and tritons are derived with the density matrix formalism, which are used to investigate the evolutions of spin eigenstate distributions of DT fuel. The depolarization of DT ions by strong self-generated magnetic fields and the mixings of DT ions with different spin states can be captured by the spin transport equation. The simulation results show that triton polarizations are sensitive to large scale magnetic fields generated by polar mode asymmetries. It is also found that the depolarization of tritons can be reduced by an optimized spin alignment of the polarized fuel. The methods and results can be used to optimize the design of polarized fusion targets and interpret polarized fusion experiments.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UL, UM, UPUK
Hybrid fluid-PIC simulations aimed at a better understanding of the implosion physics and the material mixing into the hot spot are described. The application of a hybrid fluid-PIC code is motivated ...by the difficulty of modeling the material mixing by the commonly used radiation hydrodynamic simulations. Hybrid fluid-PIC techniques, which treat the ions with the traditional particle-in-cell method, and electrons with a massless fluid, are more adaptable to handle the heating of DT fuel through PdV work and the material mixing near the DT ice-gas interface and ablator-fuel interface of a compressed capsule. During implosion shock convergence, significant reactant temperature separation and a noticeable amount of material mixing are observed, both of which have important consequences for estimating neutron yield and the understanding of implosions. Physical explanations for these phenomena are discussed, with the non-equilibrium effect in the hotspot and hydrodynamic instabilities at the interface as the likely explanation, respectively. The hybrid fluid-PIC method would be helpful to test the phenomenological fluid model describing the material mixing in ICF implosion.
Extrapolation of implosion performance between different laser energy scales is investigated for indirect drive through a semi-hydro-equivalent design. Since radiation transport is ...non-hydro-equivalent, the peak radiation temperature of the hohlraum and the ablation velocity of the capsule ablator are not scale-invariant when the sizes of the hohlraum and the capsule are scale-varied. A semi-hydro-equivalent design method that keeps the implosion velocity Vi, adiabat αF, and PL/Rhc2 (where PL is the laser power and Rhc is the hohlraum and capsule scale length) scale-invariant, is proposed to create hydrodynamically similar implosions. The semi-hydro-equivalent design and the scaled implosion performance are investigated for the 100 kJ Laser Facility (100 kJ-scale) and the National Ignition Facility (NIF-scale) with about 2 MJ laser energy. It is found that the one-dimensional implosion performance is approximately hydro-equivalent when Vi and αF are kept the same. Owing to the non-hydro-equivalent radiation transport, the yield-over-clean without α-particle heating (YOCnoα) is slightly lower at 100 kJ-scale than at NIF-scale for the same scaled radiation asymmetry or the same initial perturbation of the hydrodynamic instability. The overall scaled two-dimensional implosion performance is slightly lower at 100 kJ-scale. The general Lawson criterion factor scales as χnoα2D∼S1.06±0.04 (where S is the scale-variation factor) for the semi-hydro-equivalent implosion design with a moderate YOCnoα. Our study indicates that χnoα ≈ 0.379 is the minimum requirement for the 100 kJ-scale implosion to demonstrate the ability to achieve marginal ignition at NIF-scale.
PDF
