5 Search Results

The self-similar nonlinear evolution of the multimode ablative Rayleigh-Taylor instability (RTI) and the ablation-generated vorticity effect are studied for a range of initial conditions. We show that, unlike classical RTI, the nonlinear multimode bubble-front evolution remains in the bubble competition regime due to ablation-generated vorticity which accelerates the bubbles, thereby preventing a transition into the bubble-merger regime. We develop an analytical bubble competition model to describe the linear and nonlinear stages of ablative RTI. We show that vorticity inside the multimode bubbles is most significant at small scales with large initial perturbation. Since these small scales persist in the bubblemore » competition regime, the self-similar growth coefficient α_b can be enhanced by up to 30% relative to ablative bubble competition without vorticity effects. We use the ablative bubble competition model to explain the hydrodynamic stability boundary observed in OMEGA low-adiabat implosion experiments.« less

In the shock ignition (SI) laser fusion scheme, hot electrons generated by the laser spike pulse can either preheat the fuel or strengthen the ignition shock, depending on the hot electron characteristics. We conducted a planar target experiment on the OMEGA-EP laser facility and characterized the temperature and total energy of hot electrons generated from a kilojoule-class 100-ps infrared (IR) or a 1-ns ultraviolet (UV) laser interacting with a large (L_n~330-450 μm) and hot (T_e~1-2 keV) coronal plasma at the SI-relevant intensities (~10¹⁶ W/cm²). The IR laser converts ~2.5% energy into hot electrons with T_hot~ 60–90 keV, while the UVmore » laser couples 0.8% ± 0.7% energy into T_hot=27 ± 9 keV hot electrons. The IR-produced hot electrons yield five times higher Cu Kα emission than the UV case, confirming the higher electron conversion efficiency with the IR laser. The low energy conversion from the UV laser to hot electrons may be due to the refraction of the off-normal incident laser in the large coronal plasma. These findings are the first comparisons of hot electron generation between the IR and UV pulses at kilojoule scales in SI-relevant large-scale plasmas. The findings may expand the SI design space to include IR lasers as the possible spike lasers.« less

In this work, the coupling and evolution of two-mode ablative Rayleigh-Taylor instability (ARTI) in two-dimensional geometry are studied via numerical simulations. We focus primarily on two scenarios: Coupling and bubble competition of a long and a short wavelength mode and of two short-wavelength modes. It is found that the long-wavelength modes tend to dominate in the nonlinear phase of the long-short coupling cases. The presence of the short-wavelength mode in the long-short cases enhances the total ARTI bubble vertex velocity. However, due to the formation of enclosed bubbles, this enhancement does not increase monotonically with the initial short-wavelength amplitude. Couplingmore » of two short-wavelength modes forms a long-wavelength component which grows faster than each individual short-wavelength mode.« less

The two-plasmon decay (TPD) and stimulated Raman scattering (SRS) instabilities have been studied in the region near the quarter-critical density in the plasmas of the laser-driven inertial confinement fusion for a wide range of laser angles of incidence. The theoretical analysis of the TPD for oblique laser incidence has been carried out. The theoretical growth rates and thresholds have been compared with the results of the fluid-type simulations, and a good agreement has been found. Here, in the modeling including both TPD and SRS, the spectra of the growing perturbations have multiple peaks, and the maximum growth may be influencedmore » by the interplay between TPD and SRS.« less

In this work, we study stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in shock ignition by comparing fluid and particle-in-cell (PIC) simulations. Under typical parameters for the OMEGA experiments [Theobald et al., Phys. Plasmas 19, 102706 (2012)], a series of 1D fluid simulations with laser intensities ranging between 2 x 10¹⁵ and 2 x 10¹⁶W/cm² finds that SBS is the dominant instability, which increases significantly with the incident intensity. Strong pump depletion caused by SBS and SRS limits the transmitted intensity at the 0.17n_c to be less than 3.5 x 10¹⁵W/cm². The PIC simulations show similar physics butmore » with higher saturation levels for SBS and SRS convective modes and stronger pump depletion due to higher seed levels for the electromagnetic fields in PIC codes. Plasma flow profiles are found to be important in proper modeling of SBS and limiting its reflectivity in both the fluid and PIC simulations.« less