Formation of jet-like spikes from the ablative Rayleigh-Taylor instability

Abstract

The mechanism of jet-like spike formation from the ablative Rayleigh-Taylor instability (ARTI) in the presence of preheating is reported. It is found that the preheating plays an essential role in the formation of the jet-like spikes. In the early stage, the preheating significantly increases the plasma density gradient, which can reduce the linear growth of ARTI and suppress its harmonics. In the middle stage, the preheating can markedly increase the vorticity convection and effectively reduce the vorticity intensity resulting in a broadened velocity shear layer near the spikes. Then the growth of ablative Kelvin-Helmholtz instability is dramatically suppressed and the ARTI remains dominant. In the late stage, nonlinear bubble acceleration further elongates the bubble-spike amplitude and eventually leads to the formation of jet-like spikes. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759161]Control of the ablative Rayleigh-Taylor instability (ARTI) is crucially important for the success of inertial confinement fusion (ICF). 1 This inherent physical instability can limit the implosion velocity and the size of the hot spot, and in some cases even break up the implosion shell or make hot spot formation impossible, resulting in the auto-ignition failure. So far, nonlinear evolution of ARTI is still not very clear, especially the formation of jet-like structures, which are widely observed in high energy density hydrodynamic instability experiments and simulations in ICF research. [2][3][4] It is well documented that in the linear growth regime the ablation stabilizes the ARTI and a cutoff wavelength appears when the perturbation wavelength is sufficiently short. 5-10 The linear growth rate of Bodner-Takabe-Lindl c BTL ¼ a ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi kg=ð1 þ kL m Þ p À bkv a approximately fits the twodimensional numerical simulations and experiments, where k is the wave number, g is the acceleration, L m is the minimum scale length of density gradient at the ablation surface, and v a is the ablation velocity. 6 The parameters a and b depends basically on the flow parameters, and usually a ¼ 0:9 À 0:95 and b ¼ 1 À 3. A self-consistent theory 7 shows that the linear growth rate of the ARTI takes the formwhere the parameters are given by Eq. (8) in Ref. 7, which is an asymptotic formula valid for arbitrary Froude numbers.The self-consistent theory contains the effects of the dynamic overpressure due to the thermal conduction, of the damping due to the fire polishing and the vorticity convection, and of the density gradients of the ablation surface. The physical explanation of the stabilization of the ARTI has been described clearly by Piriz et al., 8 and references therein. The recent experiments 9,10 suggest that the preheating by energetic electrons and the nonlocal electron heat transport plays an important role in the suppression of ARTI at the ablation front. The observed density profiles in the directdrive experiments 11 and simulation...

Three-dimensional single-mode nonlinear ablative Rayleigh-Taylor instability

“…As the ablated plasma fills the volume within the bubbles, the vorticity accumulates, providing a centrifugal force that acts on the bubble's wall. Similar vorticity accumulation and bubble acceleration phenomena were also reported by Wang et al 13 In 2D geometry, 12 the bubble acceleration vanishes asymptotically and the bubble's velocity saturates at a value above its classical value U cl2D b . Another numerical study by Igumenshchev et al 14 showed that a small defect on the target surface with a 10 lm radius and 1 lm height drives an RTI bubble that quickly penetrates through the shell.…”

Interface coupling effects of weakly nonlinear Rayleigh–Taylor instability with double interfaces*

“…[11] Especially, in the implosion process of ICF, the ablation effect makes the material distribution in the capsule more complicated. [12][13][14][15][16] The classic RTI models which only include a single material interface are not good enough for depicting the perturbation growth of the RTI with multiple interfaces. However, to understand the mechanism of the RTI with multiple interfaces, some efforts have been done by researchers.…”

Rayleigh-Taylor instability at spherical interfaces of incompressible fluids

“…The Rayleigh-Taylor instability has been investigated analytically, [6][7][8][9][10][11][12][13][14][15][16][17][18] numerically, [19][20][21] and experimentally [22,23] by many researchers. The classical RTI is first given by Rayleigh, [1] where two semi-infinite fluids are separated by a perturbed interface.…”