Direct Observation of Mass Oscillations Due to Ablative Richtmyer-Meshkov Instability in Plastic Targets

Abstract

We report the first direct experimental observation of the ablative RichtmyerMeshkov instability. It manifests itself in oscillations of areal mass that occur during the shock transit time, which are caused by the rocket effect or dynamic overpressure characteristic of interaction between the laser absorption zone and the ablation front.With the 4 ns long Nike KrF laser pulse and our novel diagnostic technique (monochromatic x-ray imaging coupled to a streak camera) we were able to register a peak and a valley of the areal mass variation before the observed onset of the RayleighTaylor growth.PACS numbers: 52.57. Fg, 52.70.La, Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. ABSTRACTWe report the first direct experimental observation of the ablative Richtmyer-Meshkov instability. It manifests itself in oscillations of areal mass that occur during the shock transit time, which are caused by the rocket effect or dynamic overpressure characteristic of interaction between the laser absorption zone and the ablation front. With the 4 ns long Nike KrF laser pulse and our novel diagnostic technique (monochromatic x-ray imaging coupled to a streak camera) we were able to register a peak and a valley of the areal mass variation before the observed onset of the Rayleigh-Taylor growth. Most of our knowledge in this field still comes from simulations and theory. The seed amplitudes are known to form during the early-time period that includes a shock wave transit from the front to the rear surface of the target, and a rarefaction wave transit in the opposite direction. During this time interval, the areal mass perturbations caused by all sources of non-uniformity are expected to oscillate [4][5][6][7][8][9]. The physical mechanisms driving the oscillations depend on whether the perturbations are initially at the front surface of the target (laser imprint, front surface roughness) or at its rear surface (feedout, see [9]). Here, we limit ourselves to the former case, where the oscillations are caused by the rocket effect, or the dynamic overpressure [6,8,10,11]. The oscillatory behavior is consistent with the expression for the growth rate Γ of ablative RT instability, which has recently been established [10,11] for the case of large Froude number (low acceleration): perturbed, and a part of it gets closer to the hot laser absorption zone, the temperature at the ablation front does not increase, but the temperature gradient in its vicinity, T ∇ , does. This, in turn, increases the local heat flux to the ablation front, T ∇ − κ , hence, the rate of mass ablation from it, thereby increasing the ablative pressure and pushing this part of the ablation front back. The physics of this rocket effect is explained in detail inRefs. 6,8,10. The rocket effect rat...

Basic hydrodynamics of Richtmyer–Meshkov-type growth and oscillations in the inertial confinement fusion-relevant conditions

“…These two conditions require high laser energy delivered to the target on a nanosecond time scale. In direct-drive experiments such as those reported by Aglitskiy et al (2001aAglitskiy et al ( ,b, 2002Aglitskiy et al ( , 2006, Glendinning et al (2003), Weaver et al (2004) and Gotchev et al (2006), approximately 0.5-2.5 kJ of laser energy needs to be deposited into the focal spot. Table 1.…”

“…The first direct experimental observations of the ablative RM oscillations have been made on the Nike laser at intensities about 60 TW cm −2 and reported by Aglitskiy et al (2001a). They are illustrated in figure 5.…”

“…These experiments have been made at an irradiation intensity of 420 TW cm −2 , much higher than that in Aglitskiy et al (2001aAglitskiy et al ( , 2002, which made it possible to detect the phase reversal of dm within 1 ns. Results of these accurate measurements turned out to be incompatible with modelling of laser-target interaction incorporating flux-limited Spitzer thermal conductivity.…”

Potential SiO2/CRF bilayer perturbation aerogel target for ICF hydrodynamic instability experiment

“…During the initial part of the implosion, as the capsule is compressed and accelerated inward, the interface between two adjacent layers is hydrodynamic instability which is seeded by interface roughness and grows via the Rayleigh-Taylor instability (RTI) [2,3]. In recent years, many different kinds of targets had been designed and fabricated to control and investigate the growth of RTI, such as single layer targets [4][5][6][7][8][9], dual-layer targets [10,11] and multilayer targets [12,13]. However, a little attention had been paid to bilayer aerogel target with perturbation patterns in the interface.…”

Suppression of Laser Nonuniformity Imprinting Using a Thin High- Z Coating

“…It was measured by radiographing the target face on (Fig. 1) using a Si backlighter and monochromatic curved crystal imaging at 1.86 keV coupled to an x-ray streak camera [22]. The angle of the radiography is 25°with respect to target normal.…”