Saturation of perturbation growth in ablatively driven planar laser targets

Abstract

Saturation of the mass variation growth during the shock transit time, theoretically predicted for the surface roughness case by Ishizaki and Nishihara ͓Phys. Rev. Lett. 78, 1920 ͑1997͔͒ and for the laser imprint case by Taylor et al. ͓Phys. Rev. Lett. 79, 1861 ͑1997͔͒, is studied analytically and numerically. The saturation is demonstrated to be essentially the same effect in both cases, caused by the stabilizing action of mass ablation. Scalings of saturation time and saturation level for the two cases are related. For lower-density foam targets, the peak level of mass variation is proportional approximately to 0 1/2 and exactly to 0 for the cases of laser imprint and surface roughness, respectively.

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

“…These oscillations during the shock transit time were first predicted in our simulations [5], then explained by the theory of Ref. 6 (where the term ablative RM instability was introduced), and later confirmed by many other simulations [7,8], but never actually observed experimentally.…”

“…Our face-on diagnostics does not allow us to evaluate any of these contributions separately. Theory and simulations indicate that the contribution of the rippled ablation front rapidly becomes dominant [5][6][7][8]. Figure 4 shows the data obtained for the thickest target that we probed with our diagnostics, 90 µm, 2a 0 = 2 µm.…”

“…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]).…”

Perturbation evolution started by Richtmyer-Meshkov instability in planar laser targets

“…1(a) . Solid plastic is less compressible by the transmitted shock wavecompression ratio of no more than 3 -and therefore a larger value of is appropriate; 10,18 we take here Side-on measurements of the interfacial modulation amplitude x in laser-driven solid targets are difficult, but the total mass modulation amplitude m is directly measurable by the face-on radiographic diagnostics. [6][7][8][9]13 Time history of m is plotted in Transition to an accelerated target provides additional complications.…”

“…6, 7 we reported the first direct observations of the early-time seeding phase caused by the outer -or front, in planar geometry, -surface roughness of the target, the oscillatory process called ablative Richtmyer-Meshkov (RM) instability. 10,11 The inner -or rear, in planar geometry, -surface roughness of the target has also been studied as a source of RT seeds. Experiments 7,8 demonstrated the theoretically predicted 12 feedout-triggered areal mass oscillations in a rippled rarefaction wave that is produced in a target rippled on the rear side; see also Ref.…”