Ablation Front Research Articles

Direct observation of density gradients in ICF capsule implosions via streaked Refraction Enhanced Radiography (RER)

We have performed refraction enhanced radiography (RER) measurements of indirect drive layered high density carbon capsule implosions relevant for inertial confinement fusion. Streaked RER data using a 5 µm wide imaging slit, backlit by a 7.8 keV Ni He-α laser driven x-ray source, shows features due to inflight density gradients in the ablator and fuel ice that are not visible in absorption only radiographs. To avoid motional blurring at smaller in-flight radii, we concentrate our probe to the main compression and N+1 shock phases. During the main compression phase, both simulations and data show fringes consistent with steep density gradients. During the later N+1 shock phase we see loss of fringes and hence infer less fuel compression than simulated, that may be caused by ice-ablator mix. Quantitative analysis shows that at the ablation front during the main compression phase, the simulated density differential agrees with the data, but the inferred scale length is 50% longer than simulated. During the N+1 shock phase, the data shows 2x higher ablation front density than simulated for similar scale lengths that may be affected by data cross talk with the reduced fuel compression at the ice-ablator interface.

Investigation of supersonic heat-conductivity hyperbolic waves in radiative ablation flows.

We carry out a numerical investigation of three-dimensional linear perturbations in a self-similar ablation wave in slab symmetry, representative of the shock transit phase of a pellet implosion in inertial confinement fusion (ICF). The physics of ablation is modeled by the equation of gas dynamics with a nonlinear heat conduction as an approximation for radiation transport. Linear perturbation responses of the flow, its external surface, and shock-wave front, when excited by external pressure or heat-flux perturbation pulses, are computed by fully taking into account the flow compressibility, nonuniformity, and unsteadiness. These responses show the effective propagation, at supersonic speeds, of perturbations from the flow external surface through the whole conduction region of the ablation wave, beyond its Chapman-Jouguet point, and up to the ablation front, after the birth of the ablation wave. This supersonic forward propagation of perturbations is evidenced by means of a set of appropriate pseudocharacteristic variables and is analyzed to be associated to the "heat-conductivity" waves previously identified by Clarisse etal. [J. Fluid Mech. 848, 219 (2018)JFLSA70022-112010.1017/jfm.2018.343]. Such heat-conductivity linear waves are found to prevail over heat diffusion as a feedthrough mechanism [Aglitskiy et al., Philos. Trans. R. Soc. A 368, 1739 (2010)PTRMAD1364-503X10.1098/rsta.2009.0131] for perturbations of longitudinal characteristic lengths of the order of-or larger than-the conduction region size, and long transverse wavelengths with respect to this region size, and over time scales shorter to much shorter than the shock transit phase duration. This mechanism, which results from the dependency of the heat conductivity on temperature and density in conjunction with a flow temperature stratification, is expected to occur for other types of nonlinear heat conductions-e.g., electron heat conduction-as well as to be efficient at transmitting large scale perturbations from the surrounding of an ICF pellet to its inner compressed core at later times of its implosion. Besides, the proposed set of pseudocharacteristic variables are recommended for analyzing perturbation dynamics in an ablation flow as it furnishes additional propagation information over the fundamental linear modes of fluid dynamics [Kovásznay, J. Aeronautic. Sci. 20, 657 (1953)1936-995610.2514/8.2793], especially in the flow conduction region.

Self-focusing of UV radiation in 1 mm scale plasma in a deep ablative crater produced by 100 ns, 1 GW KrF laser pulse in the context of ICF

Experiments at the GARPUN KrF laser facility and 2D simulations using the NUTCY code were performed to study the irradiation of metal and polymethyl methacrylate (PMMA) targets by 100 ns UV pulses at intensities up to 5 × 1012 W cm−2. In both targets, a deep crater of length 1 mm was produced owing to the 2D geometry of the supersonic propagation of the ablation front in condensed matter that was pushed sideways by a conical shock wave. Small-scale filamentation of the laser beam caused by thermal self-focusing of radiation in the crater-confined plasma was evidenced by the presence of a microcrater relief on the bottom of the main crater. In translucent PMMA, with a penetration depth for UV light of several hundred micrometers, a long narrow channel of length 1 mm and diameter 30 μm was observed emerging from the crater vertex. Similar channels with a length-to-diameter aspect ratio of ∼1000 were produced by a repeated-pulse KrF laser in PMMA and fused silica glass at an intensity of ∼109 W cm−2. This channel formation is attributed to the effects of radiation self-focusing in the plasma and Kerr self-focusing in a partially transparent target material after shallow-angle reflection by the crater wall. Experimental modeling of the initial stage of inertial confinement fusion-scale direct-drive KrF laser interaction with subcritical coronal plasmas from spherical and cone-type targets using crater-confined plasmas seems to be feasible with increased laser intensity above 1014 W cm−2.

Implementation of a Talbot-Lau x-ray deflectometer diagnostic platform for the OMEGA EP laser.

A Talbot-Lau X-ray Deflectometer (TXD) was implemented in the OMEGA EP laser facility to characterize the evolution of an irradiated foil ablation front by mapping electron densities >1022 cm-3 by means of Moiré deflectometry. The experiment used a short-pulse laser (30-100 J, 10 ps) and a foil copper target as an x-ray backlighter source. In the first experimental tests performed to benchmark the diagnostic platform, grating survival was demonstrated and x-ray backlighter laser parameters that deliver Moiré images were described. The necessary modifications to accurately probe the ablation front through TXD using the EP-TXD diagnostic platform are discussed.

Understanding ICF hohlraums using NIF gated laser-entrance-hole images

The newly available ns-gated laser-entrance-hole (LEH) imager on the National Ignition Facility provides routine, non-perturbative measurements of the x-ray emission from laser-heated plasmas inside the hohlraum as viewed at 19° to the hohlraum axis through one of its LEHs. Multiple images are acquired for a series of times and filter-selected x-ray energy bands within a single shot. The images provide time dependent data on phenomena including the effective radius of the LEH, the length of the gold-plasma “bubble” evolving off the interior wall surface heated by the outer beams, the evolving radius of the x-ray heated hohlraum wall, and the radius of the ablation front of the fusion capsule. These measurements are explained and illustrated with sample data. These techniques are then applied to understand hohlraum behavior as a function of gas fill. For hohlraums with helium gas fill densities of 0.15 to 0.30 mg/cm3, synthetic images computed from simulations agree well with experimental gated LEH images when an inhibited heat transport model [Jones et al., Phys. Plasmas 24, 056312 (2017)] is used. This model can be adjusted to reproduce the expansion rate of the laser-heated plasma bubble in such a way as to improve agreement with the images. At the higher 0.6 mg/cc gas fill, the experimental images show more pronounced 3D features, resulting in slightly less good agreement with the 2D simulations.

Ablation of polyimide thin-film on carrier glass using 355 nm and 37 ns laser pulses

Ablation of the polyimide (PI) thin-film deposited on glass substrate was investigated theoretically and experimentally to find out the important characteristics during the laser-lift off (LLO) process for the fabrication of flexible electronics. Instead of using conventional ablation induced by the front irradiation on the specimen, the LLO process employs the backside irradiation on the specimen transferred through the glass substrate, assuming that the ablation occurs volumetrically in the narrow region inside the PI film near the PI and glass interface. Experimentally the PI film on glass was exposed to a pulse laser beam emitted from UV laser of 355 nm in wavelength and 37 ns in duration. Depths and morphologies of the ablation spots were observed by the surface profiler and the FE-SEM. Theoretically the ablation process was estimated by the photothermal mechanism, using the one-dimensional heat conduction equation with volumetric absorption of the laser beam. It was shown that the experimental ablation could be described through the theoretical analysis based on the surface and the bulk photothermal models. While the former assumed that the ablation occurred only on the ablation front surface, the latter assumed the ablation was accompanied by the thermal and volumetric bond breakage. In general, the theoretical calculations agreed well with the experimental observations. In particular, the experimental observations of the ablation for the backside irradiation case that was the core phenomenon in the LLO process was explained appropriately by the features predicted using the bulk model.

Review of hydrodynamic instability experiments in inertially confined fusion implosions on National Ignition Facility

Hydrodynamic instabilities are a major factor in degradation of inertial confinement fusion (ICF) implosions. In the highest performing implosions on National Ignition Facility, yield amplification (YA) due to alpha particle heating approached ∼3, while YA of ∼15–30 is needed for ignition. Understanding and mitigation of the instabilities are critical to achieving ignition. This article reviews several experimental platforms that have been developed to directly measure these instabilities in all phases of ICF implosions. Measurements of ripple-shock propagation at OMEGA laser has provided results on initial seeds for the instabilities in three ablators—plastic (CH), beryllium, and high-density carbon. At the ablation front, instability growth of pre-imposed modulations was measured in the linear regime using the hydrodynamic growth radiography platform. This platform was extended for modulation growth of ‘native roughness’ modulations and engineering features (fill tubes and capsule support membranes or ‘tents’). Several new experimental platforms have or are being developed to measure instability growth at the ablator–ice interface. In the deceleration phase of implosions, complementary ‘self-emission’ and ‘self-backlighting’ platforms were developed to measure low-mode asymmetries and high-mode perturbations near peak compression.

Long-duration direct drive hydrodynamics experiments on the National Ignition Facility: Platform development and numerical modeling with CHIC

We report on a novel planar direct-drive platform for hydrodynamics experiments on the National Ignition Facility (NIF). Its commissioning has been performed as part of the NIF Discovery Science Program. This platform enables the use of a 30 ns drive at an average intensity of 200 TW/cm2, creating a planar shock and ablation front over a 2 mm radius. To benchmark the performance of this design, the planarity of both the shock and ablation fronts has been measured between 26 ns and 28 ns after the start of the laser drive in a 3 mm-thick CH foil. The platform was then used to measure late-time Rayleigh-Taylor instability (RTI) growth at the ablation front for a 2D-rippled 300 μm-thick CH foil. Simultaneously, a numerical platform has been developed with the CHIC radiation hydrodynamics code at the CELIA laboratory. The CHIC numerical platform allows, for the first time, a complete simulation of the experiments over 30 ns to be performed. Large-scale simulations recover the trajectory and the 2D RTI growth measurements. They are further compared with half-mode simulations performed with identical parameters. We show that both numerical techniques fit with analytical modeling of RTI growth and discuss plans for future campaigns.

Three-dimensional numerical simulation of the HECLA-4 transient MCCI experiment by improved MPS method

The molten corium-concrete interaction (MCCI) is an important phenomenon after the failure of pressure vessel in a severe accident of nuclear reactor. It has been verified that the original Moving Particle Semi-implicit (MPS) method has the capacity to simulate some MCCI experiments. In this study, the original MPS method has been improved by including the explicit pressure calculation model to reduce the computational cost and enhance the computational speed. Then, the improved MPS method was validated by simulating the classical dam break problem, and the results agreed well with that of the original MPS method. Afterwards, the HECLA-4 transient MCCI test performed by VTT was simulated by the improved MPS method with a three dimensional particle configuration of about one million particles. The basemat and sidewall ablation fronts, melt pool temperature and concrete temperature at different positions predicted by MPS were in good agreement with the experimental results. All the above-mentioned simulations proved that MPS method using explicit pressure model is capable of simulating MCCI and related heat and mass transfer in multicomponent phase flow.

From ICF to laboratory astrophysics: ablative and classical Rayleigh–Taylor instability experiments in turbulent-like regimes

Rayleigh–Taylor instability (RTI) occurs whenever fluids of different densities are accelerated against the density gradient, as is the case for the target ablator in ICF implosions. The advent of megajoule class lasers, like the National Ignition Facility (NIF) or Laser Mégajoule, offers novel opportunities to study turbulent mixing flows in high energy density plasmas for fundamental hydrodynamics or laboratory astrophysics experiments. Here, we review different RTI experiments, performed either at the ablation front or at a classical embedded interface. A two-dimensional bubble-merger, bubble-competition regime was evidenced for the first time at the ablation front in indirect-drive on the NIF thanks to an unprecedented long x-ray drive. Similarly, a novel large-area, planar platform enables the capabilities to perform long duration direct drive hydrodynamics experiments on NIF. Starting from imprinted seeds, a three-dimensional bubble-merger regime was also observed in direct-drive, as larger bubbles overtook and merged with smaller bubbles. In the astrophysical context, RTI also plays a role in supernova (SN) explosions, either of Type Ia or II. We report on experiments performed on the LULI2000 facility studying RTI in scaled laboratory conditions relevant for the physics of young SN remnants. Using a light CH foam as a deceleration medium, we measured, for the first time, the RTI mixing zone by PW transverse radiography.

Progress toward a self-consistent set of 1D ignition capsule metrics in ICF

One-dimensional metrics can be considered an upper bound on the performance that can be achieved given an implosion with an imploding mass Mimp and a given adiabat α, driven to an implosion velocity V by a specified ablation pressure Pa for single shell capsules with low-opacity ablators. The quantitative value of an ignition metric depends on the definition of ignition. We review the choices that have been made by various authors before settling on a definition based on yield amplification of 30 where yield amplification is the ratio of the yield from an implosion that includes the alpha particle and neutron deposition relative to the yield obtained from PdV work alone. We then derive improved 1D ignition metrics for radiation-driven inertial confinement fusion targets that span a wide range of drive pressures, adiabats, and scales. This includes emphasizing the importance of the total imploding mass and kinetic energy inside the ablation front, including the remaining ablator mass and kinetic energy, on the 1D ignition metrics. We have also explicitly included the sensitivity to ablation pressure driving the implosion, as well as the sensitivity to the coupling efficiency between the total incoming kinetic energy and the hot spot. For implosions in which the remaining ablator mass contributes to the total stagnated mass, we have developed an approach based on defining an effective implosion adiabat which incorporates the properties and the mass of the stagnated ablator as well as the cold DT fuel. We have added a dependence on total ρr to account for the fact that the time available for self-heating increases as the total ρr increases. This extends previous work in that the value of the product of stagnation pressure times burn width required for ignition depends on the total ρr as well as on the fuel ion temperature. Additionally, we show that ignition metrics derived from a requirement on the product of the hotspot ρr and the hot spot temperature are equivalent to ignition metrics derived from a requirement on the product of stagnation pressure and burn width. Further, we discuss those ignition metrics, developed as metrics for the underlying hydrodynamics in the absence of alpha heating, which can be used when alpha heating is present.

Stabilization of thin-shell implosions using a high-foot adiabat-shaped drive at the National Ignition Facility

The thin-shell adiabat-shaped implosions proposed in this paper are designed to combine the ablation front stability benefits of the High Foot (HF) pulses with the demonstrated high fuel compressibility of the low foot implosions to reach the alpha-heating regime. This is accomplished by both lowering the drive between the first and second shocks and tailoring the rise-to-peak drive. Two-dimensional radiation hydrodynamics simulations show that while weakening the growth of low-mode number perturbations at the ablation front, this approach also introduces negative lobes to the growth factor spectrum at high mode numbers. A very-high foot picketless drive, characterized by an intermediate fuel adiabat level, is proposed to suppress negative perturbation growth. Moreover, the picketless feature of this design and the shorter duration of the through reduce the hohlraum wall motion allowing us to keep the capsule implosion symmetry under control. Introducing an accurately tuned dopant fraction in the outer ablator suggests that the stabilization of the ablation front may be even further improved. This study has shown that the smaller oscillation amplitude and the frequency of ablative Richtmyer-Meshkov instability reduce the initial perturbation seed at the beginning of the acceleration phase. The combination of a thin-shell design and a very high-foot picketless radiation drive has enlightened the calculated benefits of this intermediate fuel adiabat design: high implosion performance, more predictive low-mode implosion symmetry, and a similar stability at the ablation front than that of HF designs.

X-ray streaked refraction enhanced radiography for inferring inflight density gradients in ICF capsule implosions.

In the quest for reaching ignition of deuterium-tritium (DT) fuel capsule implosions, experiments on the National Ignition Facility (NIF) have shown lower final fuel areal densities than simulated. Possible explanations for reduced compression are higher preheat that can increase the ablator-DT ice density jump and induce mix at that interface or reverberating shocks. We are hence developing x-ray Refraction Enhanced Radiography (RER) to infer the inflight density profiles in layered fuel capsule implosions. We use a 5 μm slit backlit by a Ni 7.8 keV He-α NIF laser driven x-ray source positioned at 20 mm from the capsule to cast refracted images of the inflight capsule onto a streak camera in a high magnification (M ∼ 60×) setup. Our first experiments have validated our setup that recorded a streaked x-ray fringe pattern from an undriven high density carbon (HDC) capsule consistent with ray tracing calculations at the required ∼6 μm and 25 ps resolution. Streaked RER was then applied to inflight layered HDC capsule implosions using a hydrogen-tritium fuel mix rather than DT to reduce neutron yields and associated backgrounds. The first RER of an imploding capsule revealed strong features associated with the ablation front and ice-ablator interface that are not visible in standard absorption radiographs.

Increasing stagnation pressure and thermonuclear performance of inertial confinement fusion capsules by the introduction of a high-Z dopant

Inertial confinement fusion requires the inertia of the imploding mass to provide the necessary confinement such that the core reaches adequate high density, temperature, and pressure. Experiments utilize low-Z capsules filled with hydrogenic fuel, which are subject to multiple instabilities at the interfaces during the implosion. To improve the stability of the fuel:capsule interface and narrow the imploding shell profile, capsules are doped with a small atomic percentage of a high-Z material. A series of recent indirect-drive experiments executed at the National Ignition Facility with tungsten-doped high density carbon capsules has demonstrated that the presence of this dopant serves to increase the in-flight aspect ratio of the shell and increase the compression and neutron yield performance of both gas-filled and deuterium-tritium cryogenically layered targets. These experiments definitively demonstrate that benefits accrued by the introduction of a high-Z dopant into the capsule can outweigh the detrimentally reduced stability of the ablation front, avoiding shell breakup or significant radiative cooling of the hot spot. Future experiments will utilize these types of capsules to further increase nuclear performance.

Towards a novel stellar opacity measurement scheme using stability properties of double ablation front structures

A novel method for bringing sample elements to hydrodynamic conditions relevant to the base of the solar convection zone is investigated. The method is designed in the framework of opacity measurements and exploits the temporal and spatial stability of hydrodynamic parameters in counter-propagating Double Ablation Front (DAF) structures. The physics of symmetric DAF structures is first studied in 1D geometries to assess the influence of tracer layers in the target. These results are used to propose an experimental design compatible with the OMEGA [Boehly et al., Opt. Commun. 133(1-6), 495–506 (1997)] laser. Radiative-hydrodynamic simulations conducted using the Chic code [Breil et al., Comput. Fluids 46, 161–167 (2011).] in 2D-axisymmetric geometries suggest that a Fe sample can be brought to an electron temperature of ∼160 eV and electron number density of ∼1.35 × 1023 cm−3. These parameters are reached during a 500 ps window with temporal variations of the order of 10 eV and 1022 cm−3, respectively. This allows for potential time-integrated spectral measurements. During that time, the sample is almost at local thermal equilibrium and 2D spatial gradients in the sample are less than 5% in a 360 μm diameter cylindrical volume, including the potential effects of Hot Electrons (HE) and typical uncertainties related to target fabrication and laser performances. The effects of HEs are assessed using an inline model in Chic. The HEs are found to deposit most of their energy in the cold and dense ablator between the two fronts, leading to a small efficiency loss on the DAF parameters. The calculations also suggest that negligible amounts of unabsorbed HEs are present in the probed volume, thus not affecting the atomic properties of the sample. Potential extensions of the current design to higher sample temperatures within the OMEGA capabilities are briefly discussed.

Rayleigh–Taylor instabilities in high-energy density settings on the National Ignition Facility

The Rayleigh-Taylor (RT) instability occurs at an interface between two fluids of differing density during an acceleration. These instabilities can occur in very diverse settings, from inertial confinement fusion (ICF) implosions over spatial scales of [Formula: see text] cm (10-1,000 μm) to supernova explosions at spatial scales of [Formula: see text] cm and larger. We describe experiments and techniques for reducing ("stabilizing") RT growth in high-energy density (HED) settings on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. Three unique regimes of stabilization are described: (i) at an ablation front, (ii) behind a radiative shock, and (iii) due to material strength. For comparison, we also show results from nonstabilized "classical" RT instability evolution in HED regimes on the NIF. Examples from experiments on the NIF in each regime are given. These phenomena also occur in several astrophysical scenarios and planetary science [Drake R (2005) Plasma Phys Controlled Fusion 47:B419-B440; Dahl TW, Stevenson DJ (2010) Earth Planet Sci Lett 295:177-186].

A hydrodynamic analysis of self-similar radiative ablation flows

Self-similar solutions to the compressible Euler equations with nonlinear conduction are considered as particular instances of unsteady radiative deflagration – or ‘ablation’ – waves with the goal of characterizing the actual hydrodynamic properties that such flows may present. The chosen family of solutions, corresponding to the ablation of an initially quiescent perfectly cold and homogeneous semi-infinite slab of inviscid compressible gas under the action of increasing external pressures and radiation fluxes, is well suited to the description of the early ablation of a target by gas-filled cavity X-rays in experiments of high energy density physics. These solutions are presently computed by means of a highly accurate numerical method for the radiative conduction model of a fully ionized plasma under the approximation of a non-isothermal leading shock wave. The resulting set of solutions is unique for its high fidelity description of the flows down to their finest scales and its extensive exploration of external pressure and radiative flux ranges. Two different dimensionless formulations of the equations of motion are put forth, yielding two classifications of these solutions which are used for carrying out a quantitative hydrodynamic analysis of the corresponding flows. Based on the main flow characteristic lengths and on standard characteristic numbers (Mach, Péclet, stratification and Froude numbers), this analysis points out the compressibility and inhomogeneity of the present ablative waves. This compressibility is further analysed to be too high, whether in terms of flow speed or stratification, for the low Mach number approximation, often used in hydrodynamic stability analyses of ablation fronts in inertial confinement fusion (ICF), to be relevant for describing these waves, and more specifically those with fast expansions which are of interest in ICF. Temperature stratification is also shown to induce, through the nonlinear conductivity, supersonic upstream propagation of heat-flux waves, besides a modified propagation of quasi-isothermal acoustic waves, in the flow conduction regions. This description significantly departs from the commonly admitted depiction of a quasi-isothermal conduction region where wave propagation is exclusively ascribed to isothermal acoustics and temperature fluctuations are only diffused.

Absolute Hugoniot measurements for CH foams in the 2–9 Mbar range

Absolute Hugoniot measurements for empty plastic foams at ∼10% of solid polystyrene density and supporting rad-hydro simulation results are reported. Planar foam slabs, ∼400 μm thick and ∼500 μm wide, some of which were covered with a 10 μm solid plastic ablator, were directly driven by 4 ns long Nike krypton-fluoride 248 nm wavelength laser pulses that produced strong shock waves in the foam. The shock and mass velocities in our experiments were up to 104 km/s and 84 km/s, respectively, and the shock pressures up to ∼9 Mbar. The motion of the shock and ablation fronts was recorded using side-on monochromatic x-ray imaging radiography. The steadiness of the observed shock and ablation fronts within ∼1% has been verified. The Hugoniot data inferred from our velocity measurements agree with the predictions of the SESAME and CALEOS equation-of-state models near the highest pressure ∼9 Mbar and density compression ratio ∼5. In the lower pressure range 2–5 Mbar, a lower shock density compression is observed than that predicted by the models. Possible causes for this discrepancy are discussed.

Key stages of material expansion in dielectrics upon femtosecond laser ablation revealed by double-color illumination time-resolved microscopy

The physical origin of material removal in dielectrics upon femtosecond laser pulse irradiation (800 nm, 120 fs pulse duration) has been investigated at fluences slightly above ablation threshold. Making use of a versatile pump–probe microscopy setup, the dynamics and different key stages of the ablation process in lithium niobate have been monitored. The use of two different illumination wavelengths, 400 and 800 nm, and a rigorous image analysis combined with theoretical modelling, enables drawing a clear picture of the material excitation and expansion stages. Immediately after excitation, a dense electron plasma is generated. Few picoseconds later, direct evidence of a rarefaction wave propagating into the bulk is obtained, with an estimated speed of 3650 m/s. This process marks the onset of material expansion, which is confirmed by the appearance of transient Newton rings, which dynamically change during the expansion up to approximately 1 ns. Exploring delays up to 15 ns, a second dynamic Newton ring pattern is observed, consistent with the formation of a second ablation front propagating five times slower than the first one.

Using the ROSS optical streak camera as a tool to understand laboratory experiments of laser-driven magnetized shock waves

Supersonic flows with high Mach number are ubiquitous in astrophysics. High-powered lasers also have the ability to drive high Mach number, radiating shock waves in laboratory plasmas, and recent experiments along these lines have made it possible to recreate analogs of high Mach-number astrophysical flows under controlled conditions. Streak cameras such as the Rochester optical streak system (ROSS) are particularly helpful in diagnosing such experiments, because they acquire spatially resolved measurements of the radiating gas continuously over a large time interval, making it easy to observe how any shock waves and ablation fronts present in the system evolve with time. This paper summarizes new ROSS observations of a laboratory analog of the collision of a stellar wind with an ablating planetary atmosphere embedded within a magnetosphere. We find good agreement between the observed ROSS data and numerical models obtained with the FLASH code, but only when the effects of optical depth are properly taken into account.