Laboratoire pour l’utilisation des intenses Unité Mixte de Recherche n° 7605 CNRS – CEA – Ecole Polytechnique – UPMC

LULI, Ecole Polytechnique, 91128 Palaiseau cedex, France [email protected] – www.luli.polytechnique.fr

LULI Program Committee - applications for experimental time on the LULI2000/nano2000 (salle 2) facility

* 2012/2013 *

12‐NS‐E1 lulinano2000001757

APPLICATION to access the LULI laser facilities

May 2012 – April 2013

Title of the experiment: Laboratory simulation of magnetic field generation and amplification at protogalactic shock fronts

Principal Investigator (PI): Dr Gianluca GREGORI Clarendon Laboratory, Oxford University, Oxford OX1 3PU, UK [UNIversity]

[email protected]

phone: +44 77 56 26 46 58 fax: ‐ status: EXP (experienced researcher) citizenship: IT birth date: 20/01/1970

LULI co‐PI: Alessandra RAVASIO

Summary: The generation of magnetic field at cosmological shocks associated with the large scale structure in the universe far from being understood. A number of mechanisms have been proposed in the literature to generate intergalactic magnetic seeds. These include return currents, the Biermann battery effect, and Weibel instability among others. Such sseed are expected to be amplified (particularly in the first two scenarios) and reorganized macroscopically to the observed strengths and coherence lengths by the effect of turbulence. In this experiment we will continue investigating the generation of magnetic field at shock fronts, a process initially investigated on our 2010 LULI access. In our previous access, we have demonstrated the generation of B‐fields at shock front via vorticity. Here we want to focus on the amplification of these fields by means of turbulence, a mechanism evocated to explain the observed cosmic field strengths.

Number of access weeks requested: 3 on: LULI2000/salle 2 (nano2000)

Proposed access route: LASERLAB / standard access

Do you have received any support for this proposal? Yes from: ERC Starting Grant #256973 COSMOLAB

Keywords: laboratory astrophysics, cosmic magnetic field

Co‐investigators (not including LULI researchers): 1. Francesco MINIATI, EXP, ETH Zurich, [email protected], IT 2. Antony BELL, EXP, Oxford University, [email protected], UK 3. Dmitri RYUTOV, EXP, Lawrence Livermore National Laboratory, [email protected], USA 4. Bruce REMINGTON, EXP, Lawrence Livermore National Laboratory, [email protected], USA 5. Hye‐Sook PARK, EXP, Lawrence Livermore National Laboratory, [email protected], USA Laboratory simulation of magnetic field generation and amplification at protogalactic shock fronts

SCIENTIFIC CASE

The advent of high‐power laser systems in the past two decades has opened a new field of research where astrophysical environments can be scaled down to laboratory dimensions, yet preserving the essential physics 1. This is due to the invariance of the equations of ideal magneto‐hydrodynamics MHD) to a class of self‐similar transformations 2. Through such transformations, laboratory experiments can be utilised to study the origins of magnetic fields in the early universe. Although at the beginning of cosmic evolution matter was nearly homogenously distributed, today, as a result of gravitational instability, it forms a web‐like structure made of filaments and clusters. Gas continues to accrete supersonically onto these collapsed structures, thus producing high Mach number shocks 3. Indeed, large scale magnetic fields (with strengths from a few nG to a few μG) have been found in galaxy clusters and filaments as revealed by either Mpc wide diffuse radio‐synchrotron emission there, or Faraday Rotation Measure (RM) experiments 4. Very recent studies have also suggested the existence of magnetic fields in voids 5 with intensity ~0.01‐1 fG. The presence of magnetic fields in cosmological plasmas requires a mechanism for their generation. Various scenarios have been proposed for this purpose, including Weibel’s instability at shocks 6, Biermann’s mechanism at cosmic shocks 7, return currents due to galactic cosmic rays 8, as well as photon drag during reionization 9. In this experiment we will continue investigating the generation of magnetic field at shock fronts, a process initially investigated on our 2010 LULI access 10. In our previous access, we have demonstrated the generation of B‐fields at shock front via vorticity. Here we want to focus on the amplification of these fields by means of turbulence, a mechanism evocated to explain the observed cosmic field strengths.

EXPERIMENTAL METHODS

Either one or two frequency‐doubled (527 nm), 1.5 ns‐long laser beams were focused on the tip of a 500 μm diameter carbon rod (see figure 1), achieving a peak intensity of 2 1014 W/cm2. The interaction chamber was filled with helium gas at pressure p~1‐10 mbar. As energy is impulsively deposited, the sample initially undergoes a ballistic expansion until the shocked mass is roughly equal to the ejected mass and then the shock transitions to a Sedov‐Taylor blast wave 11. The measurement of the magnetic field was performed with 3‐axis magnetic induction coils 12, giving both the magnetic field components along the shock normal and perpendicular to it. In addition, several optical diagnostics where fielded to monitor the shock properties in time. In the experiment, we have attributed the generation of magnetic field by the vorticity induced by the non‐radial expansion (see figure 1). To corroborate our interpretation we carried out a numerical experiment in which we solve the resistive MHD equations in 2‐dimensions with a baroclinic source term for the magnetic field, and for the initial conditions we use the results from a 1D radiation‐hydrodynamics code to account for non‐uniform laser deposition. Indeed, the MHD simulation shows good agreement with the measured magnetic fields. Moreover, by applying the scaling relation between the laboratory and the astrophysical systems, we were able to show that curved intergalactic shocks in protogalactic structures, with changing curvature at the level of a few tens of ta percen on scales ~1 Mpc can generate magnetic field with values ~10‐21 G. This confirms for the first time in a direct experimental setting, the numerical estimates of Kulsrud et al. 13. While these seed fields are relatively large, their value still remains significantly below what is observed in galaxy clusters. It is expected that such seeds will be amplified and reorganized macroscopically to the observed strengths and coherence lengths by the effect of turbulence 14.

1 B.A. Remington et al., Rev. Mod. Phys. 78, 755 (2006) 2 D.D. Ryutov et al., Astrophys. J. 518, 821 (1999); D.D. Ryutov et al., Astrophys. J. 127, S465 (2000) 3 F. Miniati et al., Astrophys. J. 542, 608 (2000) 4 C.L. Carilli and G.B. Taylor, Annu. Rev. . Astrophys. 40, 319 (2002); E. Clarke et al., Astrophys. J. 547, L111 (2001) 5 A. Neronov and I. Vovk, Science 328, 73 (2010) 6 R. Schlickeiser and P.K. Shukla, Astrophys. J 599, L60 (2003); M.V. Medvedev et al., JKAS 37, 533 (2004) 7 R.M. Kulsrud et al., Astrophys. J. 480, 481 (1997); N.Y. Gnedin et al., Astrophys. J. 539, 505 (2000) 8 F. Miniati and A. R. Bell, Astrophys. J 729, 73 (2011) 9 M. Langer et al., Astron. Astrophys. 443, 367 (2005) 10 G. Gregori et al., submitted to Nature 11 e.g. J.F. Hansen et al., Astrophys. Space Sc. 298, 61 (2005); J.F. Hansen et al., Phys. Plasmas 13, 022105 (2006) 12 E. Everson et al., Rev. Sci. Instrum. 80, 113505 (2009) 13 R.M. Kulsrud et al., Astrophys. J, 480, 481 (1997) 14 E.N. Parker, Astrophys. J. 122, 293 (1955); D. Ryu et al., Science 320, 909 (2008)

Figure 1: (left) Schlieren image of the asymmetric shock wave; (center) measured magnetic field values; (right) results from the resistive MHD simulation and inferred magnetic field.

In the proposed experiment we aim at investigating the amplification of baroclinic seeds via turbulence. We envision this will be possible by placing an no ‐conductive grid array in front of the shock 15, which simulates an in‐homogenous intergalactic medium at a specific scale, and then measure the corresponding magnetic field. Comparisons with a without the presence of the grid, as well as different grid sizes will allow us to fully characterize the effect of turbulence in amplifying the magnetic field.

Figure 2: schematic of the proposed experimental set‐up; optical diagnostics are used to measure the shock properties while induction coils and electron radiography are employed for B‐field measurements; a grid array in front of the shock will create turbulence for field amplification.

Diagnostics Two sets of diagnostics will be implemented in order to both characterize the shock wave and measure the magnetic field strength and dynamics. A first set of optical diagnostics will include optical interferometry, shadowgraphy, Thomson scattering as well as time resolve self‐emission. In this way, we access to a large ensemble of parameters (electron density, electron and ion temperature, morphology, propagation velocity, etc) which well characterizes our system, as attested in our previous experiment 10. Three‐axe compact magnetic induction coils will be used to obtain the magnetic field time evolution at a given position. Moreover, an electron gun was recently developed at LULI. A frequency tripled (or quadrupled) laser beam is focused over a photocathode; and the generated photoelectrons are extracted though a gridd an accelerated to Ep =40‐60 kV. These low energy electrons can be easily deflected by the generated magnetic

15 G. Comte‐Bellot and S. Corrsin, J. Fluid Mech. 25, 657 (1966) fields, thus providing an excellent method to image the spatial correlations of the field distribution – using a technique already applied in the laser community with MeV protons 16. Finally – Faraday rotation in the THz regime will be used to get insight on field structure. We have recently purchase a quantum cascade laser at ~1 THz to perform Faraday rotation experiments in ~1‐10 mbar pressures where long wavelengths are required in order to have enough rotation of the polarization.

SHOT PLAN

15 2 Shock generation : nano2000, 2 beams at 1 kJ, 2 ns, double frequency, Ilas ~ 10 W/cm Electron gun drive: UV mJ/ns beam, third harmonic, with variable delay at t=0‐100 ns TS beam: probe (1ω or 2ω) J/ns beam, second harmonic, with variable delay at t=0‐100 ns Faraday Rotation: THz laser supplied by the Oxford University Bdot coils: provided by the Oxford University

EXPERTISE OF THE EXPERIMENTAL CONSORTIUM

Our team has an impressive and internationally leading track‐record in the exploitation of laser‐‐based research in laboratory astrophysics, simulations of magnetic fields dynamics as well as cosmological simulations of the large scale structures in the Universe. The work the PI and co‐PI's has resulted in numerous high‐quality publications. The research proposed has a large scientific impact and could lead to the understanding of fundamental processes relevant for a large range of issues spanning from the evolution of structure in the Universe to high‐energy astrophysics. These will have a broad impact on the general public well beyond the laser‐plasma community.

Experimental team: G. Gregori, J. Meinecke, C. Murphy, A. Ravasio, A. Pelka, R. Yurchak A. Benuzzi‐Mounaix, J.R. Marques, M. Koenig, H‐S. Park Theoretical support: R. Bell, F. Miniati, D. Ryutov, B. Remington Numerical support: M. Fatenejad, D. Lamb

PREVIOUS ACCESS

10‐NS‐E1 (August/September 2010; PI: G. Gregori): Generation of cosmological magnetic fields in laboratory experiments 10 11‐NS‐E1 (November 2011; PI: G. Gregori): Magnetic field generation by return currents and their non‐resonant amplification in cosmological environments

16 L. Romagnani et al., Phys. Rev. Lett. 101, 025004 (2008) 12‐NS‐F1

APPLICATION to access the LULI laser facilities

May 2012 – April 2013

Title of the experiment: Detailed study of corona plasma flow in the context of collisionless experiments on NIF

Principal Investigator (PI): Dr Tommaso VINCI LULI [public RESearch organisation]

[email protected]

phone: +33 1 69 33 54 27 fax: +33 1 69 33 54 82 status: EXP (experienced researcher) citizenship: IT birth date: 13/12/1977

LULI co‐PI: ‐

Summary: A set of experiments on Omega, Omega EP followed by a NIF one is scheduled in the next few years within the context of collisionless shock in astrophysics. These experiments gather a large set of institutions (LLNL, LLE, ILE, LULI, U. Oxford, U. Michigan, U. Zurich …) and scientists (more than 30) in a group named ACSEL (Astrophysical Collisionless Shock Experiments with Lasers). The main objective of this proposal on the LULI2000 laser is therefore to investigate in the most complete way the plasma flow generated by a high energy and intensity (up to 2‐3 1015 W/cm2 laser to benchmark radiative‐hydrodynamic codes (including Equation of state, opacities and radiation transport). The key point of this experiment is to cross check relevant parameters such as temperature, electron and ion densities using many different diagnostics. In summary the parameters we will measure are: (i) time‐resolved electron density, (ii) 2D shape, (iii) expansion velocity, (iv) ion and electron temperature both of the corona and the low density plume, (v) pressure achieved and (vi) laser energy deposited on target. These experiments at LULI will dramatically expand our understanding of the complex plasma physics involved in the corona expansion, and will have a significant impact on the design of the future experiments on Omega and NIF.

Number of access weeks requested: 3 on: LULI2000/salle 2 (nano2000)

Note: This experiment shares most of the diagnostics with the experiment “Time‐resolved and time‐integrated L‐shell of well‐characterized mid‐Z plasmas”. If both experiments are allowed, the first week of setup could be shared.

Proposed access route: standard access

Do you have received any support for this proposal? No

Keywords: laser‐plasma interaction, collisionless shock

Co‐investigators (not including LULI researchers): 1. Gianluca GREGORI, EXP, Oxford University, [email protected], IT 2. J. MEINECKE, PhD, Oxford University, USA 3. C. MURPHY, PDOC, Oxford University, UK 4. B. REVILLE, PDOC, Oxford University, Ireland 5. Youichi SAKAWA, EXP, ILE, sakawa‐[email protected]‐u.ac.jp, JP 6. Yasuhiro KURAMITSU, EXP, ILE, kuramitsu‐[email protected]‐u.ac.jp, JP 7. T. MORITA, PDOC, ILE, morita‐[email protected]‐u.ac.jp, JP 8. Paul DRAKE, EXP, University of Michigan, [email protected], USA Detailed study of corona plasma flow in the context of collisionless experiments on NIF

SCIENTIFIC CASE

A set of experiments on OMEGA, OMEGA EP followed by a NIF one is scheduled in the next few years within the context of collisionless shock in astrophysics. The OMEGA experiments are focusing on both collisionless shock interactions in non‐magnetized mediums (PI: H.‐S. Park) and pre ‐magnetized ambient plasmas (PI: A. Spitkovski). The NIF experiments (PI: Gregori/Sakawa) concern with the self‐generation of magnetic fields driven by the Weibel instability 1. These experiments gather a large set of institutions (LLNL, LLE, ILE, LULI, U. Oxford, U. Michigan, U. Zurich …) and scientists (more than 30) in a group named ACSEL (Astrophysical Collisionless Shock Experiments with Lasers). These experiments propose to study the dynamics of high velocity plasma flows relevant to astrophysical collisionless shocks. There are many natural observations of collisionless shocks or the effects they cause, such as acceleration of cosmic rays, supernova remnant shocks, shocks produced by the solar winds, and bow shocks produced by protostellar jets. A collisionless shock occurs when the shock transition occurs on a length scale much smaller than a (Coulomb) collisional mean free path length 2. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless” and form, due to plasma instabilities, and self‐generated magnetic fields. Laser driven experiments at OMEGA and EP are ideal for creating such an extreme environment of ~1000 km/s plasmas with ion density of ~1018 ‐ 1019 cm‐3 in a controlled fashion. Through scaling relations 3, these parameters can be related to astrophysical environments. There have been initial attempts on smaller‐scale laser facilities to look for collisionless shocks 4. These collisionless shock conditions are supposed to be generated by the collision of two counterpropagating high‐velocity plasma flows resulting from laser of solid targets. Some data were already obtained on Omega facility but have shown discrepancies between 2D simulations of the plasma flow and experimental results (figure 1).

Figure 1: comparison of the evolution of the electron density (left) and temperature (right) given by DUED for different intensities and OMEGA data (dots).

The main objective of this proposal on the LULI2000 laser is therefore to investigate in the most complete way the plasma flow generated by a high energy and intensity (up to 2‐3 1015 W/cm2) laser to benchmark radiative‐hydrodynamic codes (including equation of state, opacities and radiation transport). This will be done on the present material used on OMEGA experiments (CH) but also following future tests on C (diamond) that will be performed next FY on OMEGA. Indeed due to observed discrepancies, there is a crucial need to solve the issues of in order to get the final design of the NIF experiment. We would like to stress that the key point of this experiment is to cross check relevant parameters such as temperature, electron and ion densities using many different diagnostics. This will be done within the ACSEL group. These experiments at LULI will dramatically expand our understanding of the complex plasma physics involved in the corona expansion, and will have a significant impact on the design of the future experiments on OMEGA and NIF.

1 T.N. Kato and H. Takabe, Astophys. J. Lett., 681, L93 (2008) 2 R.Z. Sagdeev and C.F. Kennel, Sci. Am. 262, 106 (1991); C.F. McKee and B.T. Draine, Science, 252, 397 (1991); S.S. Moiseev and R.Z. Sagdeev, Plasma Phys., 5, 43 (1963) 3 D.D. Ryutov et al., Astrophys. J., 518, 821 (1999); D.D. Ryutov et al., Astrophys. J., 127, S465 (2000); E. Falize et al., Astrophys. J. 730, 96 (2011) 4 H. Takabe et al., Plasma Phys. Control. Fusion, 50, 124057 (2008); W.D. Zheng et al., Chinese J. Comput. Phys. 25, 36 (2008); A.R. Bell et al., Phys. Rev. A 38, 1363 (1988); T. Morita et al., Phys. Plasmas, 17, 122702 (2010); Y. Kuramitsu et al., Phys. Rev. Lett., 106, 175002 (2011) EXPERIMENTAL SETUP

The experiment will focus the two ns beams of the onto a plastic foil thus creating a classic expansion plume as shown on the picture below.

During the laser interaction with the target, a first important measure of the Laser Parametrical Instabilities (LPI) will be performed in order to carefully measure the deposed energy. A transverse interferometer coupled to three GOI (Gated Optical Imager) will give us the 2D map of the electron density up to a 1/10 of the critical density (~1020 cm‐3). The same interferometer coupled to a streak camera will also give us the axial velocities of the plasma. At the same time the Thomson scattering spectrometer will diagnose the plasma measuring the electron and ion temperatures near the critical density. Meanwhile, a time resolved x‐ray spectrometer will give us the temperature of the plasma in the over critical part of the plume (in this case a Al‐doped plastic will be used as tracer). On the rear side of the foil a classical EOS combined (2 VISARs + SOP) cameras will measure the achieved pressure inside the material. In summary the parameters we will measure are: (i) time resolved electron density, (ii) 2D shape, (iii) expansion velocity, (iv) ion and electron temperature, (v) pressure achieved and (vi) laser energy deposited on target. The figure below shows a 2D simulation of the plasma expansion into vacuum. We present both electron (left) and ion (right) temperatures as well as the isocontour line (green) at 1019 cm‐3 that is the region that will be particularly studied by the different diagnostics. The four snapshots correspond to times of (1, 2, 3 and 4 ns).

EXPERTISE OF THE EXPERIMENTAL CONSORTIUM

The group is formed by several experimental (and experimented!) scientists and is backed up by advanced users of radiative hydrodynamical codes: ‐ S. Baton: laser parametric instabilities ‐ J.‐R. Marqués: Thomson scattering diagnostics ‐ A. Ravasio: EOS diagnostics (VISAR + SOP) ‐ T. Vinci: transverse interferometry and code benchmark ‐ S. Bastiani : x‐ray diagnostics ‐ ILE group (Y. Sakawa, Y.Kuramitsu, T. Morita): transverse interferometry, self‐emission and code benchmark ‐ CRASH group (U. Michigan: R.P.Drake): modeling and participation of some students to the experiments ‐ LLNL group (H.‐S. Park): connection of the results to the OMEGA and NIF experiments. ‐ U. Oxford (G. Gregori, B. Reville): Thomson scattering and magnetic field probing + complementary large‐scale high‐resolution 3D MHD simulations using massively parallelized codes CHARM (hybrid PIC/AMR with Chombo framework) and FLASH (which includes multigroup radiation transport, non‐ideal EOS and laser ray tracing) + experimental support for gated imaging Thomson scattering (J. Meinecke) and Faraday rotation measurement (C. Murphy)

SHOT PLAN

Since the target and laser requirements are standard for the laser team, the shot plan will be driven by the experimental setup. This means that most of the shots of the first week will be dedicated to tune and set‐up all the diagnostics (this means that the schedule will be driven by the experimental problems). The success of this experiment is strongly dependent by the simultaneous measurement of all the relevant parameters. The following two weeks will be used to shot with both beams (full energy) 6 shots per day for a total of 60 shots on target. These shots will be used to probe the plume at different distances from the target and at different times.

CONCLUSIONS

In conclusion we have proposed here an experiment to investigate the laser plume expansion oint vacuum. This experiment combines several diagnostics and several groups of scientists at LULI to perform the best possible measurements both on the laser interaction and on the plasma evolution. The experiments will be supplemented by dedicated plasma simulation work with radiation‐hydrodynamics codes. The validation of the commonly used numerical codes (EOS + opacities) of laser‐matter interaction, will be a necessary test‐bed for the dimensioning of the future experiments on collisionless shock that will be carried out on OMEGA and NIF lasers.

12‐NS‐F2

APPLICATION to access the LULI laser facilities

May 2012 – April 2013

Title of the experiment: Time‐resolved and time‐integrated L‐shell spectroscopy of well‐characterized mid‐Z plasmas

Principal Investigator (PI): Dr Serena BASTIANI‐CECCOTTI LULI [public RESearch organisation]

[email protected]

phone: +33 1 69 33 54 04 fax: +33 1 69 33 54 82 status: EXP citizenship: IT birth date: 27/07/1967

LULI co‐PI: ‐

Summary: We propose an experimental investigation, coupled with calculations and simulations, of the time‐resolved L‐shell emission from mid‐Z plasmas in NLTE conditions. There is a double interest for this kind of measurement. On one side, NLTE emission benchmark experiments are notoriously rare. However, they are the only way to test the atomic physics codes under development. On the other side, previous experiments have pointed out the sensibility of the spectral structures obtained from plasmas produced by different laser pump duration. The results of the proposed experiment will be compared with those issued from a similar experiment performed on the femtosecond ECLIPSE laser facility in Bordeaux, allowing to get a deeper insight on the processes giving rise to the spectral line shapes in this two very different regimes. As the aim of the experiment is to furnish benchmark data, great care will be devoted to the measurement of the hydrodynamic parameters of the plasma by independent diagnostics, namely time‐resolved Thomson scattering, SOP, and VISAR. The plasma will be obtained by irradiating a solid target with both the ns‐beams of the LULI2000 laser. Bromine microdot targets smaller than the laser focal spot will insure a moderate lateral expansion of the Br plasma, permitting to obtain a relatively homogeneous plasma. The Thomson scattering, SOP, and VISAR experimental data will be compared with hydrodynamic 1‐D or 1.5‐D simulations, to fix the interaction parameters. Finally, the time‐resolved spectra measured will be compared with NLTE atomic codes, like STAPEC, AVERROES/TRANSPEC and the commercial code PrismSPECT used as post‐processors of the hydrocodes constrained by the hydro measurements.

Number of access weeks requested: 3 on: LULI2000/salle 2 (nano2000)

Note: This experiment shares most of the diagnostics with the experiment “Detailed study of corona plasma flow in the context of collisionless experiments on NIF”. If both experiments are allowed, the first week of setup could be shared.

Proposed access route: pluri‐annual access (2nd year)

Do you have received any support for this proposal? No

Keywords: NLTE plasma, L‐shell spectroscopy

Co‐investigators (not including LULI researchers): 1. Fabien DORCHIES, EXP, CELIA, [email protected]‐bordeaux1.fr, FR 2. Pierre‐Marie LEGUAY, PHD, CELIA, [email protected]‐bordeaux1.fr 3. Frank GILLERON, EXP, CEA/DIF, [email protected], FR 4. Robin MARJORIBANKS, EXP, Departement of Physics, University of Toronto, [email protected], Canada 5. Christopher Bowen, EXP, CEA/DIF, [email protected],FR 6. Virginie Silvert, EXP, CEA/DIF, [email protected], FR 7. Olivier Peyrusse, EXP, CELIA, [email protected]‐bordeaux1.fr, FR 8. Patrick Renaudin, EXP, CEA/DIF, [email protected], FR 9. Hyun Chung, EXP, IAEA, [email protected], Korea Time‐resolved and time‐integrated L‐shell spectroscopy of well‐characterized mid‐Z plasmas

SCIENTIFIC CASE

Preamble This proposal is intended to be the 2nd step of the “pluriannual access” granted last year to our team. The first experiment has been performed in June 2011. It has permitted to collect a good number of Tungsten and x‐ray emission data, as well as very good Thomson scattering results thanks to the new geometry used. We have been able to simultaneously collect data from all the diagnostics (2 spectrometers, Thomson scattering and SOP) on several shots. This will allow us to study both the hydrodynamics and the M‐shell emission from coronal equilibrium plasmas. Accordingly to our program, we propose this year to move towards denser plasmas created on mid‐Z elements, and to study the L‐shell emission. The suitable hydrodynamic conditions will be some 100’s eV (Te) 21 ‐3 and 10 cm (ne). We plan to set‐up basically the same kind of experiment than the one performed in 2011, adding one more hydro diagnostic, namely a VISAR, thus improving the measurement of the hydrodynamic parameters of the plasma.

Scientific objectives The evolution of a non‐local thermodynamic equilibrium (NLTE) plasma is very complex, as many different and competing factors can cause departures from LTE. Even in a plasma with high densities, where collisional rates are much larger than radiative rates, radiation fields that are not at the local temperature can drive the plasma out of LTE. Thus, for laboratory plasmas, NLTE physics can be very hard to avoid and very hard to simulate. Unlike the modeling of LTE plasmas, where the Saha‐Boltzmann equation relies on the ionization and the temperature to the density, the challenge of modeling NLTE plasmas lays ein th determination of the electronic density and temperature, as no unique correspondence between these two quantities exists. The goal of this proposal is to measure NLTE plasma emission under well‐characterized plasma conditions. Benchmarking plasma simulations is difficult, but necessary. The ongoing international NLTE Kinetic Code Comparison workshops 1 have shown over the years systematic disagreements between the predictions of the codes even for relatively simple K‐shell test cases. As the Z of the element and the complexity of the atomic model increase, the difficulties also increase. For example, a spectroscopically accurate description of the Ti L‐shell ion requires more than 104 levels. However, the representation of this ionic species within a radiation‐hydrodynamic code requires a reduction of the number of levels to around 100 (through, for example, effective rates or level grouping). Such approximations must be experimentally validated. In this context, we stress that the international community is more and more interested by our benchmarking experimental work: the experimental results on Kr emission that we have obtained on the LULI2000 facility in 2005 will be used as a case test in the next international NLTE Kinetic Code Comparison workshop, to be held December 5‐9 2011, in Vienna, Austria. But this experiment has also an additional aim. Since several years, it has been noticed the effect of the hot electrons created during the interaction of sub‐picosecond, high‐intensity lasers with solid targets on the x‐ray radiative properties of the produced plasmas 2. These studies have demonstrated that hot electrons increase the ionization balance and the intensity of inner‐shell satellite lines (formed through direct collisional excitation), leaving characteristic signatures on the emission spectra. Nevertheless, to our knowledge, any of the previous experiment has measured the 2p‐3d transitions in near Ne‐like ions. We will perform in January 2012 an experiment on the ECLIPSE short‐duration, high‐intensity laser facility, in the CELIA laboratory (Bordeaux). In that experiment we will measure the time‐resolved L‐shell emission from Bromine and Arsenic plasmas. The irradiation conditions (45 fs – 1ps laser pulse duration, 1018 W/cm2) will lead to the generation of highly transient, non‐Maxwellian plasmas, and to the production of a hot‐electrons population. The comparison between these spectra and those recorded in this proposed experiment will allow us a deeper insight of the hot‐electron effects on the spectral line shapes.

Experimental methods and diagnostics In this experiment, a plastic foil having a KBr microdot of 200 μm in diameter will be irradiated by the two ns beams of the Nd:glass LULI laser with a pulse duration of 1.5 ns, wavelength of 0.53 μm, and a flux in the range of few 1014 W/cm2. Expected density and temperature calculated using the hydrocode CHIVAS with the experimental parameters are shown in figure 1. The focal spot is at least 400 μm in diameter so that the

1 C. Bowen, R.W. Lee, and Yu. Ralchenko, J. Quant. Spectr. Rad. Transfer 99, 102 (2006); J.G. Rubiano et al., High Energy Density Physics 3, 225 (2007); C.J. Fontes et al., High Energy Density Physics 5, 15 (2009) 2 K.B. Fournier et al., Phys. Rev. E 67, 016402 (2003); S.B. Hansen and A.S. Shlyaptseva, Phys. Rev. E 70, 036402 (2004); S. Bastiani‐Ceccotti et al., High Energy Density Physics 6, 99 (2010) microdot is overfilled, producing a surrounding plastic plasma which partially limits its lateral expansion, thus reducing the lateral gradients. A HPP (Hybrid Phase Plate) will be used to produce a more homogeneous laser irradiation and obtain a flattop intensity profile in the focal spot.

Figure 1: electronic temperature and density of a Bromine plasma calculated by the 1.5‐D hydrodynamic code CHIVAS; the irradiation parameters were: laser intensity 1014 W/cm2, laser duration 1.5 ns, laser wavelength 0.537 μm.

Two conical crystal spectrometers, one coupled to a x‐ray streak camera and the other coupled to an Imaging Plate, will allow the measurement of the time‐resolved and the time‐integrated x‐ray emission in the 7‐8 Å range, covering the 2p‐3d transitions of Br Ne‐like ions. The secondary “Blue” beam, at 1.06 μm, will permit the setup of the time‐resolved electronic and ionic Thomson scattering diagnostics. The TS beam will be delayed by about 0.5 ‐ 1 ns from the main heating beams. This will allow the plasma to reach a relatively homogeneous temperature regime. From the CHIVAS simulations, the expected hydrodynamic parameters are of the order of 300 eV and 1021 cm‐3, at 1 ns after the laser peak and 500 μm in front of the target. In the experiment, the plasma size seen by the spectrometers is of the order of 200 μm, thus limiting the contributions coming from other plasma regions. A Br spectrum calculated by the commercial, atomic physic code PrismSPECT in NLTE plasma conditions and for the plasma parameters deduced by the CHIVAS simulation is illustrated in figure 2. A key point of this kind of experiments, which aim to be a benchmark experiment for atomic physic codes, is to have an independent measurement of the hydrodynamic parameters of the plasma, in order to constrain the hydro calculations and to minimize the free parameters in the atomic physic calculations. Thus, as in the previous experiment, we will implement time‐resolved electronic and ionic Thomson scattering diagnostics, that will allow us to have an independent measurement of the hydrodynamic parameters eof th plasma (electron density and temperature). Since several years our team has brought attention to this thematic, and several development have been introduced to optimize these complementary diagnostic. In particular, a top‐view Thomson scattering diagnostic has been very recently successfully used. Besides the spectroscopic and Thomson scattering diagnostics, the plasma will also be characterized by measuring the shock propagation by time‐resolved Self Optical Pyrometry (SOP) and, for the first time, the shock speed by a VISAR interferometer, which will allow to constrain the subsequent simulations of target hydrodynamics. Figure 3 indicates the geometry of the irradiation and of the different diagnostics. This geometry has already been successfully used in the “Salle 2” experimental room of the LULI2000 facility in our previous experiment.

Figure 2: PrismSPECT calculation of NLTE Br spectrum at 300 eV and 1021 cm‐3.

Figure 3: schematic diagram of the experimental arrangement.

For comparison with theoretical ionization data, the time‐resolved and time‐integrated emission spectra, and the evolution of the average charge Z* of the plasma will be compared to STAPEC 3, PrismSPECT 4, and AVERROES/TRANSPEC 5 calculations, used as a post‐processor of the hydrodynamic code FCI2 (CEA/DIF), constrained by the hydro measurements.

SHOT PLAN

The experiment requires three weeks: one week to setup the Thomson diagnostic, the VISAR, and to synchronize the Thomson scattering streak cameras. Half a week will be necessary to calibrate they x‐ra spectrometer, and to synchronize its streak camera. One week and a half will be necessary to record the experimental data (about 30 shots). This experiment shares most of the diagnostics with the experiment “Detailed study of corona plasma flow in the context of collisionless experiments on NIF”. If both experiments are allowed, the first week of setup could be shared.

Laser beams 2 LP beams at 2ω, pulse duration of 1.5 ns and maximum energy to create the adequate plasma and 2 Hybrid Phase Plates (HPP) to obtain a flattop intensity profile in the focal spot Blue beam, to perform the Thomson scattering measurements Quantaray laser for the VISAR diagnostic

EXPERTISE

The LULI team (S. Bastiani‐Ceccotti, J.‐R. Marquès, P. Audebert), the CELIA team and R. Marjoribanks have a wide experience on experimental plasma spectroscopy and on the Thomson scattering technique. T. Vinci has a large experience on EOS diagnostics. H. Chung, O. Peyrusse, F. Gilleron and C. Bowen have a wide experience in NLTE atomic physics calculations. C. Bowen has also an expertise on large‐scale hydro‐simulation and is one of organizer of the NLTE kinetic workshops series. V. Silvert is a specialist of the FCI2 hydrocode.

References on our expertise on NLTE modeling C.J. Fontes et al., Review of the NLTE‐5 kinetics workshop, High Energy Density Physics 5, 15 (2009); J.G. Rubiano et al., Review of the 4th NLTE Code Comparison Workshop, High Energy Density Physics 3, 225 (2007); C. Bowen et al., Comparing plasma population kinetics codes: Review of Ethe NLT ‐3 Kinetics Workshop, J. Quant. Spectr. Rad. Transfer 99, 102 (2006); O. Peyrusse et al., Calculation of the charge state distribution of a highly ionized coronal Au plasma, J. Phys. B 38, L137 (2005); H.‐K. Chung et al., FLYCHK: Generalized population kinetics and spectral model for rapid spectroscopic analysis for all elements, High Energy Density Physics 1, 3 (2005; C. Bowen et al., Gold emissivities for hydrocode applications, Phys. Plasmas 11, 4641 (2004); C. Bowen and P. Kaiser, Dielectronic recombination in Au ionisation temperature calculations, J. Quant. Spectr. Rad.

3 H.‐K. Chung et al., HEDP 1, 3 (2005) 4 J.J. MacFarlane et al., IFSA2003; http://www.prism‐cs.com/Software/PrismSpect/PrismSPECT.htm 5 O. Peyrusse, J. Phys. B 33, 4303 (2000) ; O. Peyrusse, J. Quant. Spectr. Rad. Transfer 71, 571 (2001) ; O. Peyrusse, C. Bauche‐Arnoult and J. Bauche, J. Phys. B 38, L137 (2005) Transfer 81, 85 (2003); O. Peyrusse, On the superconfiguration approach to model NLTE plasma emission, J. Quant. Spectr. Rad. Transfer 71, 571 (2001); O. Peyrusse, A superconfiguration model for broadband spectroscopy of non‐LTE plasmas, J. Phys. B 33, 4303 (2000)

References on our experimental expertise S. Bastiani‐Ceccotti et al., Temporal and spectral behavior of sub‐picosecond laser‐created x‐ray sources from low‐ to moderate‐Z elements, High Energy Density Physics 6, 99 (2010); A. Levy et al., Double conical crystal x‐ray spectrometer for high resolution ultrafast x‐ray absorption near‐edge spectroscopy of Al K edge, Rev. Sci. Instrum. 81, 063107 (2010); G. Loisel et al., Absorption spectroscopy of mid and neighboring Z plasmas : iron, , and germanium, High Energy Density Physics 5, 173 (2009); M. Harmand et al., Broad M‐band x‐ray emission from created by short laser pulses, Phys. Plasmas 16, 063301 (2009); S. Bastiani‐Ceccotti et al., Analysis of the x‐ray and time‐resolved XUV emission of laser produced Xe and Kr plasmas, High Energy Density Physics 3, 20 (2007); C. Bonté et al., High dynamic range streak camera for subpicosecond time‐resolved x‐ray spectroscopy, Rev. Sci. Instrum. 78, 043503 (2007) ; C. Chenais‐Popovics et al., X‐ray emission of a xenon gas jet plasma diagnosed with Thomson scattering, Phys. Rev. E 65, 046418 (2002)

PREVIOUS ACCESS

In June 2011 we have performed the first experiment of this pluriannual project. We have collected about 20 experimental shots with all the diagnostic simultaneously working: x‐ray spectroscopy, electronic and ionic Thomson scattering and SOP. We have used dot targets of Tungsten and Niobium, on a three‐layer substrate of CH/Al/CH. We have obtained good quality W and Nb M‐spectra, both time‐resolved and time‐integrated. The Thomson scattering spectra have a very good signal‐to‐noise ratio, thanks to the top‐view geometry chosen. The SOP data are also of good quality. We are currently completing the analysis of all these data. The next step will be to perform hydro simulations with the FCI2 code, constrained by the SOP and Thomson measurements, and use the AVERROES/TRANSPEC package as a post‐processor to reproduce the measured spectra.

12‐NS‐F3

APPLICATION to access the LULI laser facilities

May 2012 – April 2013

Title of the experiment: Crystallization of liquid tin under isentropic compression

Principal Investigator (PI): Dr Thierry JALINAUD CEA/DIF [public RESearch organisation]

[email protected]

phone: +33 1 69 26 74 15 fax: +33 1 69 26 70 57 status: EXP (experienced researcher) citizenship: FR birth date: 15/08/1964

LULI co‐PI: Erik BRAMBRINK

Summary: The purpose of this experiment is to validate the kinetic model of the phase transition (liquid‐solid) introduced in the hydrodynamic code ESTHER. According to this, the time variation of the solid mass fraction in the material is directly proportional to the Gibbs energies difference between the both phases (solid and liquid) and inversely proportional to a characteristic time of evolution. In the case of tin, theses characteristic times for the melting and crystallization transitions are taken to 50ps and 1.5ns. In the case of a mixture phases (solid and liquid), we have to involve an equation of state for each phase of the mixture in order to determine temperature and pressure of the mixture from the internal energy and the specific volume. These equations of state and specific algorithms have been introduced into the hydrodynamic code ESTHER. This work was publishing in an internal report. The objective of this experiment is to observe the dynamic of the liquid tin crystallization under isentropic compression. In this experiment, tin crystallizes under the isentropic compression and become liquid again when the relaxation wave fallows. In the result from the thermal equilibrium model, the mass fraction is beginning to change from the beginning of the isentropic compression on the front of the tin (t = 5 ns). In the case of kinetic calculation, this development does not start before 13 ns, when the difference between the Gibbs energies of the two phases becomes important. The both side (laser side and VISAR side) of the liquid tin has become solid, but it is significantly different inside tin target: full crystallization for the thermal equilibrium calculation and a mixture in the kinetic model. An x‐ray diffraction analysis should be able to discriminate the two results and to assess, by comparison with numerical simulations, the value of the characteristic time of crystallization.

Number of access weeks requested: 1 on: LULI2000/salle 2 (nano2000)

Proposed access route: standard access

Do you have received any support for this proposal? No

Keywords: Warm Dense Matter, VISAR, isentropic compression, x‐ray diffraction

Co‐investigators (not including LULI researchers): 1. Florent OCCELLI, EXP, CEA/DIF, [email protected], FR 2. Laurent BERTHE, EXP, LPIMM, [email protected], FR 3. Charles REVERDIN, EXP, CEA/DIF, [email protected], FR

Crystallization of liquid tin under isentropic compression

SCIENTIFIC CASE

Context and objectives The purpose of this experiment is to validate the kinetic model of the phase transition (liquid‐solid), from the D. B. Hayes model 1, introduced in the hydrodynamic code ESTHER. According to this, the time variation of the solid mass fraction in the material is directly proportional to the Gibbs energies difference between the both phases (solid and liquid) and inversely proportional to a characteristic time of evolution. In the case of tin, theses characteristic times for the melting and crystallization transitions are taken to 50ps and 1.5ns. In the case of a mixture phases (solid and liquid), we have to involve an equation of state for each phase of the mixture in order to determine temperature and pressure of the mixture from the internal energy and the specific volume. These equations of state and specific algorithms have been introduced into the hydrodynamic code ESTHER. This work was publishing in an internal report 2. The objective of this experiment is to observe the dynamic of the liquid tin crystallization under isentropic compression.

Hydrodynamic models The hydrodynamic results show a crystallization delay between a local thermal and pressure equilibrium result and a kinetic calculation. The two next results are obtained with PS1 and 2µm carbon (North Beam side) + 50µm of diamond + 20 µm of liquid tin + 100 µm of diamond (VISAR side).

Fraction mass evolution in liquid tin (left: thermal equilibrium calculation; right: kinetic model)

In this experiment, tin crystallize under the isentropic compression 3 and become liquid again when the relaxation wave fallows. In the result from the thermal equilibrium model, the mass fraction is beginning to change from the beginning of the isentropic compression on the front of the tin (t = 5 ns). In the case of kinetic calculation, this development does not start before 13 ns, when the difference between the Gibbs energies of the two phases becomes important. The both side (laser side and VISAR side) of the liquid tin has become solid (dark red), but it is significantly different inside tin target: full crystallization for the thermal equilibrium calculation and a mixture in the kinetic model. An x‐ray diffraction analysis 4,5 should be able to discriminate the two results and to assess, by comparison with numerical simulations, the value of the characteristic time of crystallization.

1 D. B. Hayes, J. Appl. Phys. 46, 3438‐3443 (1975) 2 P. Combis and L. Videau, La cinétique des changements de phases dans le code ESTHER, internal report (2011) 3 J. P. Davis et al., Shock Compression of Condensed Matter CP620, 221, (2001) 4 J. Dean Barnett et al., J. Appl. Phys. 37, 875 (1966); S. Desgreniers et al., Phys. Rev. B 39, 10359 (1989); S. Bernard and J. B. Maillet, Phys. Rev. B 66, 012103 (2002) 5 J.D. Kress et al., Shock Compression of Condensed Matter CP955, 1433 (2007) Targeted measurements We propose to measure (i) the sound velocity 6 in the tin (crystallized or not) with the VISAR (to do this, we will need the North beam to produce an isentropic compression and the South beam to produce a shock wave in the tin), (ii) the temporal change of the tin reflectivity 5,7 with an Active Shock Break Out (ASBO) diagnostic without shock breaking and (iii) the x‐ray diffraction analysis to discriminate between partial or total crystallization in the liquid tin.

Experimental set‐up

North beam: isentropic compression Pulse duration see below: laser pulse shape Temporal shape Focal length 1600 mm Focal spot size ~1 mm (with appropriate RPP) Wavelength 2ω Energy on target PS1 : from 50 to 150 Joules PS2 : from 40 to 60 Joules

South beam: shock wave in solid tin or x‐ray diffraction source [delay: from 5 to 10 ns] Pulse duration 1 ns to 5 ns Temporal shape square Focal length 1600 mm Focal spot size ~1 mm (with appropriate RPP) for the shock wave propagation ~150 µm (with appropriate RPP) for the x‐ray diffraction source Wavelength 2ω Energy on target from 40 to 60 Joules for the shock wave in solid tin 300 Joules for the x‐ray diffraction source

Blue beam: no request

Liquid tin We will use a specific heating cell to have liquid tin up to ~600°K (figures below).

~6 cm Ø ~1mm

~4 cm ~10 cm

Laser side (front side). Target aperture.

6 J. Hu et al., J. Appl. Phys. 104, 083520 (2008); J. Hu et al., Appl. Phys. Lett. 92, 111905 (2008); A.I. Funtikov, High Temperature 49, 439 (2011) 7 S.L. Pistinner, Shock Compression of Condensed Matter CP955, 181 (2007) and 185 (2007)

VISAR side (back side).

The target will be composed of a stack of diamond and liquid tin, with some carbon on the laser side (figures below).

50 µm diamond 2µm C 10 or 20 µm liquid Sn

10 µm steps in diamond

100 µm diamond

North beam target side. VISAR target side.

North beam laser pulse shape

4 5  t  t    t  t   )( ItI      )( ItI      0 0  ns  55 ns    ns  88 ns  

Preferred laser pulse shape (PS1). Palliative laser pulse shape (PS2).

Thermodynamic work in the liquid tin target for PS1 and PS2 compare to the melting curve.

Measurements Diagnostics Range of measurement Sound speed 3.5 – 5.5 km/s Reflectivity 10 – 70 % X‐ray diffraction 3.0 – 3.5 keV analysis 7.0 – 9.0 keV

SHOT PLAN

Days Objectives Shot number 1 Laser pulse shape validation with solid tin, aluminium or iron targets ‐ VISAR & reflectivity measurements 5‐10 shots Objective: determine a sufficient pulse shaping with the sound (not full energy) speed measurement in the solid target. 2 Solid and liquid tin under isentropic compression ‐ VISAR and 10‐15 shots 3 reflectivity measurements (not full energy) 4 Objective : measure of the sound speed in the crystallized tin 5 X‐ray diffraction analysis ‐ VISAR and reflectivity measurement 5 shots and x‐ray diffraction analysis (full energy on in collaboration with the PIMM laboratory. South Beam)

Palliative shot plan: in case of troubles with the liquid tin experiment, we will make measurements (sound speed, reflectivity measurement and x‐ray diffraction analysis) with solid tin targets (same characteristics than the liquid target) under isentropic compression and shock wave (Hugoniot measurements); the request will be the same for the North andr South lase beams.

Days Objectives Shot number 2 Solid tin under isentropic compression or shock wave ‐ VISAR and 15‐20 shots 3 reflectivity measurements (not full energy) 4 Objective : measure of the sound speed in the solid tin 5 X‐ray diffraction analysis on solid tin ‐ VISAR and reflectivity 5 shots measurement and x‐ray diffraction analysis (full energy on in collaboration with the PIMM laboratory. South Beam)

EXPERTISE OF THE EXPERIMENTAL CONSORTIUM

The proposal is based on the joint expertise of the LULI, PIMM and CEA teams in studying the dynamic material behaviour by laser driven shocks (laser‐matter interaction, wave propagation...) [VISAR diagnostic practice: CEA, PIMM, LULI; 1D hydrodynamic radiative code: CEA; molecular‐dynamics simulations: CEA] and on specific expertises related to over parts of the experiment : development of a x‐ray source produced by laser (CEA, LULI) and heating cell (CEA).

12‐NS‐F4

APPLICATION to access the LULI laser facilities

May 2012 – April 2013

Title of the experiment: Microjetting under laser driven shocks

Principal Investigator (PI): Dr Thibaut De RESSEGUIER Institut Pprime, ENSMA, 1 av. Clément Ader, 86961 Futuroscope Cedex, France [public RESearch organisation]

[email protected]

phone: +33 5 49 49 81 73 fax: +33 5 49 49 81 76 status: EXP (experienced researcher) citizenship: FR birth date: 14/06/1967

LULI co‐PI: ‐

Summary: When a shock wave propagating in a solid sample breaks out at a free surface, geometrical effects predominantly governed by the roughness and defects of that surface may lead to the ejection of tiny jets that may break apart into µm‐size (or smaller) fragments. This process usually called microjetting is a major safety issue for engineering applications involving pyrotechnics or armour design. We propose the first investigation of microjetting in the specific loading conditions associated to laser shocks (short duration of pressure application, very high strain rates, small spatial scales, low damage to the setup and nearby equipment), to provide quantitative data that would complement those obtained under explosive loading. The goal will be to characterize the jets ejected from various types of surfaces of controlled roughness or with calibrated grooves, using mainly two complementary diagnostics: (i) transverse shadowgraphy to visualize the ejecta at successive times after the laser shot, and (ii) time‐resolved velocity measurements with a heterodyne interferometer. Additionally, an attempt will be made to evaluate the momentum involved in the jetting process by measuring the acceleration of a distant foil impacted by the ejecta. The main parameters (nature of the , groove angle, roughness, shock pressure…) governing jet formation, ejection velocity and jet fragmentation, will be identified and their influences will be quantified. The experimental data will be used to test the predictive capability of both analytical and numerical models in 2D and/or 3D simulations, in collaboration with the CEA.

Number of access weeks requested: 1 on: LULI2000/salle 2 (nano2000)

Proposed access route: standard access

Do you have received any support for this proposal? No

Keywords: laser shock, microjetting, dynamic fragmentation

Co‐investigators (not including LULI researchers): 1. Michel BOUSTIE, EXP, Institut P’, [email protected], FR 2. Emilien LESCOUTE, PDOC, CEA/DIF, [email protected], FR 3. Arnaud SOLLIER, EXP, CEA/DIF, [email protected], FR 4. Laurent BERTHE, EXP, LPIMM, ENSAM Paris, [email protected], FR Microjetting under laser driven shocks

SCIENTIFIC CASE

Motivation When a shock wave propagating in a solid sample breaks out at a free surface, geometrical effects predominantly governed by the roughness and defects of that surface may lead to the ejection of tiny jets that may break apart into µm‐size (or smaller) fragments. This process, which is referred to microjetting herein, is a major safety issue for engineering applications involving pyrotechnics or armor design. Hence, it has been widely studied for several decades, in particular by the CEA in France (figure 1 left) and by the LANL in the USA 1,2,3. In our ongoing effort to characterize laser shock‐induced fragmentation 4,5,6,7 it has appeared as one of the mechanisms leading to high velocity ejecta (figure 1 right). Discussions of our preliminary results with several CEA colleagues (Bruyères‐le‐Châtel and Valduc) and with W.T. Buttler at LANL have prompted us to consider new experiments specifically oriented toward a detailed characterization and some predictive modeling of microjetting under laser driven shocks.

Figure 1: (left) transverse radiograph of a tin target subjected to a detonation wave breaking up at the (top) free surface [Chapron, 1991]; the left half of this surface was carefully polished, while 100 µm‐deep triangular grooves were machined in the right half, leading to the ejection of microjets; (right) transverse shadowgraph of a 200 µm‐thick aluminum foil subjected to a laser shock 7 applied from left to right; shock breakout produces the ejection of microjets from the free surface.

International context As mentioned above, microjetting has been widely studied in the past and it is still the main subject of interest for groups like that of W.T. Buttler at Los Alamos National Laboratory 3. However, all existing data have been obtained using explosive loading or plate impacts. Because laser shocks offer several advantages over those conventional techniques, they appear as a promising way to get new data on this process. In particular, they allow easy access to the high pressures required to produce microjets, las wel as relatively simple sample preparation due to the small dimensions involved. Finally, their extremely short duration leads to a range of unusually high strain rates, typically 107 s‐1. To date, we are the first team to propose a systematic investigation of microjetting under laser shock loading.

Background Since 2005, we have been conducting on the “nano2000” facility of the LULI and on the “Alisé” facility of the CESTA (now closed) an extensive investigation of the different processes governing dynamic fragmentation of at extremely high strain rates: spallation, micro‐spalling after shock‐induced melting, dynamic punching, and microjetting (figure 1 right). Our campaigns have allowed the development and use of several complementary experimental diagnostics, including . transverse shadowgraphy at successive times to characterize the motion of the ejecta, . soft recovery of the fragments in a low density gel, . time‐resolved velocity measurements using a new Heterodyne interferometer, in collaboration with the CEA Bruyères‐le‐Châtel.

1 J.R. Asay et al., Ejection of material from shock surfaces, Appl. Phys. Lett. 29, 284 (1976); J.R. Asay, Thick‐plate technique for measuring ejecta from shock surfaces, J. Appl. Phys. 49, 6173 (1978) 2 C.L. Mader, LASL Phermex Data, University of California Press (1980); P. Andriot, P. Chapron and R. Olive, AIP Conf. Proc. 78, 505 (1981); D.S. Sorenson et al., J. Appl. Phys. 92, 5830 (2002) 3 W.S. Vogan et al., J. Appl. Phys. 98, 113508 (2005); M.B. Zellner et al., J. Appl. Phys. 102, 013522 (2007) 4 T. de Rességuier et al., On the dynamic fragmentation of laser shock‐melted tin, Appl. Phys. Lett. 92, 131910 (2008) 5 E. Lescoute et al., Appl. Phys. Lett. 95, 211905 (2009); T. de Rességuier et al., Int. J. Fracture, 163, 109 (2010); L. Signor et al., Int. J. Impact Engineering 37, 887 (2010); G. Morard et al., Phys. Rev. B 82, 174102 (2010) 6 E. Lescoute et al., Comput. Mat. Continua 22, 219 (2011) 7 E. Lescoute et al., J. Appl. Phys. 108, 093510 (2010) The possibility to apply those techniques for a specific study of microjetting has been tested in one particular shot on a 200 µm‐thick aluminum sample where approximately triangular, ~70 µm‐deep parallel grooves had been dug in the free surface. The emergence of a 58 GPa laser shock at this free surface clearly shows the ejection of distinct jets from the tip of the grooves (figure 2). Their velocity, which is mainly conditioned by the groove angle, has been determined. Because of the asymmetry of the groove, this angle varies during shock breakout, which leads to unexpected changes in the jet direction.

Figure 2: transverse shadowgraphs of the breakout of a laser shock at the free surface of a 200 µm‐thick aluminum foil with two parallel grooves 7; distinct jets are ejected from the tips of the grooves at velocities of about 2.8 km/s; the apparent change in their direction is attributed to a change in the groove angle during the reflection of the shock wave.

Proposal The present project is a continuation of that preliminary, successful test. To obtain quantitative data on microjetting in the unexplored range of loading conditions associated to laser shocks, we will prepare samples of different metals with surfaces of various geometries, from controlled roughness (like in figure 1 right) to distinct defects (like in figure 2) carefully prepared by laser micromachining. Transverse optical shadowgraphy and time‐resolved measurements (heterodyne interferometry) will provide ejection velocities. The ratio between jet velocity and smooth free surface velocity will be determined. The effects of the asymmetry of the defects (like in figure 2) will be investigated. An effort to evaluate the mass of the ejecta will be undertaken, since this is a major practical question for the applications involved. The results will be compared to data reported under explosive loading, and they will be used to test the predictive capability of both analytical and numerical models in 2D and/or 3D simulations.

Experimental methods and diagnostics Samples will be foils of aluminum, tin and tantalum. Sample surface will be machined to achieve controlled defects or roughness. Two complementary diagnostics will be used. Transverse shadowgraphy will provide quasi‐instantaneous images of the ejecta at successive times, using several cameras with different delay times (see schematic setup below). Thiss ha been successfully tested in the LULI 4, then our technique has been improved (illumination by a flash lamp to remove the speckle due to coherent laser light, use of three cameras) 6,7. Time‐resolved velocity measurements will be performed with a heterodyne interferometer developed by the CEA/DIF. The unique capability of this technique to measure several velocities throughout an expanding cloud of particles is particularly well suited for studying microjetting. To try to evaluate the momentum involved in the jetting process, somes shot will be designed in order to measure the acceleration of a distant foil upon impact of the ejecta coming from the free surface of the laser shock‐loaded sample (using the so‐called “Asay window” principle 1). The experimental results will be compared to the predictions of analytical and modeling tools.

Experimental set‐up The nano2000 laser will be shot at  (1.06 µm), ~1000 J and 5 ns, alternatively with the south and north beams, focused onto a ~3 mm‐diameter spot, in secondary vacuum. Our program should require approximately 20 experiments, as mentioned in the shot plan. A schematic of the setup is shown below.

SHOT PLAN

The influences of the most relevant parameters will be explored: shapes and dimensions of the grooves in the sample free surface, nature of the metal, shock pressure (i.e. laser intensity). This program will require approximately 20 experiments, after some preliminary shots to set up and test the diagnostics.

EXPERTISE OF THE EXPERIMENTAL CONSORTIUM

Our group has many years of expertise at the international level in the field of shock waves in condensed matter, and more particularly laser‐driven shocks. Original experiments have been designed and performed (mainly in the LULI, for about 20 years) on a wide variety of materials (metals, polymers, glasses, rocks, water…), with specific diagnostics, both time‐resolved (velocimetry, piezoelectric gauges, shadowgraphy…) and post‐shot (analysis of recovered materials). The results have been used to model various processes governing the response of materials to shock loading (densification, brittle and ductile damage, fragmentation, debonding of multilayered targets, structural changes in minerals, phase transitions…). These studies are of key interest to both basic science (dynamic behaviour of matter at extremely high strain rates, geophysics…) and engineering applications (damage under dynamic loading, structural strength, synthesis of ultra‐hard materials, debonding tests …). The experiments will be performed by Emilien Lescoute (post‐doctoral student at CEA, designer of the transverse shadowgraphy setup in our latest campaigns), Thibaut de Rességuier, Michel Boustie and Laurent Berthe (permanent researchers, CNRS). Some shots will be connected to the work of Didier Loison (PhD student, ENSMA) so that he will participate to the campaign ifl he is stil available then. At the beginning of (or slightly before) the campaign, the heterodyne interferometer will be set up by colleagues from CEA (Jacky Bénier, Patrick Mercier and Arnaud Sollier), best French experts on this technique, and they will help us running it afterwards. The analysis of the experimental data will be coupled with 2D and 3D simulations in collaboration with the CEA (Arnaud Sollier and Emilien Lescoute).

PREVIOUS ACCESS

Our group has been offered a regular and continuous access to the LULI laser facilities for more than 15 years, 2 to 3 weeks per year in average. In the past two years, two one‐week campaigns have been dedicated to the study of a process called “micro‐spalling”, governing dynamic failure after shock‐induced melting, another two have been made on the simulation of a coating process based on the use of laser shocks, and two campaigns have been conducted on the ELFIE facility on the effects of ultra‐short shock loading. Some selected publications of our results obtained on these facilities since 2008 are listed below. J. Gattacceca et al., On the efficiency of shock magnetization processes, Phys. Earth Planet. Interiors 166, 1 (2008); T. de Rességuier and M. Hallouin, Effects of the a‐e phase transition on wave propagation and spallation in laser shock‐loaded iron, Phys. Rev. B 77 , 174107 (2008); L. Signor et al., Dynamic fragmentation of melted metals upon intense shock wave loading. Some modelling issues applied to a tin target, Archives Mechanics 60, 323 (2008); J.P. Cuq‐Lelandais et al., Spallation generated by femtosecond laser driven shocks in thin metallic targets, J. Phys. D: Applied Physics 42, 065402 (2009); T. de Rességuier et al., Transformations of graphite‐like B‐C phases under dynamic laser‐driven pressure loading, Phys. Rev. B 79, 144105 (2009) T. de Rességuier et al., Wave propagation and dynamic fracture in laser shock‐loaded solid materials 22, 419, in Wave Propagation in Materials for Modern Applications, A. Petrin ed., INTECH Croatia (2010) L. Signor, Contribution à la caractérisation et à la modélisation du micro‐écaillage de l’étain fondu sous choc, thèse de doctorat ENSMA (2008) ; J.P. Cuq‐Lelandais, Etude de l’endommagement dynamique de matériaux sous choc laser subpicoseconde, thèse de doctorat ENSMA (2010); E. Lescoute, Etude de la fragmentation dynamique de métaux sous choc laser, thèse de doctorat ENSMA (2010) and references 4‐7 hereby