Article submitted to: High Power Laser Science and Engineering, 2018 April 10, 2018 Laboratory radiative accretion shocks on GEKKO XII laser facility for POLAR project L. VanBox Som1,2,3, E.´ Falize1,3, M. Koenig4,5, Y.Sakawa6, B. Albertazzi4, P.Barroso9, J.-M. Bonnet- Bidaud3, C. Busschaert1, A. Ciardi2, Y.Hara6, N. Katsuki7, R. Kumar6, F. Lefevre4, C. Michaut10, Th. Michel4, T. Miura7, T. Morita7, M. Mouchet10, G. Rigon4, T. Sano6, S. Shiiba7, H. Shimogawara6, and S. Tomiya8 1CEA-DAM-DIF, F-91297 Arpajon, France 2LERMA, Sorbonne Universit´e,Observatoire de Paris, Universit´ePSL, CNRS, F-75005, Paris, France 3CEA Saclay, DSM/Irfu/Service d’Astrophysique, F-91191 Gif-sur-Yvette, France 4LULI - CNRS, Ecole Polytechnique, CEA : Universit Paris-Saclay ; UPMC Univ Paris 06 : Sorbonne Universit´e- F-91128 Palaiseau Cedex, France 5Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan 6Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan 7Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan 8Aoyamagakuin University, Japan 9GEPI, Observatoire de Paris, PSL Research University, CNRS, Universit´eParis Diderot, Sorbonne Paris Cit´e,F-75014 Paris, France 10LUTH, Observatoire de Paris, PSL Research University, CNRS, Universit´eParis Diderot, Sorbonne Paris Cit´e,F-92195 Meudon, France Abstract A new target design is presented to model high-energy radiative accretion shocks in polars. In this paper, we present the experimental results obtained on the GEKKO XII laser facility for the POLAR project. The experimental results are compared with 2D FCI2 simulations to characterize the dynamics and the structure of plasma flow before and after the collision. The good agreement between simulations and experimental data confirm the formation of a reverse shock where cooling losses start modifying the post-shock region. With the multi-material structure of the target, a hydrodynamic collimation is exhibited and a radiative structure coupled with the reverse shock is highlighted in both experimental data and simulations. The flexibility on the laser energy produced on GEKKO XII, allowed us to produce high-velocity flows and study new and interesting radiation hydrodynamic regimes between those obtained on the LULI2000 and Orion laser facilities. Keywords: Laboratory Astrophysics; accretion processes; high power laser; hydrodynamics arXiv:1804.02714v1 [astro-ph.HE] 8 Apr 2018 1. Introduction surrounding luminosities. They are close binary systems composed of a strongly magnetized white dwarf accreting Accretion processes are main sources of high-energy radia- matter from a low-mass companion star [4]. The intense tions in several binary systems [1], in particular in cataclysmic magnetic field of the white dwarf (B > 10 MG) locks variables. As potential progenitors of type Ia supernovae [2], WD the whole system into synchronous rotation, prevents the understanding these complex systems is crucial to explain formation of accretion disks and guides the accreted flow the initial conditions of these explosions which are used as an accretion column onto the white dwarf magnetic to study the acceleration of the Universe [3]. Among these poles [5,6]. The supersonic accreted flow coming from the objects, polars are remarkable ones to study accretion pro- companion and channelled by the dipolar magnetic field, cesses in isolation since the high-energy radiation coming strikes the white dwarf photosphere at the free-fall velocity from accretion processes are not contaminated by other −1 (vff ∼ 3000 km s ), creating an accretion shock. This shock is counter propagating counter to the incoming flow Correspondence to: Address of corresp. Email: [email protected] 1 2 L. VanBox Som et al. and heats the matter to temperatures of about 10 keV. Thus success of these experiments and their relevance for the study the intense emitted radiation shapes the post-shock region of high-energy processes, the plasma flow collimation using and slows down the accretion shock which reaches a height the tube can generate various wall shocks before impact of about 100 km above the white dwarf photosphere. Con- and afterwards, it can corrupt the reverse shock formation sequently, the small size of this post-shock region prevents and propagation. Besides, to implement X-ray radiography direct inference of spatial profiles of the radiative zone which diagnostics, holes are introduced in the tube which induce are relevant to determine the white dwarf mass. Besides, 3D effects on the reverse shock structure. Consequently a the observed flux can not be reconciled with the current design is therefore under study to remove the tube where standard model of accretion columns which introduce strong the collimation of the supersonic flow is produced by a disagreements between theory and observations [7–10]. Thus, dynamical nozzle-like effect. powerful facilities provide an alternative approach which In this paper, experimental results, obtained with the will help clarify outstanding questions related to radiative GEKKO XII laser facility with a new target design, are processes in accretion column models. presented and compared to 2D FCI2 laser simulations. Many Based on similarity properties of this high-energy envi- optical diagnostics have been implemented to probe the ronment, millimetre-sized models of accretion columns can dynamics and the structure of the plasma flow and the be built with powerful lasers. The radiation hydrodynamics post-shock region. In the first section, the target design physics of these structures admits exact scaling laws which is presented. In the second one, the dynamics of the demonstrate that measurable-scale models of radiative incident flow and the properties of the post-shock region accretion columns could be produced in laboratory for are presented through experimental data and 2D simulations. the main accretion shock regimes: the bremsstrahlung- Finally, the similarity properties of the target are investigated dominant regime [11], the two radiative processes regime with the GEKKO XII laser conditions. (bremsstrahlung and cyclotron processes) [12], the two- temperature regime [13], and the ideal magnetohydrodynami- 2. Target design and experimental setup cal (MHD) regime [14]. These scaling laws for many different shock regimes offer new opportunities to study astrophysical The target design, which models the astrophysical scale, is objects at laboratory scale. To assure the relevance of shown in Figure 1. It is composed of a pusher playing the such experiments, the key dimensionless numbers of the role of the accreted matter, and an obstacle modelling the laboratory and astrophysical plasmas have to be equal which white dwarf photosphere. The two parts are separated by implies that the two physical regimes are similar. In this a vacuum zone and fixed on a plastic support. The main case, powerful lasers become microscopes to study radiative difference from previous POLAR targets is the absence of processes at laboratory scale relevant to the astrophysical the tube. one. Based on the Alfven similarity [14], the calculated magnetic p field at laboratory scale is given by BL = PL=PABA where PL=PA is the ratio of the laboratory and astrophys- ical pressures and BA is the intensity of the astrophysical magnetic field. For typical values in polars, the laboratory magnetic field should be equal to about 1 GG. This intensity of magnetic field given by the scaling laws is obviously inaccessible in the laboratory, except in very particular configurations [15]. This prevents from studying the main polars regime with the multiple radiative processes. Con- sequently, the only accessible accretion regime with lasers is the bremsstrahlung-dominant regime when the white dwarf magnetic field is expected to be weak (BWD = 10 − 30 MG). In this regime, the only magnetic effect is the Figure 1. Target’s schematic. The laser is coming from the right and it magnetic collimation of the column due to the white dwarf interacts with the pusher. A plasma is created due to the interaction between magnetic field and others magnetic effects, such as the the laser and the pusher. This supersonic plasma expands in the vacuum and impacts an obstacle. This leads to the creation of a reverse shock. cyclotron radiation, can be neglected [4]. At laboratory scale, to replace magnetic collimation, a tube was used The pusher is composed of a 25 µm thickness plastic to mechanically collimate the flow. This design has been −3 [16,17] [18] layer (1:29 g cm ), which converts laser energy into kinetic successfully experimented on LULI2000 and Orion [21] laser facilities in the POLAR project [16] and simultaneously energy by rocket effect , and a 4 µm thickness tin layer, on the OMEGA facility [19,20] laser facility. In spite of the which has two important roles: it protects the obstacle against X-ray radiation produced by the hot coronal plasma Laboratory radiative accretion shocks on GEKKO XII 3 and increases the radiative properties of the target. All these layers are stuck to an aluminium washer. Experiments were performed on the GEKKO XII facility. Depending of the chosen laser configuration, the energy delivered on target can range from 300 J to 1100 J at 3! wavelength, with a Gaussian focal spot of around 350 − 400 µm FWHM and a Gaussian pulse width of 500 ps. The associated intensities can vary from 4 × 1014 W cm−2 to 2:5 × 1015 W cm−2 on target. Seven optical diagnostics (1D and 2D SOP, 1D and 2D shadowgraphies and interferometry) have been implemented to probe the incoming plasma and the post-shock region. An optical probe beam (∼mJ, 2! and 10 ns duration) has been installed in the perpendicular direc- tion to the incoming flow propagation direction. Transverse optical pyrometry (SOP) diagnostics image the self-emission from the incoming flow and the reverse shock structure. In addition, shadowgraphy diagnostics record the global shape Figure 2. Spatial evolution of the density as function of time extracted from of the plasma and interferometers enable inference of the 1D simulation performed with FCI2 code.