New probing techniques of radiative shocks

doi:10.1016/j.optcom.2011.09.008

Optics Communications

Volume 285, Issue 1, 1 January 2012, Pages 64-69

https://doi.org/10.1016/j.optcom.2011.09.008 Get rights and content

Abstract

Radiative shock waves propagating in xenon at a low pressure have been produced using 60 joules of iodine laser (λ = 1.315 μm) at PALS center. The shocks have been probed by XUV imaging using a Zn X-raylaser (λ = 21 nm) generated with a 20-ns delay after the shock creating pulse. Auxiliary high-speed silicon diodes allowed performing space- and time-resolved measurement of plasma self-emission in the visible and XUV. The results show the generation of a shock wave propagating at 60 km/s preceded by a radiative precursor. This demonstrates the feasibility of radiative shock generation using high power infrared lasers and the use of XRL backlighting as a suitable diagnostic for shock imaging.

Introduction

The study of strong radiative shock waves in gases has been stimulated within the last 10 years by the availability of kJ laser installations, which allow the launching of shock waves with velocities larger than 60 km/s and enabling the study of the coupling between radiation and hydrodynamics in simple geometries [1], [2], [3], [4]. This topic is very important for understanding the physics of astrophysical shock waves in processes occurring during stellar formation. For instance, shock waves are produced during the gravitational collapse from a dense molecular cloud. Later on, in young stars, an accretion shock is formed when matter falls from the circumstellar disk into the stellar photosphere. In addition, shocks are also observed at the head of high velocity protostellar jets.

A radiative shock is a very strong shock wave, in which the shock front is heated at a very high temperature and thus presents a strong self-emission. Depending on the opacity, the photons emitted from this zone can be reabsorbed by the cold gas in which the shock propagates. This generates a noticeable ionization wave, also called a radiative precursor [5], [6], [7]. The complexity of the physical description of these flows, which results from radiation effects [8], is enhanced by the non ideal effects in the gas (e.g. ionization, equation of state) [9] and by the geometrical effects even in cylindrical or parallelepipedic shock tubes [10], [11].

Most of the experiments have focused on the study of the radiative precursor, e.g. by using optical laser probing (shadowgraphy and interferometry) [1], [4], [11]. Hard X-ray back-lighting has been used at Rochester [2] to image the front shock. From these types of studies, several points can be highlighted: (i) radiation cooling strongly affects the geometry and temperature of the precursor, (ii) the interactions between the shock and the walls may be appreciable [12], (iii) the shock wave may be observed at late times (~ 40 ns) in a regime where the velocities of the ionization and shock fronts are comparable [4]. However, none of these experiments were able to probe the whole shock structure, i.e. the precursor, the shock front and the postshock. Moreover, the study of the radiative signatures of these shocks remains very challenging [4]. The experiments presented here aim at testing for the first time the feasibility of two new diagnostics: instantaneous imaging of the whole shock structure with an auxiliary X-ray laser (XRL) at 21.2 nm and a photometric, spatially resolved, study of the plasma self-emission which will also be useful to follow the shock chronometry.

Section snippets

Experimental setup

The experiments were conducted at the Prague Asterix Laser System (PALS) [13]. The core of the experimental setup (Fig. 1) consists of two beams from the iodine laser (λ = 1.315 μm), with a pulse duration of 0.3 ns. The first beam (hereafter called AUX beam, 60 J) drives the shock-wave inside a target filled with xenon at low pressure (~ 0.3 bar, i.e. 1.5 10^− 3 g/cm³). The second beam (hereafter called MAIN beam, 500 J) generates a laser beam at 21.2 nm (λ = 21.2 nm, hereafter called XRL beam), with a pulse

XUV radiography of the shock

Monochromatic XRL radiography offers several advantages compared to other techniques: the number of photons, a parallel beam, and the possibility to probe dense matter up to a fraction of the electron critical density (N_cr [cm^− 3] = 1.1 10²⁷λ^− 2 [nm], i.e. 2.5 10²⁴ cm^− 3 at λ = 21 nm). In our case, this technique will provide an image of the whole shock structure. It will allow us to probe the dense part of the shock, but also the heated low-density precursor, where absorption occurs through the rich

Result from diodes

The shock velocity estimated from the XRL probing is confirmed by the plasma self-emission registered by the two fast diodes, PERP and OBL. Both of them recorded the plasma self-emission coming from a section of the shock tube located at ~ 2.2 mm from initial position of the piston, and with an emitting surface of about 0.2 × 0.4 mm². The diodes were not filtered and no analogical attenuators were used on them. The resulting record is reported in Fig. 8. In both cases, we observe a strong peak at

Conclusions

We present the first experimental study of radiative shock waves in gases by simultaneous XRL imaging and spatially resolved plasma self-emission. The results obtained by both diagnostics are consistent and in qualitative agreement with the chronometry expected from 1-D simulations.

XRL backlighting is particularly interesting as it allows probing not only the shock front but also a part of the radiative precursor. Using high quality imaging mirrors this XRL diagnostic will allow to visualize

Acknowledgments

The authors acknowledge the vital contributions of the PALS technical staff, the target fabrication staff of Observatoire de Paris, the important contribution of V. Petitbon at IPN for the piston manufacturing, and illuminating discussions with F. Delmotte and S. de Rossi (Institut d'Optique, University Paris XI), B. Rus (Institute of Physics in Prague), F. Delahaye, A. Ciardi (LERMA), M. Gonzalez (IRFU). The work was supported by LASERLAB access program, French ANR, under grant 08-BLAN-0263-07

References (23)

H. Fiedorowicz et al.
Optics Communication
(1999)
R. Ramis et al.
Computer Physics Communications
(1988)
B.L. Henke et al.
Atomic Data and Nuclear Data Tables
(1993)
S. Bouquet et al.
Physical Review Letters
(2004)
A.B. Reighard et al.
Physics of Plasmas
(2006)
M. González et al.
Astronomy and Astrophysics
(2009)
C. Stehlé et al.
Laser and Particle Beams
(2010)
Y.B. Zeldovich et al.
Physics of Shock Waves and High Temperature Hydrodynamic Phenomena
(1996)
A. Penzo et al.
Journal de Mécanique Théorique et Appliquée
(1984)
Yu N. Kiselev et al.
Computational Mathematics and Mathematical Physics
(1991)

S. Bouquet et al.

Astrophysics Journal Supplement

(2000)

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New probing techniques of radiative shocks

Abstract

Introduction

Section snippets

Experimental setup

XUV radiography of the shock

Result from diodes

Conclusions

Acknowledgments

Optics Communication

Computer Physics Communications

Atomic Data and Nuclear Data Tables

Physical Review Letters

Physics of Plasmas

Astronomy and Astrophysics

Laser and Particle Beams

Physics of Shock Waves and High Temperature Hydrodynamic Phenomena

Journal de Mécanique Théorique et Appliquée

Computational Mathematics and Mathematical Physics

Astrophysics Journal Supplement