Astrophys Space Sci (2009) 322: 25–29 DOI 10.1007/s10509-009-0025-7
O R I G I N A L A RT I C L E
Experimental results to study astrophysical plasma jets using Intense Lasers B. Loupias · C.D. Gregory · E. Falize · J. Waugh · D. Seiichi · S. Pikuz · Y. Kuramitsu · A. Ravasio · S. Bouquet · C. Michaut · P. Barroso · M. Rabec le Gloahec · W. Nazarov · H. Takabe · Y. Sakawa · N. Woolsey · M. Koenig
Received: 17 June 2008 / Accepted: 9 March 2009 / Published online: 19 March 2009 © Springer Science+Business Media B.V. 2009
Abstract We present experimental results of plasma jet, interacted with an ambient medium, using intense lasers to investigate the complex features of astrophysical jets. This experiment was performed in France at the LULI facility, Ecole Polytechnique, using one long pulse laser to generate the jet and a short pulse laser to probe it by proton radiography. A foam filled cone target was used to generate high velocity plasma jet, and a gas jet nozzle produced the well B. Loupias () · C.D. Gregory · A. Ravasio · M. Rabec le Gloahec · M. Koenig LULI, Ecole Polytechnique, CNRS, CEA, UPMC, Route de Saclay, 91128 Palaiseau, France e-mail:
[email protected] E. Falize · S. Bouquet CEA-DAM-DIF, BP 12 91680 Arpagon, France E. Falize · S. Bouquet · C. Michaut Laboratoire de l’Univers et de ses Théories, UMR8102, Observatoire de Paris, 92195 Meudon, France P. Barroso GEPI, Observatoire de Paris, CNRS, Universite Paris Diderot, Place Jules Janssen, 92190 Meudon, France J. Waugh · N. Woolsey Department of Physics, University of York, York, YO10 5DD, UK D. Seiichi · Y. Kuramitsu · H. Takabe · Y. Sakawa Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita 565-0871, Japan S. Pikuz Joint Institute for High Temperatures of RAS, Izhorskaya 13/19, Moscow 125412, Russia W. Nazarov School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, UK
known ambient medium. Using visible pyrometry and interferometry, we were able to measure the jet velocity and electronic density. We get a panel of measurements at various gas density and time delay. From these measurements, we could underline the growth of a perturbed shape of the jet interaction with the ambient medium. The reason of this last observation is still in debate and will be presented in the article. Keywords Plasma · Laser · Astrophysical jet · Laboratory astrophysics · Young stellar objects
1 Introduction Experimental research using high-energy density facilities, like intense laser or Z-pinch facilities, is a recent promising area to investigate astrophysical phenomena reviewed in Remington et al. (2006). Several approaches are possible such as comparison or similarity to answer astrophysical questions. In the term “comparison”, we arise experiments which can create the exact conditions of matter observed in astrophysical objects, for instance in the core of giant planets (Chabrier et al. 2007). The experimental measurements provide crucial input for astrophysical theoretical or numerical models. Whereas, the meaning for “similarity” between two area (experiment and astrophysics) highlights the possibility to simulate the whole evolution of the system and to keep it invariant compare to its astrophysical counterpart. The measurements of these evolution, when properly diagnosed, allow theoretical models and computer simulation codes to be tested. This last branch of experiments required the existence of scaling laws for the read up system. Scaling laws has been already demonstrated for phenomena which
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follow a set of equations, like pure hydrodynamic (Ryutov et al. 1999), magnetohydrodynamic (Ryutov et al. 2001) or radiative hydrodynamic systems (optically thin and thick medium) (Falize et al. 2008). The scaling law ensures the complete similarity between the astrophysical object and the experiment assuming some conditions. These conditions are the dimensionless numbers which define, by their order of magnitude, the limits of the scaling laws application. Strong shock driven turbulent dynamics relevant to supernovae explosions was investigated following this method in Ryutov et al. (2001) and can be applied to study astrophysical jets. The collimated supersonic jets observed around stars in formation (YSO) can be studied by similarity in experiments and has been already studied following this way using intense lasers (Blue et al. 2005; Gregory et al. 2008) and Zpinch facilities (Lebedev et al. 2005). The jets associated with YSO are often seen to have a chain of high visible emission knots and to terminate with a bow shock. The whole structure is known as Herbig-Haro (HH) objects (Blondin et al. 1989). The nature and complexity of astrophysical jets is such that it is very complex for an experiment to probe the whole of their dynamics. In this experiment, we decided to select and examine part of the physics follow by the astrophysical jet: its interaction with the interstellar medium (ISM). The bow shock structure observed in astronomical data appears with a very perturbed and fragmented shape (Hartigan 1989). We hence have decided in the presented experiment to pursue studies of astrophysical jets using intense laser (Loupias et al. 2007) to study the interaction of the jet with the ambient medium. To check the similarity via the dimensionless parameters we have to measure all the jet parameters such as velocity, temperature and density to calculate the dimensionless parameters. This last point is crucial to experimentally check the similarity of the experimental plasma jet to its astrophysical counter part. The analyses are still in progress so we present in this article only the transverse diagnostics results and the related estimation concerning the dimensionless parameter η corresponding to the ratio of the jet density to the ambient medium density.
2 Experimental setup For this experiment we used a long pulse to generate the plasma jet and a short pulse beam to produce protons for radiography. The lasers characteristics used was 1 kJ in 1.5 ns for the shock driven beams, and 100 J in 1 ps for the backlight beam. We used to generate plasma jet a brominate doped foam filled cone. This target was well characterized in previous experiments and the jet parameters were measured for its propagation in vacuum (Loupias et al. 2007). The 1.5 ns pulse was converted to the second harmonic, and
Astrophys Space Sci (2009) 322: 25–29
Fig. 1 Experimental setup from the top view. Using the rear side SOP, we measured the jet radius evolution as well as its equivalent black body temperature using absolute calibration
Fig. 2 Experimental setup from the transverse side
focused through a phase plate resulting in around 300 J in a 400 µm focal spot, with a pulse duration of 1.5 ns. The target is comprised of a conical former with entrance hole of diameter 500 µm, an exit hole of diameter 100 µm, and length 250 µm. A solid target as a pusher, from laser side 9 µm of plastic and 2 µm of titan, is placed over the entrance hole to drive a strong shock through the target. This shock is guided by the cone walls and results in the expulsion of a plasma jet from the exit hole. A washer of length 100 µm, and hole diameter 100 µm, is attached to the rear side of the target to increase the collimation of the flow for the initial stage of the expansion. The foam density was 50 mg·cm−3 or 100 mg·cm−3 30% brominated in mass. The experimental setup is presented in Fig. 1 for the top view of the diagnostics implementation and 2 for the transverse view. Thanks to the cylindrical symmetry of the jet, the tilted probe beam allowed to have simultaneously a visible transverse view of the jet and its proton radiography.
Astrophys Space Sci (2009) 322: 25–29
The experiment was diagnosed through time resolved optical emission at a wavelength of 450 ± 10 nm, both transverse and end-on to the jet propagation direction, a transverse 532 nm probe, analysed with a modified Normarski interferometer, and transverse proton radiography using radiochromic film stack as detector (RCF). In this case, the probe beam delivered an 8 ns pulse, and the interferograms were recorded with a gated optical imager (GOI), with a temporal resolution of 250 ps. The ambient medium was characterized in the gas jet facility experimental room at LULI. A Mach-Zehnder interferometer was used to measure the neutral density at various initial pressure of the argon gas used to simulate the ISM. The nozzle pressure was varied between 5 bar and 80 bar, resulting in an Argon (Ar) ambient number density of between 6 × 1017 cm−3 and 1 × 1019 cm−3 . The ambient medium density is proportional to the initial input pressure to the nozzle. The plasma jet generation at the rear side of the target is a very important point, to limit any jet or ambient medium interaction with the laser beam. In fact the laser beam itself can induce spurious effects like in Edens et al. (2004) modifying the jet evolution and disturbing the shock launching from the jet propagation into the ambient medium (bow shock). That are the reasons why we privilege jet generation from the rear side of the target.
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for a 50 mg/cm−3 30% brominated foam density target and 5 bar input pressure of argon gas. We observe the emission length evolution by time. Without ambient medium the velocity is the jet velocity, whereas with an ambient medium it supposes to be the gas emission, heated by a shock. In fact, we notice different emission profile with and without gas. The profile from Fig. 3, show an intensity increasing along the length. For instance at ∼17 ns, the intensity increases from 0 µm (the target side) to ∼1300 µm to reach its maximum. In comparison, for a shot without gas, we observe roughly constant emission level all along the jet. Furthermore, by increasing the density, the measured emission velocity decreases. This observation is consistent with the dependence of the shock velocity with the ambient medium density. Thanks to these observations we conclude that the velocity measured with an ambient medium comes from the shock front generated by the jet interaction with the gas: the bow shock. Its velocity (VBS ) varies according to the jet density (nj ), the ambient medium density (na ) and the jet velocity (Vj ) (Hartigan 1989): 2 βnj (Vj − VBS )2 = na VBS ,
(1)
The jet velocity was measured from the transverse Self Optical Pyrometer (SOP). In the Fig. 3 we present the result
where β correspond to the momentum transfer efficiency (from 0.4 to 0.8). By measuring the bow shock velocity at different ambient medium density (Fig. 4) and using the best fit following equation 1 (red curve) we can estimate the dimensionless parameters at the early stage of the phenomenon: η = nj /na . Astrophysical conditions correspond to η = 1–10. In the presented experimental conditions we obtained: 2.5 η 22 (using limited values of β).
Fig. 3 Jet length evolution and velocity measurement from the transverse SOP diagnostic. The shot is with 6 × 1017 cm−3 of argon neutral density and 50 mg/cm−3 30% brominated foam density. The black line represents the linear fit of the length evolution and corresponds to 123 km·s−1
Fig. 4 Velocity measurements using transverse SOP (initial foam density 100 mg/cc) at different ambient gas density (blue points). The red curve corresponds to the best fit using (1)
3 Experimental results 3.1 Jet and shock velocity
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Fig. 5 Interferogram for a 50 mg·cm−3 30% brominated foam density target (30 ns delay, 5 bar gas pressure)
3.2 Interferometry results In the Fig. 5, the probe delay is 30 ns from the arrival of the drive laser beam, and we observe the effect of introducing Ar gas. Differences are seen from the vacuum case: a high density feature is seen to propagate in front of the jet, as the flow acts as a piston driving a shock through the ambient gas. The shock front is not uniform, and small scale perturbations are seen around the leading edge. Due to the higher density in this shock front, the optical probe is not able to provide any quantitative measurement of the electronic density for this case. Whereas in the case without ambient medium, we observe a regular shape of the plasma jet. Earlier in time, the jet highly compressed by the cone guiding effect absorb and refract the probe beam. The shadow profile of the jet seems not perturbed in the limit of our spatial resolution (
