Dynamics of colliding aluminium plasmas produced by laser ablation
Introduction
Colliding laser produced plasmas have been investigated in a wide range of laser irradiance regimes (1010–1015 W/cm2) and for several purposes: by employing them as possible fuels for ICF-inertial fusion confinement or in order to reproduce, on a laboratory-scale, astronomical processes as for example collisionless shock waves production [1], [2]. In a low laser energy regime (1011–1012 W/cm2) some studies have already been made by different authors in order to study the physical interaction between two counter-propagating plumes (called seeds) expanding at a distance of a few mm from each other [3], [4]. For these conditions consistently interpenetration between the seeds does not occur and at the interaction region a so-called stagnation layer is formed [5]. It can be studied as a third layer of plasma with peculiar properties: the single plasmas rapidly accumulate at the collision region leading to the formation of a dense layer where a high collisional regime is established. The collisional parameter is then ξ = D/λii ≫ 1, where D is the distance between the two seeds and λii is the Spizter mean free path, given by the formula [5]:where ν12 is the relative velocity of the ions coming from the two plumes, Z is the average charge state of the plasma, ni is the average plasma ion density and lnΛ12 is the Coulomb logarithm [6]. Therefore, the characteristics of the stagnation layer depend on the distance between the two seeds and of their dynamics expansion properties and consequently, on the electron and ion densities of the collisional front. The purpose of this work is to demonstrate that colliding plasmas could be used to study fusion reaction rates in a low energy domain and phenomena of astrophysical interest [7], [8]. Many large scale phenomena are studied through laboratory plasmas and the results are then related to space plasmas through scale laws. Numerical simulations already demonstrated that laser produced plasmas (LPPs) have the unique property of having high enough ion temperatures to favor a certain number of nuclear fusion events. At the same time, the presence of clouds of cold electrons, provides to a non negligible electron screening factor whose value is similar to the stellar one: it is on the order of 1.2, calculated with the typical density and temperature values of laser plasmas produced at a power density of about 1011 W/cm2. Its weight is on the order of 20% in determining the total number of fusions events [9], [10]. Since the ES scales with the ratio of ne/Te, in order to have a consistent screening effect one needs high electron densities and at the same time low electron temperatures: the characteristics of the stagnation layer could be used for this purpose. Hough et al. already demonstrated that at a laser fluence of 1.6 × 103 J/cm2, the first collision front, observed at ∼30 ns, is composed mainly of electrons followed 10 ns later by ionized Al ions. A significant screening effect could be induced from the stagnation of the electrons at the mid-plane collision front which permits the highly charged ions to approach each other quite closely leading to a tight ion stagnation layer [4]. In this paper measurements were carried out with Al colliding plumes, by combining time and spatially resolved spectroscopy and Langmuir probe measurements. Fast imaging and emission spectroscopy measurements were performed in the visible range and revealed detailed information about the dynamics of spectral emission of the atomic species (neutral atoms and ions) which compose the seeds and the colliding region. Moreover, the Langmuir probe measurements permitted us to obtain the time of flight signals (TOF) at the stagnation front: by placing the probe very close to the target surface a current plateau was detected in the signals due to the stagnation layer contribution. The geometrical configuration employed to obtain the collision between the two plasmas (flat-target configuration) allowed to distinguish and study the properties of both seeds and the stagnation layer.
Section snippets
Experimental set-up
The experimental apparatus used in this work is described in detail by Hough in Ref. [11]. The laser is a Surelite III-10 Laser system: it operates at the fundamental wavelength of 1064 nm and emission is in pulsed mode with a maximum energy per pulse of 800 mJ, a FWHM of 6 ns and a maximum repetition rate of 10 Hz. The Laser system was synchronized temporally with the main diagnostic systems using two Stanford DG-535 delay generators with a maximum temporal jitter of 1 ns. All measurements were
Time and spectral resolved emission imaging
A broadband spectral image sequence for different times of expansion is shown in Fig. 5. The three images display the more interesting stages of the collision front evolution: at t = 30 ns we can observe the first collisions between the seed plasma particles that occurs closer to the lower seed. This means that fast particles coming from the upper seed move with higher velocity. At t = 50 ns the stagnation layer is definitively formed, hence determining its glow. At t = 100 ns it further evolves and
Discussion
The flat target colliding configuration used in this work, by employing a new distance D = 2.6 mm between the two seeds plasmas as compared to previously published studies [4], provided a plasma geometry that enabled the study of both seed and stagnation layer evolution separately in time and space. Moreover, higher density layers were obtained comparable to those ones obtained by other authors in similar experimental conditions [15]. From the imaging early stagnation was observed at 30 ns: the
Conclusions
By combining high time and spatial resolved optical spectroscopy with Langmuir probe measurements, we were able to characterize, in terms of electron density and temperature, both the stagnation layer and one seed plasma. The results presented in this paper show that experiments realized with laser produced plasmas in order to study the ES in a stellar-like environment are possible and physically meaningful. Future experiments are needed in order to test the validity of the classical
Acknowledgments
The Authors thank the technical and scientific staff of NCPST, Dr. S. Gammino and the 5th National Committee of the NTA project of LNS-INFN of Catania and acknowledge the Dipartimento di Metodologie Fisiche e Chimiche per lÍngegneria of Catania and the Science Foundation Ireland, under award 07/IN.1/I1771, for financial support.
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