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Plasma jets collimation by strong magnetic fields

An experiment was carried out using the ELFIE laser facility of LULI at Ecole Polytechnique (France) in order to test collimation of high-power produced plasma jets by strong magnetic fields, a topic of interest for so-called laboratory astrophysics.

For this experiment we have designed and build a 40 T split pair magnet, designed to operate around 300 K, with two cylindrical accesses under vacuum of 5 mm diameter. One, in the magnet bore, will allow the target fixation and the laser access. The other, in the mid plane, is for diagnostic.

The laser beam (pulse 50 J of 700 ps FWHM, 1.053 µm) was focused, at the centre of the split coil, on a planar target. Various configurations were used, to study the influence of the intensity, in the range 1012-1014, on the formation and evolution, with or without magnetic field, of the plasma. By varying the delay between the main laser beam and the probe laser beam, we were able to follow the evolution in time of the plasma jet. As evidenced in the figure below, a strong collimation of the plasma was observed when applying strong magnetic field to the expanding plasma.

(a) Picture of the coil compartment inside the vacuum chamber on the laser line and raw interferometric data obtained for Cu 250 µm thickness. The image is taken 5 ns after the beginning of the interaction (b) without external magnetic field (c) with a 20 T magnetic field added parallel to the plasma expansion axis. Plasma density computed using Abel inversion (d) for the image shown in of (b) and (e) for the image shown in (c).

These first results demonstrate the importance of strong magnetic field in collimating high-energy density plasma jet, a topic of interest to test models of astrophysical jets evolution.

Contact: Jérôme Béard