Measurements by Faraday rotation of magnetic fields internal to plasmas created by interaction with a neodymium laser beam on a plane target

One of the physical parameters, that plays an important role in the dense plasma created by the interaction of a pulsed laser beam with matter, is the magnetic field generated by the medium which reaches theoretically a few megagauss. The aim of this work is to measure the magnitude of such magnetic fields, their location in the plasma using the optical method of Faraday rotation of the angle of polarisation, and that from experimental results obtained by our groups of research. We used as principal laser beam the neodyme glass doped pulsed laser of the 'GRECO' at 'l'Ecole Polytechnique de Palaiseau' with the following characteristics: - 1.06 μm of wavelength; - 100 ps (with possibility to have a prepulse) of time of impulsion; - 9 joules of mean energy. We used as laser probe beam the retrodiffused stimulated Raman-Stockes beam with the following characteristics: - 632.9 nm of wavelength; - 20 ps of time of impulsion; - 1 mjoule of mean energy. The optical experimental arrangement permitted as to measure the Faraday effect by means of photographic plates. The digitalisation of these was done by a microdensitometer with a pitch of 100 μm. The numerical values were recorded on cassettes. A method of programmation was than used to reach the values of the magnetic field versus its position in the plasma. For each plan of incidence of the probe beam, the platting of the magnitude of the magnetic field in the plasma shows a 3 megagauss magnetic field localised in the sub-critical region of the plasma where the electronic density gradient is majored