*2.4. VISAR*

The rear free-surface velocity of pure aluminum foils was measured with VISAR [16]. It is an optical diagnostic tool allowing the characterization of shock waves via the obtention of the back face velocity of the sample. It is a non-contact acquisition method which can be used with water or solid confinement. The device is a Michelson-type interferometer measuring the Doppler shift in the wavelength of a probe laser reflected on the rear free surface of a target which is moved as a result of the induced shock wave.

The PIMM VISAR is composed of two main parts: the laser probe, which is a continuous-wave laser (532 nm, 5 W) from the Coherent company (Santa Clara, CA, USA), and a home-made interferometer. The laser probe is focused on the back face of the target on the center of the laser spot of the incident laser (Figure 1). The wavelength shift induced by the rear free-surface deformation with the shock-wave propagation is then transmitted to the interferometer part, where the signal interferes with itself. This device has already been used and described for plasma characterization in the direct regime [17] and in the water-confined regime [18].
