Observation of Density Dip in a Pre-Formed Plasma: Toward Axial Pumping X-Ray Laser

We report the observation of electron density dip in a laser-produced molybdenum plasma pumped by double laser pulses which are linearly focused on the molybdenum slab target. Interferometer based on Wollaston prism was employed to diagnose the plasma, and we obtained interference pattern together with backlighting image of the plasma. The electron density dip was clearly formed at the time of 1 similar to 1.5 ns after the second pulse, and the electron density of the dip was estimated to be around 2x10(18) cm(-3) - 4x10(18) cm(-3). The existence of the density dip was also verified by the backlighting image, which showed that the probe beam was guided in the plasma column with the plasma length of up to 4.3 mm. The experimental result implies that the present plasma can be used as the pre-formed plasma for the axial pumping x-ray laser.