Measurement of the electron density produced by the prepulse in an experiment of high energy proton beam generation

The characteristics of the preplasma, such as the electron density and its profile, were investigated from a measurement of the phase shift due to the preplasma in an experiment on high energy proton beam generation. The electron density and its profile were calculated from the measured phase shifts, and their dependences on the prepulse intensity and the time delay were investigated. Although the size of the electron density depended on the prepulse intensity, the maximum electron density ranged from 1.5 x 10(19) to 2.1 x 10(19) cm(-3), irrespective of the prepulse intensity and the time delay in the picosecond range. The analysis showed that a preplasma having an electron density of similar to 10(19) cm(-3) existed on a target before the main laser pulse arrived at the target for the generation of high energy proton beams.