Imaging of THz oscillating electron plasma wave

Summary form only given. The oscillation period of the electron plasma wave in the "standard" laser wake field, which can be used for the laser accelerator, is longer than the pulse width of the pumping laser pulse. Then that is thought to be easier to control than the self-modulated laser wake field. In order to control the electron plasma wave, it is necessary to directly observe the plasma oscillation. Several previous papers have reported the successful observations. In the paper, we expanded their method to two-dimensional (2D) observation. A 0.6 TW Ti:sapphire laser pulse ionized the helium gas of 0.4 Torr near the focus, and excited the electron plasma oscillation. The period of the oscillation T/sub 0/ was calculated to be 680 fs. We observed the electron density perturbation in the electron plasma oscillation by the frequency domain interferometry technique.