A New Method of Laser Pulse Compression in Plasma for Next Generation High Power Lasers

M.S. Hur

The CPA (chirped pulse amplification) technique is rapidly approaching its technological limitation. Currently up to petawatt peak power carried by a single laser pulse is available from CPA. However, the compression gratings of meter scale is vulnerable to the damage already in petawatt regime, and going toward exawatt or more by increasing the diameter of gratings to hundreds of meters is impractical. Using a plasma for pulse compression is one promising alternative to the conventional solid material gratings, because the plasma is damage-free and at the same time, dispersive. Diverse plasma-based methods have been studied: Raman backward amplification, Brillouin amplification, and transient plasma gratings. Those methods have demonstrated their potential as novel compression methods, but the efficiency is still very low and complicated experimental technique is required. Here we present a totally different approach to obtain high intensity compression in a plasma. In previous methods, the alternating layers of plasma density were produced to be used as a reflection gratings. However, the plasma is intrinsically a dispersive medium and can work as a dispersion gratings without the density modulation. Here we use a smooth gradient, high density (around critical) plasma, where a long negatively chirped pulse is incident [1]. The low frequency photons in the pulse front are reflected from the shallow where the density is low, while high frequency photons in the pulse tail are reflected from deep, high density region of the plasma, taking long reflection paths. The path difference of the photons result in concentration of photons into a narrow region. In this way, from 1D PIC simulations, we obtained 225 times pulse compression with peak intensity of order 1017W/cm2, which is orders of magnitude larger than the intensity sustainable by solid state of materials. We discuss how to suppress or detour various adverse instabilities.