Dynamics of Electric Fields Driving the Laser Acceleration of Multi-MeV Protons

Abstract

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (approximately 10(18) W/cm2) and short (approximately 1.5 ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

Investigations of Field Dynamics in Laser Plasmas with Proton Imaging

“…It was assumed that the field between the moving front and the droplet is zero. This is due to the fact that only a homogenous field between front and target exists which decreases very fast (∼ t −2 ) [47,105]. The adjacent droplets are field ionized by the field of the charged central droplet and ionized by the wings of the laser beam.…”

“…The electric field peaks directly at the ion front [47] which expands with an assumed velocity of about 10 6 − 10 7 m/s. The field is strongly localized and decreases in time (∝ t −1 ) [47,105] and is responsible for the fast and strong increasing and decreasing deflection.…”

“…It can be used to visualize density gradients, for example in shocked material [110,111] or to detect electric and magnetic fields generated by laser matter interaction (proton deflectometry) [102,112]. With proton imaging the ion acceleration process itself [105,113] was investigated recently. The electrostatic acceleration field and the field of the expanding ion front as well as toroidal magnetic fields [114][115][116] were imaged.…”

“…Up to now there is no real experimental evidence of the shape of these proton layers. In reference [103] an inverse parabolic shape is assumed in difference to other publications [98,104,105] which suppose a gaussian shape. In the central part of the beam both functions are nearly similar.…”

“…For a quantitative analysis of Fig. 11.2 C, with a 2.3 J incident laser pulse, a model was used on the basis of [105] with an additional charge of the cylinder.…”