Ultrafast Radial Transport In A Micron-Scale Aluminum Plasma Excited At Relativistic Intensity

Using femtosecond microscopy, we observe a thermal/ionization front expand radially at ~10cm/s from a λ-size spot of an aluminum target excited at >10W/cm. Numerical modeling shows transport is predominantly radiative and may be initially nonlocal. Excitation of solid targets with laser pulses of relativistic intensity has recently emerged as a promising method for producing collimated beams of MeV protons and ions [1,2]. In order to tailor the energy, energy spread and quantity of particles in a beam to a specific scientific query or engineering application the physics between initial laser energy deposition and emission of an ion beam must be fully understood. To this end, we present an experiment in which ultrafast energy deposition on any target material is optically probed to observe the dynamics of energy flow in the target with very high temporal and spatial resolution. Intense, high contrast femtosecond (fs) laser pulses deposit energy into the electrons of a solid faster than it escapes from the initially-excited volume and much faster than the target surface expands hydrodynamically. When initial electron temperature kTe exceeds several hundred eV radiative heat transport begins to dominate over collisional transport [3]. The physics of this regime underlies production of MeV proton and ion beams, as well as energy transport in stars and ultrashort pulse x-ray generation. Past experiments in this regime [3] used loosely focused, ~1J, ~1ps pump pulses and probed the target transversely in transmission, and thus were restricted to observing late stages of 1D radiative transport in an optically transparent material on a time scale of tens of picoseconds. We present new measurements using 1 mJ, 24 fs pump pulses focused to a diffraction-limited λ-size spot (1.5μm diameter) to excite a metal target surface at relativistic intensity (up to 1.8 × 10W/cm). We probe the target in reflection through microscope optics. This geometry enables us to observe the earliest stages of radiative transport in 2D on any target material on a sub-picosecond time scale. New features of radiative transport are expected on this space-time scale. For example, simple transport models in the