Abstract
Using 1D and 2D PIC simulations, we describe and model the backward ejection of electron bunches when a laser pulse reflects off an overdense plasma with a short density gradient on its front side. The dependence on the laser intensity and gradient scale length is studied. It is found that during each laser period, the incident laser pulse generates a large charge-separation field, or plasma capacitor, which accelerates an attosecond bunch of electrons toward vacuum. This process is maximized for short gradient scale lengths and collapses when the gradient scale length is comparable to the laser wavelength. We develop a model that reproduces the electron dynamics and the dependence on laser intensity and gradient scale length. This process is shown to be strongly linked with high harmonic generation via the Relativistic Oscillating Mirror mechanism.
Highlights
The interaction of ultra-high intensity lasers with overdense plasmas is of particular importance for applications such as fast ignition for inertial confinement fusion,1,2 ion acceleration,3,4 and high harmonic generation (HHG)
High harmonic generation in this regime is described by the Relativistic Oscillating Mirror (ROM): electrons from the front surface of the solid target form a very dense sheet of electrons which is driven by the intense laser pulse and radiates high harmonics
Harmonic emission via the ROM mechanism is efficient in the plasma mirror regime, i.e., when the laser impinges the target at oblique incidence in p-polarization on an overdense plasma with a very short density gradient
Summary
The interaction of ultra-high intensity lasers with overdense plasmas is of particular importance for applications such as fast ignition for inertial confinement fusion, ion acceleration, and high harmonic generation (HHG). High harmonic generation in this regime is described by the Relativistic Oscillating Mirror (ROM): electrons from the front surface of the solid target form a very dense sheet of electrons which is driven by the intense laser pulse and radiates high harmonics. Harmonic emission via the ROM mechanism is efficient in the plasma mirror regime, i.e., when the laser impinges the target at oblique incidence in p-polarization on an overdense plasma with a very short density gradient. Experiments have shown that the electron emission is efficient when the gradient scale length is around L ’ k0=10, where k0 is the laser wavelength.10 Such solid density plasmas with very short gradients (L < k0) are referred to as plasma mirrors because they essentially behave like mirrors that reflect the incident laser pulse with high reflectivity and little spatial deformation, provided the intensity is not too large..
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