Abstract
Ultraintense laser-driven relativistic electrons provide a way of heating matter to high energy density states related to many applications. However, the transport of relativistic electrons in solid targets has not been understood well yet, especially in dielectric targets. We present the first detailed two-dimensional particle-in-cell simulations of relativistic electron transport in a silicon target by including the field ionization and collisional ionization processes. An ionization wave is found propagating in the insulator, with a velocity dependent on laser intensity and slower than the relativistic electron velocity. Widely spread electric fields in front of the sheath fields are observed due to the collective effect of free electrons and ions. The electric fields are much weaker than the threshold electric field of field ionization. Two-stream instability behind the ionization front arises for the cases with laser intensity greater than $5\times 10^{19}~\text{W}/\text{cm}^{2}$ that produce high relativistic electron current densities.
Highlights
The transport of high-current relativistic electron beams driven by ultraintense laser interactions with plasmas is relevant to many applications of high energy density physics, in areas of the fast ignition scheme for inertial confinement fusion[1], laser-driven ion acceleration[2, 3] and production of ultrashort radiation sources[4,5,6,7]
A target can be ionized by relativistic electrons both through field ionization and collisional ionization, inducing nonlinear and collective effects that can feed back to the transport of relativistic electrons
The velocity of the ionization wave increases with laser intensity but is much less than the speed of light and that from the 1D theoretical analysis, indicating that 2D3V numerical simulations are better to describe the relativistic electron transport in dielectric targets
Summary
The transport of high-current relativistic electron beams driven by ultraintense laser interactions with plasmas is relevant to many applications of high energy density physics, in areas of the fast ignition scheme for inertial confinement fusion[1], laser-driven ion acceleration[2, 3] and production of ultrashort radiation sources[4,5,6,7]. It is important to comprehensively investigate the transport process of relativistic electrons in such a target, especially in insulators that are without free electrons initially. The transport of relativistic electrons can be inhibited by charge separation fields until a cold return current is generated or the electrons from ionization neutralize space charge fields. The propagation velocity of the ionization front in insulators depends on the relativistic electron energy and current density. The previous numerical simulations on high-current relativistic electron beam inducing ionization in targets were mainly limited to 1D PIC simulations due to the highly computing resource requirement.
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