Supersonic Ionization Wave Driven by Radiation Transport in a Short-Pulse Laser-Produced Plasma

Abstract

Through the use of an ultrashort (2 ps) optical probe, we have time resolved the propagation of an ionization wave into solid fused silica. This ionization wave results when a plasma is created by the intense irradiation of a solid target with a 2 ps laser pulse. We find that the velocity of the ionization wave is consistent with radiation driven thermal transport, exceeding the velocity expected from simple electron thermal conduction by nearly an order of magnitude. [S0031-9007(96)00542-X] PACS numbers: 52.50.Jm, 52.25.Fi, 52.40.Nk, 52.70.Kz Progress in understanding the interaction of intense, ultrashort laser pulses with plasmas has been quite dramatic in the previous decade [1]. This progress has been fueled by a desire to study plasma conditions at very high temperatures and electron densities, a condition made possible by the development of intense picosecond lasers which can rapidly heat a plasma before significant hydrodynamic expansion can take place [2]. Understanding of the physics behind energy transport mechanisms in these plasmas is central to these studies. This is not only because of the fundamental nature of these mechanisms to the understanding of plasma physics in general, but also because a clear picture of energy transport is required to fully understand the dynamics of ultrashort pulse x-ray generation [3], a very important application of short-pulse produced plasmas [4].One important ultrafast process driven by energy transport is the formation of an ionization wave that propagates with a supersonic velocity into the solid after the target surface has been heated by an intense short pulse [5]. This process has been studied previously at intensities of ,5 3 10 14 W͞cm 2 where the plasmas formed exhibited temperatures of 50 eV or less [6]. In these studies the velocity of the ionization wave was inferred from the Doppler shift exhibited by a probe beam reflected off the expanding ionization front within the cold fused silica substrate. This experiment found that the velocity of the ionization wave could be well explained by standard electron thermal conduction in this low intensity regime.In this Letter we report the first time resolved measurements of the spatial extent of an ionization wave produced with high intensity (up to 10 17 W͞cm 2 ) picosecond pulses. At this intensity we find that the velocity of the ionization wave cannot be explained by simple electron thermal conduction. We find that our measurements are, instead, consistent with radiative thermal conduction. The resulting velocity and depth of penetration of the radiatively driven ionization wave is 1 to 2 orders of magnitude greater than that observed at low intensity. Previous measurements of radiative transport effects have been largely composed of long pulse ͑ϳ1 ns͒ studies where the effects of radiative transport have been inferred by indirect means [7] or by direct measurement of radiative transport effects in thin foils [8,9] or foams [10] driven by a separate lasercreated x-ray source. In these experiments the drive...

Ultrafast radial transport in a micron-scale aluminum plasma excited at relativistic intensity

“…When initial electron temperature kT e 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.…”

“…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 2 -size spot (1.5µm diameter) to excite a metal target surface at relativistic intensity (up to 1.8 × 10…”

“…We modeled the evolution of the electron temperature profile T e (r, z ≤ 0, t) at constant solid density ρ by numerically solving the nonlinear diffusion equation ∂T e /∂t = ∇•(χ∇T e ) with a temperature-dependent thermal diffusivity χ = (κ SH + κ R )/ρc v that included collisional (Spitzer-Härm) conductivity κ SH ~ (kT e ) 5/2 /(Z+1) and radiative conductivity κ R = 16σT e 3 λ R /3 [5], where σ is the Stefan-Boltzman constant and [3,5]. The initial condition was defined by partitioning absorbed pump energy (~ 1 mJ), with Gaussian radial profile, between electron thermal energy kT e and ionization Z(kT e ) assuming Saha equilibrium.…”

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

“…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 2 -size spot (1.5µm diameter) to excite a metal target surface at relativistic intensity (up to 1.8 × 10 18 W/cm 2 ).…”

“…We modeled the evolution of the electron temperature profile T e (r, z ≤ 0, t) at constant solid density ρ by numerically solving the nonlinear diffusion equation ∂T e /∂t = ∇•(χ∇T e ) with a temperature-dependent thermal diffusivity χ = (κ SH + κ R )/ρc v that included collisional (Spitzer-Härm) conductivity κ SH ~ (kT e ) 5/2 /(Z+1) and radiative conductivity κ R = 16σT e 3 λ R /3 [5], where σ is the Stefan-Boltzman constant and λ R [cm] = (9 x 10 6 )T e [ o K] 2 /Zn e [cm -3 ] is a simplified radiative mean free path valid for hydrogenic ions [3,5]. The initial condition was defined by partitioning absorbed pump energy (~ 1 mJ), with Gaussian radial profile, between electron thermal energy kT e and ionization Z(kT e ) assuming Saha equilibrium.…”