Abstract
The creation of well-thermalized, hot and dense plasmas is attractive for warm dense matter studies. We investigate collisionally induced energy absorption of an ultraintense and ultrashort laser pulse in a solid copper target using particle-in-cell simulations. We find that, upon irradiation by a$2\times 10^{20}~\text{W}\,\text{cm}^{-2}$intensity, 60 fs duration, circularly polarized laser pulse, the electrons in the collisional simulation rapidly reach a well-thermalized distribution with${\sim}3.5~\text{keV}$temperature, while in the collisionless simulation the absorption is several orders of magnitude weaker. Circular polarization inhibits the generation of suprathermal electrons, while ensuring efficient bulk heating through inverse bremsstrahlung, a mechanism usually overlooked at relativistic laser intensity. An additional simulation, taking account of both collisional and field ionization, yields similar results: the bulk electrons are heated to${\sim}2.5~\text{keV}$, but with a somewhat lower degree of thermalization than in the pre-set, fixed-ionization case. The collisional absorption mechanism is found to be robust against variations in the laser parameters. At fixed laser pulse energy, increasing the pulse duration rather than the intensity leads to a higher electron temperature.
Highlights
The creation of warm dense matter (WDM) or hot dense matter (HDM) in a laboratory setting is of high interest for a broad field of research disciplines such as laboratory astrophysics (Remington 2005; Bailey et al 2007; Fujioka et al 2009), studies of planetary interiors (Ross 1981; Knudson, Desjarlais & Dolan 2008), inertial confinement fusion (Drake 2018; Le Pape et al 2018), understanding the equations of state under such extreme conditions
We demonstrate that the energy absorption of an intense short laser pulse in a high-Z∗ solid-density target is mainly due to inverse bremsstrahlung electron heating within the plasma skin layer, and that this scenario holds in a broad range of experimentally relevant parameters
The collisionless simulations only reach an electron temperature of ∼10–100 eV; these electrons are, far from being thermalized and only their energetic tails are visible in the figure. The fact that both circular polarization (CP) and linear polarization (LP) reach very similar bulk electron temperatures when collisions are enabled indicates that the laser absorption mechanism is the same in both cases
Summary
The creation of warm dense matter (WDM) or hot dense matter (HDM) in a laboratory setting is of high interest for a broad field of research disciplines such as laboratory astrophysics (Remington 2005; Bailey et al 2007; Fujioka et al 2009), studies of planetary interiors (Ross 1981; Knudson, Desjarlais & Dolan 2008), inertial confinement fusion (Drake 2018; Le Pape et al 2018), understanding the equations of state under such extreme conditions The generation of WDM/HDM at uniform near-solid density requires that the sample be heated rapidly, i.e. before any significant hydrodynamic expansion Such isochoric heating can be achieved using ultrahigh-intensity, short-pulse lasers, as has been done at various high-power systems (Evans et al 2005; Gregori et al 2005; Martinolli et al 2006; Chen et al 2007; Nilson et al 2009; Pérez et al 2010; Brown et al 2011; Hoarty et al 2013a). Most laser-based isochoric heating experiments conducted so far have exploited the fast electrons driven by a linearly polarized laser pulse (Nilson et al 2010; Santos et al 2017; Sawada et al 2019) Their energy dissipation through the plasma bulk enables heating to high temperatures (0.1–1 keV) at solid-range plasma densities, but usually at the expense of poor spatial uniformity (Dervieux et al 2015) and relatively slow thermalization. Inside the plasma, where the laser field is negligible, collisions cause fast relaxation of the electron distribution to a Maxwellian
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