Enhanced laser-energy coupling to dense plasmas driven by recirculating electron currents

Abstract

The absorption of laser energy and dynamics of energetic electrons in dense plasma is fundamental to a range of intense laser-driven particle and radiation generation mechanisms. We measure the total reflected and scattered laser energy as a function of intensity, distinguishing between the influence of pulse energy and focal spot size on total energy absorption, in the interaction with thin foils. We confirm a previously published scaling of absorption with intensity by variation of laser pulse energy, but find a slower scaling when changing the focal spot size. 2D particle-in-cell simulations show that the measured differences arise due to energetic electrons recirculating within the target and undergoing multiple interactions with the laser pulse, which enhances absorption in the case of large focal spots. This effect is also shown to be dependent on the laser pulse duration, the target thickness and the electron beam divergence. The parameter space over which this absorption enhancement occurs is explored via an analytical model. The results impact our understanding of the fundamental physics of laser energy absorption in solids and thus the development of particle and radiation sources driven by intense laser-solid interactions.

Energy absorption and coupling to electrons in the transition from surface- to volume-dominant intense laser–plasma interaction regimes

“…The measured photostimulated luminescence (PSL) values of the energy stored in the Image Plates is linearly correlated to the number of electrons depositing energy at each of these threshold values [31]. Further details of the experiment, including the calibration of the energy response of the diagnostics, is provided in reference [20].…”

“…Figures 5(c) and (d) show the corresponding numbers of electrons with energy >1 MeV (integrated in the Y dimension from −2.5 μm to 2.5 μm) for the l = 376 nm and l = 40 nm cases, respectively. For l = 376 nm, these high energy electrons are generally trapped within the target, refluxing or recirculating between the electrostatic fields that build up on the surfaces [20,21,33]. For the l = 40 nm target, shortly before the arrival of the peak of the laser pulse a population of relativistic electrons are accelerated and propagate forward with the laser light (this corresponds to the time at which the target becomes relativistically transparent).…”

“…45 • incidence. Gray et al [20] reports on total energy absorption measurements for Al foils with thickness in the range 6-20 μm, for a similar intensity range, but for longer laser pulse duration (∼700 fs) to explore the role of the recirculation of electrons within the foil on absorption [21]. In both of these studies, the targets remain opaque to the laser light, resulting in reflection and energy coupling to electrons via processes such as resonance absorption and J × B heating [22], primarily in the region of the critical density surface.…”

Inverse Bremsstrahlung absorption coefficient in inhomogeneous plasma with nonextensive electron velocity distribution and Tsallis exponential electron density profile

“…Different computer simulation research works are carried out on the IB absorption of the laser in plasmas [13][14][15] to describe how the laser energy is absorbed by plasma, all based on the assumption that plasma electrons are in equilibrium and obey Maxwellian distribution function, an equilibrium which be held when there is no laser irradiation. The laser will certainly alter the underlying plasma conditions.…”

Effect of rear surface fields on hot, refluxing and escaping electron populations via numerical simulations

“…The laser to electron conversion efficiency has been found to scale as a function of intensity [1,2] . The absorption efficiency depends on numerous laser pulse and plasma parameters, such as scale length of the preformed plasma [3,4] and focal spot size [5] , each of which changes how the laser energy is coupled to the electrons. The accelerated electrons typically have a thermal/Maxwellian or relativistic Maxwellian distribution of energies whose temperature directly scales to the on-shot laser intensity [6][7][8] , with temperatures from ≈100 keV to several MeV for the highest achievable intensities [9][10][11][12][13][14] .…”