Time-Resolved Observation of Ultrahigh Intensity Laser-Produced Electron Jets Propagating through Transparent Solid Targets

Abstract

We report on shadowgraphic measurements showing the first space-and time-resolved snapshots of ultraintense laser pulse-generated fast electrons propagating through a solid target. A remarkable result is the formation of highly collimated jets (,20-mm) traveling at the velocity of light and extending up to 1 mm. This feature clearly indicates a magnetically assisted regime of electron transport, of critical interest for the fast ignitor scheme. Along with these jets, we detect a slower (ഠc͞2) and broader (up to 1 mm) ionization front consistent with collisional hot electron energy transport. 52.60. + h The fast ignitor scheme, which claims to relax some of the constraints hampering the standard approaches to inertial confinement fusion, has triggered a worldwide interest since its inception [1]. It hinges on the rapid additional heating of the core of a precompressed thermonuclear pellet due to the slowing down of a bunch of relativistic electrons generated by an ultraintense laser pulse. Now, the highly overcritical plasma surrounding the core should prevent any laser pulse from reaching it, whatever highintensity penetration mechanisms are at work (relativistic self-induced transparency [2] or ponderomotive hole boring [3]). An encouraging point is that particle-in-cell simulations predict a rather peaked hot electron distribution in the vicinity of the laser-solid interaction zone [4]. However, an efficient heating of the core requires the electron beam to remain collimated up to its final absorption zone, i.e., on a distance of several hundreds of microns. This can be achieved only through the pinching effect of the beam-driven magnetic field competing with multiple scattering. Therefore, fast electron transport from moderately to extremely dense regions appears as a key issue for the success of fast ignition, which must be thoroughly tackled both experimentally and theoretically.Over the past year, there has been a growing body of experimental evidence pointing to the existence of very collimated high intensity laser-produced electron jets traveling through solid targets. Tatarakis et al. have recently observed a narrow expanding plasma at the rear surface of thick plastic slabs irradiated by a 1 ps, 10 19 W͞cm 2 laser pulse [5]. By using a 2D Fokker-Planck hybrid code, they interpreted this localized rear heating as a magnetic field-enhanced electron energy deposition at the target/vacuum interface [6]. This effect has also been detected in other experiments [7]. Though very encouraging, these studies still provide an incomplete experimental picture of the phenomena arising in the bulk of the target.In the present paper, we report on optical shadowgraphic results showing what is, to our knowledge, the first comprehensive set of space-and time-resolved snapshots of fast electrons propagating through a solid target. In order to bypass the classical limitation of optical probing into an overcritical solid target, we use transparent glass slides. Our measurements pinpoint the existence of two types of fast...

High-energy ion generation in interaction. of short laser pulse with high-density plasma

“…The latter experiments have been performed using green light of approximately the same pulse duration with laser intensities up to 4 × 10 18 W/cm 2 and 10 19 W/cm 2 , correspondingly. However, the penetration depth of hot electrons in [24] was more than two orders of magnitude longer than in [25]. Two possible disparate scenarios may explain these controversies in high-energy electron and ion generation.…”

“…For instance, in [27], which models the transport of fast electrons generated by a 20-J laser, it was shown that only 0.6 J are required to ionize a target of 250-µm thickness. Note that the experiment in [24] was performed with an order of magnitude higher laser energy (and so larger prepulse energy) than the experiment in [25], which displayed a dramatic difference in the hot-electron penetration depths. The aim here is to see how both underdense preplasma scale length and solid dense plasma thickness determine the efficiency of ion generation.…”

“…We believe both situations are possible, depending on laser-pulse characteristics and foil-material conductivity. There is not only a controversy about the origin of the high-energy ions but also a dramatic disagreement on the hot-electron penetration in solid targets [24,25]. The latter experiments have been performed using green light of approximately the same pulse duration with laser intensities up to 4 × 10 18 W/cm 2 and 10 19 W/cm 2 , correspondingly.…”

Direct observation of electron propagation and dielectric screening on the atomic length scale

“…resolving studies on electron propagation in condensed matter employed laser-based spectroscopic techniques to reveal ballistic currents and drift motion of charge carriers [6][7][8][9] .…”

Investigations of Field Dynamics in Laser Plasmas with Proton Imaging

“…Additionally, electromagnetic and electrostatic instabilities can arise, e.g. the Weibel instability [59][60][61][62][63] and electron filaments can be generated. When the electrons reach the rear side of the target only the fastest electrons (precursor electrons) can escape since a potential will be build up which hinder the electrons to escape.…”