Generation and optimization of electron currents along the walls of a conical target for fast ignition

Abstract

The interaction of an ultraintense laser pulse with a conical target is studied by means of numerical particle-in-cell simulations in the context of fast ignition. The divergence of the fast electron beam generated at the tip of the cone has been shown to be a crucial parameter for the efficient coupling of the ignition laser pulse to the precompressed fusion pellet. In this paper, we demonstrate that a focused hot electron beam is produced at the cone tip, provided that electron currents flowing along the surfaces of the cone sidewalls are efficiently generated. The influence of various interaction parameters over the formation of these wall currents is investigated. It is found that the strength of the electron flows is enhanced for high laser intensities, low density targets, and steep density gradients inside the cone. The hot electron energy distribution obeys a power law for energies of up to a few MeV, with the addition of a high-energy Maxwellian tail.

High proton energies from cone targets: electron acceleration mechanisms

“…The inset shows the trajectory of an electron (black) injected in a homogeneous quasi-static magnetic field at an angle α, following a circular path with the cyclotron radius R β small enough to be fully confined laterally in the interaction region close to the cone wall surface. cone [6] or the resonant acceleration of electrons along the cone surface [8,12] cannot explain the electron dynamics and temperature seen in these simulations [3].…”

“…Curved-wall hollow micro-cone targets, with a flat top at the tip, were recently shown to increase the proton acceleration compared to flat foil targets and reduced mass targets [3]. We attribute this increase in maximum proton energy to the formation of surface currents along the cone wall, which are observed when the laser is aligned tangentially to the inner cone wall, as seen, for example, in [4][5][6]. These currents are flowing toward the tip and are made up of high-energy electrons.…”

“…This in turn can enhance the proton acceleration from the top as compared to regular flat foils [3]; see figure 1. The conditions necessary to create such currents are a high laser contrast, a high laser pulse intensity and the use of low-density, small Z-targets [5], while the mechanism responsible for the energy increase has remained a subject of debate. Sentoku et al [6] showed that for a certain class of experiments-i.e.…”

High-flux high-energy ion beam production from stable collisionless shock acceleration by intense petawatt-picosecond laser pulses

“…Since the tube only interacts with the wings of the laser where the laser intensity is rather low, the ionization of the tube is also low while the mass-to-charge ratio is rather large, which means ions are almost immobile during the acceleration progress and the expansion of the tube does not influence the near-axis shock formation area. Such assumptions have also been widely used in former research works [49][50][51]. In order to check the simulation convergence, a high resolution (about twice higher) case is also conducted, where the results are almost the same as the results with relative low resolution, as shown in figure 7(c) the gray line.…”

Enhanced Relativistic-Electron-Beam Energy Loss in Warm Dense Aluminum

“…The corresponding fast electron mean energy over the 0.05 ≤ ε h ≤ 20 MeV range isε h = 1 MeV. The power law component of the electron spectrum, already observed in other kinetic simulations [23][24][25][26], significantly impacts K α emission and must be taken into account to reproduce the experiment. This high sensitivity to the low energy part of the electron spectrum is due to the high collisional ionization rate of the inner shells for energies around a 100 keV [see for example the Sn K-shell ionization cross section [31], thin solid line in Fig.…”