Enhancing the Number of High-Energy Electrons Deposited to a Compressed Pellet via Double Cones in Fast Ignition

Abstract

Particle-in-cell simulations aimed at improving the coupling efficiency of input laser energy deposited to a compressed core by using a double cone are described. It is found that the number of high-energy electrons escaping from the sides of the cone is greatly reduced by the vacuum gap inside the wing of the double cone. Two main mechanisms to confine high-energy electrons are found. These mechanisms are the sheath electric field at the rear of the inner cone wing and the quasistatic magnetic field inside the vacuum gap. The generation mechanism for the quasistatic magnetic fields is discussed in detail. It is found that the quasistatic fields continue to confine the high-energy electrons for longer than a few picoseconds. The double cones provide confinement and focusing of about 15% of the input energy for deposition in the compressed core.

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

“…Such targets have indeed been shown to enhance the number of electrons emitted toward the precompressed core. 30 In order to characterize the fast electron beam emitted at the cone tip, we follow the trajectories of about 8 ϫ 10 6 quasiparticles, which initially belong to the cone or to the preplasma. At each time step, we extract the particles crossing the cone tip ͑at z = 30.6 m͒ with an energy threshold of 500 keV.…”

“…They are finally emitted toward the cone axis and participate in the enhancement of the hot electrons yield. 30 The energy spectra of the fast electron beam crossing the cone tip are shown in Fig. 9 for the three different intensities.…”

Control of an electron beam using strong magnetic field for efficient core heating in fast ignition

“…As for the beam divergence, however, it is difficult to control the angular spread of fast electrons since laserplasma interactions are the strongly-non-linear phenomena. Instead of reducing the angular spread, some ideas of the guiding of the fast electron beam with large angular spread have been proposed, e.g., the double cone [4][5][6] and the resistive guiding [7][8][9][10][11][12][13]. Those are based on using of self-generated magnetic fields.…”

Integrated simulation of magnetic-field-assist fast ignition laser fusion

“…By improving the contrast ratio of heating laser pulse, we have eliminated the plasma formation inside the guiding cone before the main part of the heating pulse comes, and successfully reduced the energy (or temperature) of fast electrons with keeping the energy convergence from laser to relativistic electron beam (REB) as constant [4]. As for the beam divergence, instead of the control of laser-plasma interactions to supress the angular spread of the fast electron distribution function, some guiding ideas for fast-electron beam with large beam divergence have been proposed, e.g., guiding schemes using self-generated magnetic fields (resistive guiding [5][6][7][8][9][10][11] and vacuum gap guiding [2,12,13]) and using the externally-applied magnetic fields [14][15][16][17][18]. In the present paper, the latter scheme is discussed.…”