The effect of pre-plasma on core heating in cone-guiding fast ignition is evaluated by two-dimensional particle-in-cell (PIC) and Fokker–Planck (FP) simulations. If the long-scale pre-plasma exists in the cone, the generated fast electron energy becomes too high for effective core heating. As a result, the energy coupling from laser to core ηL→core is reduced by 80% compared with the case without a pre-plasma. Even for the case without a pre-plasma, ηL→core obtained in the simulation is smaller than that required for 5 keV heating in FIREX-I. In order to enhance ηL→core, we propose a new target design ‘extended double cone with short inner cone wall’ for fast electron guiding to imploded core and show sufficient improvement of heating efficiency using this new cone on the basis of PIC and FP hydro-simulations.
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.
The energy distribution of the produced high energy electrons in the interaction of ultraintense picosecond laser pulses with high-Z solid targets is shown to be sensitive to the preformed plasma created by the prepulse and the amplified spontaneous emission pedestal. The created preformed plasmas, which are obtained by radiation hydrodynamic simulations for the present heating laser system at ILE, Osaka University, are seen to extend up to 30–100 μm just before the arrival of the main pulse. The dependences of the coupling efficiency of the laser energy to high energy electrons, and the energy spectra of these accelerated electrons, on this preformed plasma, are studied via a two-dimensional particle-in-cell simulation code. It is found that in a small preformed plasma case, J×B heating is dominant and the produced electron temperature agrees well with Haines’ scaling law [Haines et al., Phys. Rev. Lett., 102, 045008 (2009)]. While in a large preformed plasma case, in addition to J×B heating and/or vacuum heating, other acceleration mechanisms, such as stochastic heating, can accelerate electrons to very high energies, carrying a significant fraction of input laser energy. Even after several picoseconds, the number of high energy electrons (0.5 MeV<E<5 MeV) generated in a small preformed plasma case can be several times larger than that of a large preformed plasma case.
Hot electrons generated by short-pulse-laser interaction with nanolayered target (NT) are investigated using two-dimensional particle-in-cell simulation. Compared to the planar target, the NT leads to more efficient conversion of laser energy to the kinetic energy of the accelerated electrons. However, the energy absorption by the NT decreases at both too-low and too-high laser intensities. At lower laser intensities it is because of the weaker electric and magnetic fields generated by the hot-electron jets and smaller relativistic skin depth. At higher laser intensities it is because of the damage or destruction of the layered structure by the laser field. On the other hand, the dependence of the conversion efficiency and hot-electron number on the duration of the (short) laser pulse and the nanolayer length is weak. Control of the hot-electron characteristics by tailoring the parameters of the laser and the NT is discussed.
Channeling by a train of laser pulses into homogeneous and inhomogeneous plasmas is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy. The increased penetration distance can be attributed to the repeated actions of the ponderomotive force, the continuous between-pulse channel lengthening by the inertially evacuating ions, and the suppression of laser-driven plasma instabilities by the intermittent laser-energy cut-offs.
An analytical fluid model is proposed for artificially collimating fast electron beams produced in the interaction of ultraintense laser pulses with specially engineered low-density-core-high-density-cladding structure targets. Since this theory clearly predicts the characteristics of the spontaneously generated magnetic field and its dependence on the plasma parameters of the targets transporting fast electrons, it is of substantial relevance to the target design for fast ignition. The theory also reveals that the rapid changing of the flow velocity of the background electrons in a transverse direction (perpendicular to the flow velocity) caused by the density jump dominates the generation of a spontaneous interface magnetic field for these kinds of targets. It is found that the spontaneously generated magnetic field reaches as high as 100 MG, which is large enough to collimate fast electron transport in overdense plasmas. This theory is also supported by numerical simulations performed using a two-dimensional particle-in-cell code. It is found that the simulation results agree well with the theoretical analysis.
The effects of the imposed uniform magnetic field, ranging from 1 MG up to 50 MG, on the production and transport of relativistic electron beams (REBs) in overdense plasmas irradiated by ultraintense laser pulse are investigated with two-dimensional particle-in-cell numerical simulations. This study gives clear evidence that the imposed magnetic field is capable of effectively confining the relativistic electrons in space even when the source is highly divergent since it forces the electrons moving helically. In comparison, the spontaneous magnetic fields, generated by the helically moving electrons interplaying with the current filamentation instability, are dominant in scattering the relativistic electrons. As the imposed magnetic field was increased from 1 MG to 50 MG, overall coupling from laser to the relativistic electrons which have the potential to heat the compressed core in fast ignition was found to increase from 6.9% to 21.3% while the divergence of the REB increases significantly from 64 to 90 . The simulations show that imposed magnetic field of the value of 3-30 MG could be more suitable to fast-ignition inertial fusion. V C 2013 AIP Publishing LLC. [http://dx.
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