R." Horiuchi

T. "Moritaka

National Institute for Fusion Science (NIFS-Repository)

    Recent high-intensity laser technology provides a new experimental approach to high-energy plasma dynamics strongly coupled with gamma-ray emission and pair production. Such dynamics play an important role in plasma phenomena around active astronomical objects such as neutron star magnetospheres and accretion discs around black holes.  Previously, pair production experiments have been carried out using gold target1,2). Basic process leading to the pair production would be Bethe-Heitler process in nuclear field: gamma ray emission from high-energy electron via Bremsstrahlung and pair production from the emitted gamma ray3). This process is enhanced in strong nuclear field of high-Z material, such as gold. Systematic experimental data have been obtained about positron energy spectrum and their dependency on experimental conditions such as laser pulse duration and target thickness. These results are, however, not fully described by current numerical simulations. This is because traditional simulation schemes cannot address full picture of positron generation mechanism, which could be a combined process of collective plasma dynamics and quantum stochastic reactions. A new simulation scheme has been developed to demonstrate the laser-plasma interaction and resulting pair production process in solid target4). The simulation scheme is based on relativistic particle-in-cell method5). In addition, following two calculation processes are integrated: (1) a conservative semi-Lagrangian scheme (CIP method6)) for gamma ray transport and (2) a Monte-Carlo scheme for Bremsstrahlung and pair production. The gamma ray transport is described by a fluid description in 5-dimensional real + momentum space. Figure 1 gives schematic diagram of calculation cycle.  The developed scheme is applied to a test simulation relevant to the positron production experiment. The simulation results show a basic process leading to the positron ejection from the target: (1) Electron acceleration due to laser-plasma interaction in the under-dense region at the target surface (2) Bremsstrahlung from the accelerated electrons and pair production from gamma ray inside the solid target (3) Positron acceleration toward the laser propagation direction due to sheath electric field at the target rear side. Figure 2 shows electron density profile at ct = 42[µm], where c denotes light speed. Pulse laser penetrates under-dense region at the target surface and electrons are accelerated in this region. Figure 3 shows positron density profile at ct = 82[µm]. Density is normalized to critical density Ncr for the incident pulse laser. Positrons mainly locate inside the high-density target region. Some positrons are ejected from the center of target rear side where sheath electric field is dominant.   1) T. Cowan et al, Laser and Particle Beams 17 773 (1999) 2) H. Chen et al, High Energy Density Physics 7 225 (2011) 3) W. Heitler “The Quantum Theory of Radiation” (The Oxfort University Press) 1954 4) T. Moritaka et al, Journal of Physics: Conference Series 454, 012016 (2013) 5) C. K. Birdsall and A. Langdon “Plasma Physics via Computer Simulation” (Adam Hilger) 1991 6) T. Yabe et al, Mon. Wea. Rev. 129 332 (2001)   Fig. 1 : Schematic diagram of the simulation scheme Fig. 2 : Spatial profile of electron density (ct = 42[µm])   Fig. 3: Spatial profile of positron density (ct = 82[µm]) In plasmas, dust particles are negatively charged up to form so-called ``plasma crystals'' under the strong Coulomb interaction among the dust particles. There have been many experiments on the dust particles and various interesting features of dust-particle plasma have been clarified.Moreover, the dust particles have attracted a strong interest due to their important roles as contamination source in the material processing and fusion plasmas. It is an essential issue to understand and control the collective motion of dust particles levitating in plasmas in order to remove dust particles from the processing and fusion plasmas.Particularly, the control of an electrostatic field is quite important, because the large charged particles are very sensitive to the electric field in plasma, and new dust-particle structures such as vortices and void are generated and controlled by an appropriate potential structure.In order to investigate the effect of ion drag force on dust transport, we carry out a two-dimensional numerical calculation on the dust-particle orbit by solving the equation of motion of dust particles, including the effects of thepotential structure and the ion drag force. The equation of motion of dust particle is given by the following equation,First term in the right-hand side shows the electrostatic interaction among dust particles. The inter-particle potential is assumed to be the Yukawa type. Second term represents the electrostatic force acting on the dust particle due to the external potential φe. We assume a parabolic confining potential in radial direction. Third term shows the ion drag force Fi due to the Coulomb collision between ion and dust particle. Some models on Fi has been reported and discussed and we adopt the simple model by M. S. Barrnes [1]. Fi is given by the sum of the force Fic due to the collection of positive ion and the force Fio due to the Coulomb interaction (no collection of positive ion). Fic and Fio are expressed by the following equations, respectively,where vth,i is the ion thermal velocity, vi is the ion drift velocity, vis is (vth,i2 + vi2)1/2, rc is the collection impact parameter, rπ/2 is the impact parameter with orbit angle of π/2, and Γp is the Coulomb logarithm.We calculated on dust-particle motions near the metal plate. In the absence of the ion drag force, the dust particlesstop at their equilibrium positions in a potential well due to energy dissipation from the friction force. When the ion drag force Fi becomes finite, the particles are driven to have finite velocity in the steady state. Figure 1 shows the orbit of dust-particle motion for ion density ni = 7 x 109 cm-3 and dust-particle number Nd = 2000, respectively. Approaching to the metal plate, dust particles are gradually accelerated toward the metal plate edge region from the side surface region to form clockwise and anticlockwise rotations on right and left sides of the metal plate, respectively. This clearly demonstrates that the ion drag force plays an important role on dust-particle behavior in plasma, and detailed analysis of the ion drag force would be quite useful for controlling the dust particles in the processing and fusion plasma.References1) Barnes, S. et al.: Physical Review Letters 68 (1992) 313.Fig. 1: The orbit of dust-particle motion at dust-particle number Nd = 2000 and ion density ni = 7 x109 cm-3.415§13. Simulation Study on Pair Production in Laser-produced PlasmasMoritaka, T. (ILE, Osaka Univ.),  Horiuchi, R.

Simulation Study on Pair Production in Laser-produced Plasmas

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Simulation Study on Pair Production in Laser-produced Plasmas

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