Modeling Ion Flux Coefficient in High Ion Concentrated Dusty Plasmas Using Langevin Dynamics Simulations

The influence of dense and strongly coupled ions on the ion current to a particle is modeled in this work. Langevin Dynamics is used to simulate the motion of multiple ions around a negatively charged grain in a periodic domain. The ion flux coefficient is calculated using the grain-ion collision time distribution. In addition to the ion-ion electrostatic coupling strength Γ=e 2 /(4πε 0 k B T i n i -1/3 ), the ion flux coefficient is also influenced by the diffusive Knudsen number Kn D =(m i k B T i ) 1/2 /f i a p that characterizes the ion-neutral gas interactions (through the scalar friction factor f i =k B T i /D i , D i is the ion diffusion coefficient), a concentration parameter χ=a p /n i -1/3 that compares the size of the grain a p to the mean inter-ion spacing n i -1/3 , where n i is the ion number concentration and Ψ E =-z p Γ/χ that denotes the strength of particle-ion interaction. The influence of Kn D on the structure of the ions and their self-organization is demonstrated by analyzing the ion-ion pair correlation function g 2 (r) and ion velocity distribution function. It is seen that depending on the regime defined by Γ, Kn D , χ, Ψ E the ion velocity distribution function is Maxwellian for Γ<10 and the state resembles that of a gas. For higher values of Γ (strongly coupled states), liquid or solid like behavior is seen and the use of a simple screened Coulomb potential may not be valid in such regimes. The ion flux coefficient model is presented as the ratio of the computed value to that obtained using a dilute charging model to understand the relative increase or decrease due to ion correlated motion. The developed model will be validated by comparing with suitable experimental data from the literature with an intention to extend previously published dilute charging models 1 , 2 for non-thermal plasmas into the high ion concentration regime.