Neutral gas in complex plasmas causes friction and affects the dynamics of dust particle systems

Strongly coupled plasmas exhibit both short-range order and complex motion due to the interactions between different particle species within them. Warm dense matter inside giant planets and ultracold plasmas with temperatures below 10-degree K are examples of interest to astronomers and plasma scientists.

Simulating the dynamics of the many interacting particles inside these systems requires careful tracking of motion and the highly nonlinear forces between the particles. The latest paper by Hanno Kählert used a three-dimensional simulation of charged dust particles interacting with both a Yukawa force, representing the plasma-shielded electric force, and a collisional friction. The simulations revealed detailed collective effects of the plasma on the dust particles—particularly the friction induced by the neutral gas and its effect on the dynamic structure of the dust particle system.

The dynamic structure factor describes the particle distribution in a plasma in terms of spatial and temporal parameters and characterizes the strong coupling of the ions or dust particles. Kählert ran a large number of simulations, each of 3,800 dust particles, with different combinations of coupling and friction to see how these parameters affect the dynamic structure factor at different frequencies and wavenumbers.

He discovered that the dynamic structure factor at small wavenumbers is strongly affected by friction: the sound mode completely disappears, indicating the plasma’s hydrodynamic limit, or small wavenumber limit. Unlike strongly-coupled plasma without friction, dissipation causes long-wavelength fluctuations to become diffusive.

According to Kählert, the new results can be used as a starting point for modelling other types of plasmas that would include considerations of friction. He also plans to apply a similar methodology to frictionless systems, as well as work on developing a theoretical model for the dynamic structure factor in systems with friction to possibly extract transport coefficients.

Source: “Dynamic structure factor of strongly coupled Yukawa plasmas with dissipation,” by Hanno Kählert, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5099579 .