Friction force in strongly magnetized plasmas

A charged particle moving through a plasma experiences a friction force that commonly acts antiparallel to its velocity. It was recently predicted that in strongly magnetized plasmas, in which the plasma particle gyrofrequency exceeds the plasma frequency, the friction also includes a transverse component that is perpendicular to both the velocity and Lorentz force. Here, this prediction is confirmed using molecular-dynamics simulations, and it is shown that the relative magnitude of the transverse component increases with plasma coupling strength. This result influences single-particle motion and macroscopic transport in strongly magnetized plasmas found in a broad range of applications.

Figure 1

A comparison between theoretical predictions and simulation results for the transverse force ($F_{\times }$) [panels (a), (d), and (g)], stopping force ($-F_v$) [(b), (e), and (h)], and force in the Lorentz force direction ($-F_n$) [(c), (f), and (i)]. Parameters $\beta$ and $\theta$ for each row are indicated. The black dotted line marks 0. $\mathrm{\Gamma }=0.1$ for these simulations.

Figure 2

Electrostatic potential maps (wakes) from MD simulation are shown in panels (a) and (c) ($\beta =10$), and those from linear-response theory in the large-$\beta$ limit [1] are shown in panels (b) and (d). In panels (a) and (b), $\theta =0^{\circ }$. In panels (c) and (d), $\theta =22.5^{\circ }$. The positions of the projectiles are marked with white dots, and the arrows show the velocity orientation and direction ($V_0=2v_T$).

Figure 3

(a) Individual force time series (along the $z$-direction) for four different simulations (with $\beta =10$, $\theta =0^{\circ }$, and $V_0=2v_T$). (b) Distribution of $3\times {10}^4$ time-averaged force measurements compared with a best-fit Gaussian. (c) Cumulative average of the time-averaged force measurements with simulation number. $\mathrm{\Gamma }=0.1$ for all panels.

Figure 4

Simulation results (blue dots) for forces when $\mathrm{\Gamma }=1$ and $\beta =10$. The dotted line marks 0. Panels (a) and (b) show $F_{\times }$, (c) and (d) show $-F_v$, and (e) and (f) show $-F_n$. $1\times {10}^4$ particles were used for these simulations (fewer than the $\mathrm{\Gamma }=0.1$ simulations).