Ballistic and snowplow regimes in J×B plasma acceleration

Thoma, C.; Welch, D. R.; Hughes, T. P.
March 2009
Physics of Plasmas;Mar2009, Vol. 16 Issue 3, p032103
Academic Journal
The J×B acceleration of a one-dimensional uniform plasma slab is analyzed using fully electromagnetic particle-in-cell simulations. Two different regimes of ion dynamics are observed. At relatively high magnetic field values, the ions are accelerated nearly ballistically in a thin sheath at the plasma-vacuum interface and then form a beam which propagates through the downstream bulk plasma. This behavior can be explained by a simple collisionless thin-sheath model. At lower field values the sheath becomes thicker and the ions are collisional at the interface. This leads to “snowplowing” of ion density at the interface. From the electron transport equations for a simple magnetized plasma we can estimate the temperature and effective collisionality in the sheath as a function of magnetic field strength. From this theory we can qualitatively explain the existence of the two regimes. In the simulations the plasma sheath thickness is found to scale somewhat more weakly with magnetic field strength than is predicted by the simple transport theory. We propose that a high Mach number plasma slab may be obtained by the combination of a short accelerator and a strong magnetic field in the collisionless regime.


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