Author: Scott D Baalrud
Requested Type: Poster
Submitted: 2016-02-15 23:08:42
University of Iowa
413 Van Allen Hall
Iowa City, IA 52242
Electrons run away in the presence of an external field if the electrostatic driving force exceeds the friction force arising from collisions. Since the friction force associated with Coulomb collisions scales inversely with energy, it is electrons on the tail of the distribution that are susceptible to runaway. The threshold energy beyond which runaway occurs is typically characterized by either the Dreicer  or avalanche  electric fields. Previous analysis has calculated these threshold fields using Landau-type collision theories, which treat particle interactions via a Coulomb interaction that is statically screened at the Debye length. Here, these calculations are revisited using Lenard-Balescu theory, which treats screening in a more generally manner via the linear dielectric response function. For fast electrons on the tail of the distribution, this includes wake fields that substantially influences the force through which particles interact. Wakes lead to increased collision rates in comparison with the standard theories, and correspondingly larger threshold electric fields. A comparison of the results of each collision model is made for densities and temperatures characteristic of fusion plasmas.
 H. Dreicer, Phys. Rev. 115, 238 (1959).
 M. N. Rosenbluth and S. V. Putvinski, Nuclear Fusion 37, 1355 (1997).