Identification of streaming instabilities in the presheath of plasmas with two ion species

A recent theory proposes that ion-ion two-stream instabilities can arise in the presheath of plasmas with two ion species under certain conditions, and that these instabilities rapidly enhance the frictional coupling between the ion species.\footnote{Baalrud, Hegna and Callen, Phys.\ Rev.\ Lett.\ 103, 205002 (2009)} The threshold condition for instability onset along with the multi-species Bohm criterion allowed prediction of the speed of each ion species at the sheath edge. These predictions were later confirmed experimentally.\footnote{Yip, Hershkowitz and Severn, Phys.\ Rev.\ Lett.\ 104, 225003 (2010).} However, recent work has questioned the validity of this theory based on PIC simulations that did not observe instabilities under conditions similar to the experiment.\footnote{Gudmundsson and Lieberman, Phys.\ Rev.\ Lett.\ 107, 045002 (2011).} Using numerical solutions for the dispersion relation, we show that this discrepancy is due to a lower electron temperature in the simulations. This identifies an inaccuracy with an approximate instability criterion that predicted instability for the simulation parameters.\footnote{Baalrud, Hegna and Callen, Phys.\ Rev.\ Lett.} We thoroughly test this numerically. Additionally, for the first time we identify the ion-ion two-stream instabilities in the presheath using PIC simulations.