Isolated unstable Weibel modes in unmagnetized plasmas with tunable asymmetry

In this letter, an initially unmagnetized pair plasma with asymmetric velocity distributions is investigated where any unstable Weibel mode must be isolated, with discrete values for the growth rates and the unstable wavenumbers. For both a non-relativistic distribution with thermal spread and a high-relativistic two-stream distribution it is shown that isolated modes are excited and that, as the asymmetry tends to zero, the growth rate remains finite, as long as the distribution function is not precisely symmetric.