  We present the first 2X2V continuum Vlasov-Maxwell simulations of
interpenetrating, unmagnetized plasmas to study the competition between
two-stream, Oblique, and filamentation modes in the weakly relativistic regime.
We find that after nonlinear saturation of the fastest-growing two-stream and
Oblique modes, the effective temperature anisotropy, which drives current
filament formation via the secular Weibel instability, has a strong dependence
on the internal temperature of the counter-streaming plasmas. The effective
temperature anisotropy is significantly more reduced in colder than in hotter
plasmas, leading to orders of magnitude lower magnetization for colder plasmas.
A strong dependence of the energy conversion efficiency of Weibel-type
instabilities on internal beam temperature has implications for determining
their contribution to the observed magnetization of many astrophysical and
laboratory plasmas.
