  Velocity-space anisotropy can significantly modify fusion reactivity. The
nature and magnitude of this modification depends on the plasma temperature, as
well as the details of how the anisotropy is introduced. For plasmas that are
sufficiently cold compared to the peak of the fusion cross-section, anisotropic
distributions tend to have higher yields than isotropic distributions with the
same thermal energy. At higher temperatures, it is instead isotropic
distributions that have the highest yields. However, the details of this
behavior depend on exactly how the distribution differs from an isotropic
Maxwellian. This paper describes the effects of anisotropy on fusion yield for
the class of anisotropic distribution functions with the same energy
distribution as a 3D isotropic Maxwellian, and compares those results with the
yields from bi-Maxwellian distributions. In many cases, especially for plasmas
somewhat below reactor-regime temperatures, the effects of anisotropy can be
substantial.
