Note in this context that it is the collisions with small deflections which are responsible for the total Coulomb scattering cross section as becoming very large.

A number of physical considerations suggest that 0min should be chosen in accordance with a maximum impact parameter beyond which Coulomb scattering is relatively small. In a vacuum, the Coulomb field from a dominant isolated charge extends to infinity, implying therefore a scattering "deflection" interaction
at any arbitrary impact parameter. In a plasma, however, the target charge would be surrounded by a particle cloud of opposite charge. The fields due to surrounding particles will effectively "screen-out" the Coulomb field of an arbitrary charge at some maximum impact parameter distance which corresponds to some minimum scattering angle. This distance may be specified as the radius of an imaginary sphere surrounding a target ion such that the plasma electrons will reduce the target ion’s Coulombic field by 1/e at the sphere’s surface. The Debye length, AD as defined by Eq.(3.23a), evidently corresponds to this distance and we take the maximum impact parameter as

fo, max Ad *

Thus, 0mm follows from the inversion of Eq.(3.10):

Figure 3.5 displays the Coulomb scattering cross section for the case of d-t interactions and for comparison also shows the fusion cross section; as suggested previously, it is evident that Coulomb scattering events occur orders of magnitude more frequently than fusion reactions.