Coulomb Logarithm
The natural logarithm of the ratio of maximum to minimum impact parameters in small-angle Coulomb collision integrals.
Governing formula
ln Λ = ln(r_max / r_min) = ln(λ_D / b_min) ≈ 10–20
SI unit
Dimensionless Parameter (ln Λ)
In depth
Accounts for cumulative small-angle Coulomb scattering in a plasma. Cutoff upper limit is Debye length λ_D; lower limit is quantum De Broglie wavelength or classical closest approach b_min. Values typically range from 10 to 20.
Examples in the real world
Calculating plasma electrical resistivity η ∝ ln Λ / T_e^(3/2) in fusion physics.