EVALUATION OF THE COULOMB LOGARITHM USING CUTOFF AND SCREENED COULOMB INTERACTION POTENTIALS

The Coulomb logarithm is a fundamental plasma parameter which is commonly derived within the framework of the binary collision approximation. The conventional formula for the Coulomb logarithm, lambda = ln LAMBDA, takes into account a pure Coulomb interaction potential for binary collisions and is not accurate at small values (lambda<10). However, a more exact Fokker-Planck equation was recently presented by Li and Petrasso which is accurate at small Coulomb logarithm values (lambda greater than or similar to 2) [Phys. Rev. Lett. 70, 3063 (1993)]. This theory and computer simulations which are accurate for small Coulomb logarithm values provide the motivation for a more precise evaluation of the Coulomb logarithm. In the present work, the Coulomb logarithm is evaluated more precisely by using a cutoff Coulomb interaction potential. The result is compared to an exact numerical evaluation of the Coulomb logarithm which considers a screened Coulomb interaction potential. Fits to the numerical results are also provided. The fitted formula lambda=ln(0.6 LAMBDA) is recommended for most applications providing values within 4% of the exact numerical values for lambda greater than or similar to 2. This formula is easily implemented by using 0.6lambda(D) in place of lambda(D) (the Debye length) in the conventional formula for the Coulomb logarithm.