Cooling of Ions by Cold Electrons through Coulomb Collisions

This paper is concerned with plasmas where the ion temperature is larger than the electron temperature and the cooling of ions by Coulomb interaction is important. Contrary to previous treatments on Coulomb collisions where the Coulomb logarithm is assumed to be constant, it is treated here as variable to take the effect of low energy electrons more accurately into account. This correction is found to be important especially when the ion velocity is comparable to the electron thermal velocity at which the ion cooling is most efficient. Functions from which the cooling rate is derived are obtained and compared with the previous results and then the energy relaxation of ions is calculated by integrating the cooling rate over the ion velocity distribution. The cooling time is obtained as a function of ion temperature. Some applications of the result are suggested.