Stopping power calculations of fast ions penetrating a hydrogen plasma target in local thermodynamic equilibrium at arbitrary temperatures are performed. Excited state contributions to the energy loss are included in the framework of the Bethe formalism. Average ionization potentials for the excited ions are given in a quasiclassical approximation. It is shown that the net effect is an enhancement of the stopping power compared to the energy loss when assuming all atoms to be in their ground state.
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The reader should not be confused by the fact that de-exciting collisions result in an acceleration of the projectile ion, which at first glance appears to be a contradiction to the second law of thermodynamics. The acceleration of the ion is not a conversion of undirected thermal energy into kinetic energy of the projectile, but rather a conversion of potential energy into kinetic energy.
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see also: R. H. Fowler, Statistical Mechanics: The Theory of the Properties of Matter in Equilibrium, 2nd Ed., (University, Cambridge, 1966), p. 576.
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© 1991 American Institute of Physics.
1991
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