Polarization Bremsstrahlung on Ions and Atoms in Plasmas

We now examine the emission from incident particles that obey the Born approximation in a plasma [1, 2]. We shall assume that the plasma ions are partially ionized and have an electronic structure. The Debye electron “cloud” of fully ionized ions in the plasma effectively replaces the bound electrons of the atom. When a complicated many-electron atom is partially ionized in a plasma, some of its electrons form a “skeleton” of bound electrons, while the remainder are the passing (unbound) electrons in its Debye “cloud.” As a rule, the characteristic length R a within which the bound electrons are concentrated is much shorter than the Debye length r d (i.e., R a « r d ). Thus, we must distinguish the radiation for different impact parameters: ρ r d . As we showed in Chapter 1, for incident particles with given impact parameter ρ, the emitted frequencies are less than v 0/ρ. Thus, collisions with impact parameters ρ < R a will contribute to fre­quencies greater than v 0 /R a , collisions with impact parameters ρ < r d will contribute to frequencies greater than v 0 / R a , and, finally, collisions with impact parameters less than v 0/ω will contribute to frequencies less than v 0 / r d . In the last case, interference must occur between ordinary bremsstrahlung and polarization bremsstrahlung on the bound and free electrons. The difference between the bound and free electrons is not so large (in a certain sense and under certain conditions). For the bound electrons, the Coulomb field of the nucleus of the ion must, of course, be taken into exact account, rather than through perturbation theory [3].