Laboratory of Elementary Processes in Plasma

AISA experiment

The ions H₃⁺ and D₃⁺ have been the subject of a number of studies. The H₃⁺ ion and its deuterated analogues play important roles in the kinetics of media of astrophysical interest (interstellar molecular clouds, and planetary atmospheres) and also in laboratory-produced plasmas (discharges, fusion plasmas at walls etc.). The recent discovery of interstellar H₃⁺ in molecular clouds has confirmed the presence of H₃⁺ and opened a problem of observed high column density, which critically depends on the rate of recombination of H₃⁺ with electrons at low energies.

The kinetics of formation of H₃⁺ and D₃⁺ ions in hydrogen (deuterium) containing plasmas is well understood. Equally important for the hydrogen and deuterium containing plasmas is to understand the processes of destruction of these ions in collisions with electrons. However, despite enormous efforts, the results of studies determining the rate of recombination of H₃⁺ and D₃⁺ ions with electrons give values that vary by at least one order of magnitude. The recent calculations of the recombination rate coefficient give for thermal electrons and ground state H₃⁺ ions value α = 7×10^–8 cm³ s^–1.

Very recent storage ring measurements with internally cold H₃⁺ ions are in good agreement with the calculations over a broad range of collision energies. The recombination rate coefficient obtained in recent plasma studies (AISA, FALP) of H₃⁺ recombination shows a strong dependence on hydrogen partial density and on buffer gas (He) pressure indicating that the decay of H₃⁺ dominated plasma is controlled by sort of multistep process with long-lived intermediate highly excited neutral H₃^#. Additional experiments (FALP, CRDS) are carried out to characterise this process.

The most important papers: