Observation of Enhanced Electron-Ion Recombination Rates at Very Low Energies

Abstract

A strong enhancement of electron-ion recombination over expected rates at sub-eV relative energies is revealed for highly charged ions. Measured Ne' + and Ar' + recombination rates were found to be enhanced at low (&1 rneV) energies by factors of 4 and 10 over theoretical predictions of radiative recombination rates, respectively.It is shown that the majority of the enhancement in Ar' is caused by very low lying An = 0 dielectronic recombination resonances; nevertheless, the nonresonant contributions to the measured recombination rates for both systems were found to be significantly higher than predicted by presently available models.PACS numbers: 34.80.Kw, 32.80.HdAmong the fundamental electron-ion interactions which occur in plasmas [1,2], recombination at low energies has turned out to be surprisingly unpredictable.Measurements performed under the most precise conditions yet achieved have consistently yielded experimental rates far above theoretical expectations [3 -6]. No convincing explanation of these enhanced rates exists, nor have detailed quantitative spectra in the sub-meV region been presented.In models describing recombination in electron coolers one considers three basic processes: radiative recombination (RR), where energy and momentum conservation is balanced by the emission of a photon, e + a '-A" "'+ hv, three-body recombination (TBR), where the excess energy is imparted to a neighboring free electron, e +e +A+ A~'~+ e and, for nonbare ions, dielectronic recombination (DR), where a free electron is captured resonantly with the excitation of a bound electron in the ion. This process can be viewed as the time reverse of Auger emission.

Electron-ion Recombination at Sub-zero Temperature

“…Above this energy the SRR rate drops rapidly while the TSRR rate decreases slowly but is always remaining greater than the SRR rate. Below 1 meV the sum of the SRR and the TSRR rates are closer to the experimental results [8] but greater than the calculated rate coefficients based on the Kramers cross section. Above 1 meV the present calculation and Kramers results follow the experimental curve.…”

Fusion-Related Ionization and Recombination Data for Tungsten Ions in Low to Moderately High Charge States

“…The reason for the excursion of the storage-ring rate coefficient at very low energies is specific to electron-ion merged-beam experiments and depends on the electron density, the electron-beam temperatures and the magnetic and electric fields present in and around the electron-ion interaction region. The effect is known as low-energy recombination enhancement in merged-beam measurements and has been investigated and discussed in detail previously [80][81][82][83][84][85][86][87][88]. In the present case the maximum storage-ring rate coefficient reached 227 × 10 −8 cm 3 s −1 at 70 µeV.…”

Experimental dielectronic recombination rate coefficients for Na-like S VI and Na-like Ar VIII

“…Large enhancements were observed in the recombination spectra of other ions, also containing lowenergy DR resonances (see e.g. Ar xiv, Gao et al 1995; and Pb liv, Lindroth et al 2001). The AUTOSTRUCTURE calculation predicts a strong resonance in the low-energy part of the Ar viii spectrum, at ∼30 meV.…”

“…Below 1 meV, the experimental recombination rate is clearly enhanced compared to the calculated RR contribution. This low-energy enhancement is a common characteristic of storage ring experiments and typically amounts to a factor of ∼3 increase, compared to the calculated RR (Gao et al 1995).…”