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Register a new userNonextensive Interpretation Of Radiative Recombination In Electron Cooling
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An interest for the low-energy range of the nonextensive distribution function arises from the study of radiative recombination in electron cooling devices in particle accelerators, whose experimentally measured reaction rates are much above the theoretical prediction. The use of generalized distributions, that differ from the Maxwellian in the low energy part (due to subdiffusion between electron and ion bunches), may account for the observed rate enhancement. In this work, we consider the isotropic distribution function and we propose a possible experiment for verifying the existence of a cut-off in the generalized momentum distribution, by measuring the spectrum of the X-rays emitted from radiative recombination reactions.
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Electron-ion recombination of completely stripped Bi83+ was investigated at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. It was the first experiment of this kind with a bare ion heavier than argon. Absolute recombination rate coefficients have been measured for relative energies between ions and electrons from 0 up to about 125 eV. In the energy range from 15 meV to 125 eV a very good agreement is found between the experimental result and theory for radiative recombination (RR). However, below 15 meV the experimental rate increasingly exceeds the RR calculation and at Erel = 0 eV it is a factor of 5.2 above the expected value. For further investigation of this enhancement phenomenon the electron density in the interaction region was set to 1.6E6/cm3, 3.2E6/cm3 and 4.7E6/cm3. This variation had no significant influence on the recombination rate. An additional variation of the magnetic guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted in periodic oscillations of the rate which are accompanied by considerable changes of the transverse electron temperature.
Dielectronic recombination has been investigated for Delta-n = 1 resonances of ground-state Li+(1s^2) and for Delta-n = 0 resonances of metastable Li+(1s2s ^3S). The ground-state spectrum shows three prominent transitions between 53 and 64 eV, while the metastable spectrum exhibits many transitions with energies < 3.2 eV. Reasonably good agreement of R-matrix, LS coupling calculations with the measured recombination rate coefficient is obtained. The time dependence of the recombination rate yields a radiative lifetime of 52.2 +- 5.0 s for the 2 ^3S level of Li+.
The parity nonconservation effect on the radiative recombination of electrons with heavy hydrogenlike ions is studied. Calculations are performed for the recombination into the $2^1S_0$ state of helium-like thorium and gadolinium, where, due to the near-degeneracy of the opposite-parity $2^1S_0$ and $2^3P_0$ states, the effect is strongly enhanced. Two scenarios for possible experiments are studied. In the first scenario, the electron beam is assumed to be fully polarized while the H-like ions are unpolarized and the polarization of the emitted photons is not detected. In the second scenario, the linearly polarized photons are detected in an experiment with unpolarized electrons and ions. Corresponding calculations for the recombination into the $2^3P_0$ state are presented as well.
Three-body recombination, an important collisional process in plasmas, increases dramatically at low electron temperatures, with an accepted scaling of T_e^-9/2. We measure three-body recombination in an ultracold neutral xenon plasma by detecting recombination-created Rydberg atoms using a microwave-ionization technique. With the accepted theory (expected to be applicable for weakly-coupled plasmas) and our measured rates we extract the plasma temperatures, which are in reasonable agreement with previous measurements early in the plasma lifetime. The resulting electron temperatures indicate that the plasma continues to cool to temperatures below 1 K.
Photodetachment thermometry on a beam of OH$^-$ in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional, frequency and time dependent photodetachment spectroscopy over 20 minutes of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J = 1 natural lifetime (about 193 s) and determine the OH$^-$ rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.
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