Dissociative recombination and electron-impact de-excitation in CH photon emission under ITER divertor-relevant plasma conditions

For understanding carbon erosion and redeposition in nuclear fusion devices, it is important to understand the transport and chemical break-up of hydrocarbon molecules in edge plasmas, often diagnosed by emission of the CH A^2\Delta - X^2\Pi Ger\"o band around 430 nm. The CH A-level can be excited either by electron-impact or by dissociative recombination (D.R.) of hydrocarbon ions. These processes were included in the 3D Monte Carlo impurity transport code ERO. A series of methane injection experiments was performed in the high-density, low-temperature linear plasma generator Pilot-PSI, and simulated emission intensity profiles were benchmarked against these experiments. It was confirmed that excitation by D.R. dominates at T_e < 1.5 eV. The results indicate that the fraction of D.R. events that lead to a CH radical in the A-level and consequent photon emission is at least 10%. Additionally, quenching of the excited CH radicals by electron impact de-excitation was included in the modeling. This quenching is shown to be significant: depending on the electron density, it reduces the effective CH emission by a factor of 1.4 at n_e=1.3*10^20 m^-3, to 2.8 at n_e=9.3*10^20 m^-3. Its inclusion significantly improved agreement between experiment and modeling

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density, and temperature. On the other hand, observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons, the electron fraction, and the cosmic-ray ionisation rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is already known. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionisation rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres

Dissociative recombination of C

We have studied the vibrationally relaxed C2H$_3^+$ ion in the heavy – ion storage ring CRYRING in Stockholm. We measured the dissociative recombination absolute cross section over center-of-mass energies in the range between 0 and 0.1 eV by scanning the electron energy.
The rate of different neutral product channels of dissociative recombination was measured. We found the three-body channel C2H + H + H, with a branching ratio of 59%, to be the dominant one. Finally, we compare C2H$_3^+$ and C2H$_2^+$ ([CITE]) results

Absolute cross sections and final-state distributions for dissociative recombination and excitation of CO+ (v=O) using an ion storage ring

Absolute cross sections and rate coefficients have been determined for dissociative recombination of electrons and CO+ ions for energies from 1 meV to 54 eV. We found values of 4 × 10-12 cm2 at 1 meV and 10-15 cm2 at 1 eV, with an essentially 1/E energy dependence. Branching ratios over the final atomic product states have been determined using a position- and time-sensitive imaging system. At zero eV collision energy the predominant yield is to ground-state atomic fragments (76%). At higher collisional energies the branching ratio to the ground-state limit is reduced. A new limit, O(1D) + C(1D), opens up and branching to the O(3P) +C(1D) limit increases. Cross sections are also determined for dissociative excitation of CO+. Thermal rate coefficients are deduced from the dissociative recombination (DR) data, and compared with measurements in the literature. Consideration of both the theoretical and spectroscopic data in the literature giving information about the potential curves along which DR may take place reveals both a paucity and disparity of the data