47 research outputs found
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
A position- and time-sensitive particle detector with subnanosecond time resolution
A microchannel plate/CCD-camera particle detector is described that utilises gold strips deposited upon the surface of a MCP to provide particle arrival time information. The uncertainty in the timing is assessed to be ∼500 ps (FWHM) for a measured time interval between adjacent strips. The main contribution of which is the result of the slow response time of the CFD compared to the signal. The application of timing measurement is shown to be of particular benefit in the accurate determination of product state branching ratios from the dissociative recombination of diatomic ions. Finally, the possibilities of such a detector are assessed
A position- and time-sensitive particle detector with subnanosecond time resolution
A microchannel plate/CCD-camera particle detector is described that utilises gold strips deposited upon the surface of a MCP to provide particle arrival time information. The uncertainty in the timing is assessed to be ∼500 ps (FWHM) for a measured time interval between adjacent strips. The main contribution of which is the result of the slow response time of the CFD compared to the signal. The application of timing measurement is shown to be of particular benefit in the accurate determination of product state branching ratios from the dissociative recombination of diatomic ions. Finally, the possibilities of such a detector are assessed