Electron transport parameters in CO₂: a comparison of two experimental systems and measured data

Abstract

Two experimental apparatuses used to obtain electron transport coefficients in gases are compared based on measurements in CO2 over a wide range of E/N-values. The operation principles of the two experimental systems as well as their data acquisition methods are different. One operates under the time of flight (TOF) principle, where the transport coefficients are obtained by fitting the theoretical form of the electron density of a swarm in an unbounded region, n(x, t), to the measured current at different values of the drift length, I(L, t). The other experimental apparatus operates in the Pulsed Townsend (PT) mode, where the electron transport coefficients are obtained by fitting the spatial integral of n(x, t) over the drift region to the measured, time-dependent current signal, I(t). In both apparatuses, the measured E/N range was extended as much as possible to allow a large overlap for the comparison of the results. The bulk drift velocity, W, obtained by the two systems agrees well (within a few %) over a wide range of E/N values (100 Td ≤ E/N ≤ 1000 Td). The agreement between the data sets for the longitudinal component of the bulk diffusion tensor, D L , is less satisfactory, the TOF data show systematically higher values (by 10–50% depending on E/N) than the PT measurements. Significant differences are also found below 100 Td in case of the effective ionisation frequency, ν e f f , and the (steady state) Townsend ionisation coefficient, α e f f , where the TOF apparatus is unable to give accurate results. Our comparison justifies the correctness of the measured data over the range of agreement and also indicates the interval in E/N where the data obtained by each of the experimental systems can be taken to be reliable. The limits of the operating regimes of the two setups, stemming from the hardware and from the physical limits, are discussed.

Data Needs for Modeling Low-Temperature Non-Equilibrium Plasmas: The LXCat Project, History, Perspectives and a Tutorial

“…In Figure 11 are shown swarm data for a pure CO 2 gas calculated using the Biagi database [80,198] . Swarm parameters calculated using the online Boltzmann solver for a pure CO 2 gas using the Biagi database [80,198] and comparison with experimental swarm measurements [133,135, from Dutton [132], ETHZ [134], Heidelberg [154], LAPLACE [149] and UNAM [150] databases as a function of reduced electric field. The computed quantities are: (a) characteristic electron energy, (b) product of the gas number density and electron diffusion coefficient, (c) product of the gas number density and electron mobility coefficient, (d) Townsend coefficients.…”

Synergism in SF6 mixtures with C=C−C backbone compounds

“…The swarm measurements were performed with a pulsed Townsend experiment (described in [18,19]) at low pressures-ranging from 200 Pa to 25.2 kPa and electrode distances between 11 and 29 mm. Assuming a Gaussian spatial distribution of the electron swarm (boundaries at infinity), the measured electron displacement current can be evaluated according to the analytical expression:…”

“…The term in parenthesis accounts for the fraction of electrons absorbed at the anode (and no longer contributes to the current) which depends on the drift time T e (or bulk drift velocity w = d/T e with d the distance between electrodes) of the electron swarm between the electrodes and bulk longitudinal diffusion coefficient D L = 1 2 w 2 τ D . Details on the evaluation method can be found in [10,19]. The evaluated density-scaled diffusion coefficients show a pressure-dependence which is a consequence of neglecting in equation ( 1) the initial broadening of the swarm and of the limited bandwidth of the experimental system, detailed in [19].…”

“…Details on the evaluation method can be found in [10,19]. The evaluated density-scaled diffusion coefficients show a pressure-dependence which is a consequence of neglecting in equation ( 1) the initial broadening of the swarm and of the limited bandwidth of the experimental system, detailed in [19]. Therefore, diffusion parameters obtained for the lowest pressures should be considered as most reliable.…”

Discussion on the mechanism leading to positive synergism in SF6 mixtures with HFO1234ze(E)

“…The swarm measurements were performed in a pulsed Townsend (PT) setup which was described in detail in previous works [36,37]. Photoelectrons (⩾10 5 ) are extracted from a Mg/Pd coated photocathode using a 20 Hz-pulsed UV laser beam and released in the gaseous medium of interest subject to electric stress.…”