Wide Range Of Discharge Currents Research Articles

Characterization of a C12A7 electride plasma-based cathode using different keeper orifice sizes

For the operation of electric propulsion systems in space, efficient electron sources are crucial components. Hollow cathodes have been established for many applications since they allow sufficient current ranges at reasonable power requirements and have been proven to operate for several thousands of hours. New approaches are being evaluated to improve these cathodes’ general performance. This publication presents an extended characteristic of a heaterless plasma-based cathode using the emitter material C12A7 electride. The focus is on the relationship between the discharge potential and total discharge power over the discharge current. Furthermore, a characteristic of the discharge performance at lower mass flow rates is presented and discussed. The discharge potential is generally quite constant for a wide range of discharge currents, typical in the range of 30 V and only increases steeply for low discharge current ranges. Successful heaterless ignition and stable operation have been achieved down to 2 sccm krypton flow rate.

A Coupled 0-D Plasma Thermal Model for Hollow Cathodes

Hollow cathodes are essential components of dc electric thrusters for space applications, such as ion and Hall thrusters. Since the late 1970’s, several mathematical models have been proposed with the goal of characterizing the working principle and the plasma properties of such devices. Currently the more advanced 2-D models are used to effectively describe the onset of instabilities inside the plasma plume, but the increased complexity and the required calibrations make them not particularly suitable for cathode prototyping. Zero-dimensional models, on the other hand, can be used quickly and effectively to compute the main plasma parameters. In spite of their good qualities, reduced models usually do not perform well when used for very different cathodes with respect to the ones they were created for, especially due to the strong assumptions and free parameters which must be tuned to the specific case. By coupling a newly developed 0-D plasma model with a robust finite element analysis thermal simulation, the proposed model aims at filling this lack of flexibility, enabling the fast simulation of cathodes in a wide range of discharge currents and geometries. The model is able to compute self consistently all the physical quantities of interest, except the effective emission length, being the only free parameter. The model includes the approximate computation of the plasma sheath density on the emitter surface and the charge exchange heating (CEX) for the heavy species temperature. The thermal simulation is fully parametrized to quickly test different geometries and materials, while the prototyping tool allows great flexibility in terms of propellants and emissive insert materials.

The Effect of Expanded and Natural Flake Graphite Additives on Positive Active Mass Utilization of the Lead-Acid Battery

The effects of expanded and not expanded (natural flake) graphite additives were evaluated on the discharge utilization of the positive active material (PAM) in the lead-acid battery. Graphite powders were added to the paste at 2.20 vol. % and tested in model 2V battery cells under a wide range of discharge currents from 8C to C/20. The effects of graphite on the PAM pore volume and pore size distribution were measured with mercury porosimetry, and a good correlation was found between the pore volume of the PAM and utilization performance of the cells. It was shown that the powder characteristics of graphite can affect the PAM pore volume. A correlation was found between the graphite additives’ structural order and PAM utilization.

Measured cathode fall characteristics depending on the diameter of a hydrogen hollow cathode discharge

In this work, Doppler-free two photon optogalvanic spectroscopy is used to measure the electric field strength in the cathode fall region of a hollow cathode discharge, operated in pure hydrogen, via the Stark splitting of the 2S level of atomic hydrogen. The cathode fall characteristics are analysed for various pressures and in a wide range of discharge currents. Tungsten is used as the cathode material, because it allows for reliable measurements in a fairly wide range of discharge conditions and because of its minimal sputtering. Two cathode diameters (10 mm and 15 mm) are used to study the dependence of the cathode fall on discharge geometry. The measurements reveal that the cathode fall characteristics are quite independent on the cathode diameter for equal cathode current density; hence the measurements can be used to test one dimensional modelling of the cathode fall region for low pressure hydrogen discharges using e.g. plane parallel electrodes.

Analysis of various scenarios of the behavior of voltage–current characteristics of direct-current microdischarges at atmospheric pressure

The voltage–current characteristics of a discharge at atmospheric pressure in a wide range of discharge currents have been numerically studied within a hybrid model for a gas-discharge plasma with a self-consistent description of the heating of a cathode. Segments corresponding to a normal glow discharge, a transition from a glow discharge to an arc discharge, and an arc discharge have been revealed on the voltage–current characteristics. All main parameters of the plasma have been obtained for each of these segments of the voltage–current characteristics of the discharge.

Breakdown and dc discharge in low-pressure water vapour

In this paper we report studies of basic properties of breakdown, low-current Townsend discharge and high-current discharge regimes in water vapour. Paschen curves and the corresponding distributions of emission intensities at low current were recorded in the range of pd (pressure x electrode gap) from 0.1 to 10 Torrcm covering the region of Paschen minimum. From the experimental profiles we obtained effective ionization coefficient of water vapour for the E/N range 650 Td–7 kTd and fitted the results by using the extended Townsend analytical formula. Using the obtained ionization coefficient, we calculated the effective yield of secondary electrons from the copper cathode. Results of the measurements of Volt-Ampere characteristics in water vapour were presented together with the images of the axial structure of the discharge in a wide range of discharge currents for two pd values. Recorded profiles showed development of the spatial structure of the discharge in different operating regimes. We were able to identify conditions where processes induced by heavy particles, probably fast hydrogen atoms, are dominant in inducing emission from the discharge. Finally, standard scaling laws were tested for low current and glow discharges in water vapour.

Modeling and Simulation of Electrochemical Impedance Spectra in Li-Air Batteries

Recently, we have developed an analytical model for studying the Electrochemical Impedance Spectroscopy (EIS) of Li-air batteries, in which the mass transport inside the cathode is limited by oxygen diffusion (1). The model takes into consideration the effects of double layer, faradaic processes, and oxygen diffusion in the cathode, but neglects the effects of anode, separator, conductivity of Li2O2, and Li-ion transport. The model equations are relatively similar to the ones presented in (2) can be expressed in terms of a small-signal equivalent circuit like in (3). In this presentation we develop a finite element model for computing the impedance spectra of Li-air batteries with organic electrolyte and investigate the accuracy of the analytical model. The new finite element model is based on the theory of concentrated solutions, which we recently applied in (4) to compute the discharge characteristics in Li-air batteries. This model provides a complete description of lithium-ion and oxygen diffusion in the anode, separator and the cathode regions. It also takes into account the following effects: the electron conductivity in the carbon cathode, Butler-Volmer kinetics at anode and cathode, the formation and deposition of the Li2O2 at the cathode, and the change in porosity as a result of the deposition of the reaction product. The impedance spectra are computed by applying small-signal linear perturbations to the transport equations and solving the final linear system of equations numerically.The figure below compares the results obtained using the analytical model (symbols) with the more computationally expensive finite element simulations (continuous lines) for three values of the dc discharge current, 0.2mA/cm2, 0.5mA/cm2, and 2mA/cm2. The total value of the discharge current is a superposition of the dc and small-signal ac currents. In figure (a) the simulations were performed by solving only the oxygen diffusion equations, whereas in figure (b) the simulations were performed by solving the equations for oxygen diffusion, lithium-ion drift and diffusion, porosity change, and electron conductivity in the solid cathode phase. A very good agreement between the analytical computations and the finite element simulations is observed at all the dc discharge currents. This suggests that the assumptions used in the development of the analytical model hold true for a wide range of discharge currents. However, in figure (b) we observe a slight disagreement between the two models. It is important to point out that although the curve obtained using finite element simulations is slightly shifted in the positive direction of the real axis, the actual nature (or shape) of both the curves (i.e. analytical and simulation) remains the same for low discharge currents. This shift is due to the finite electric conductivity of the solid phase and electrolyte. For current densities larger than 2mA/cm2, the general shape of the simulation curve deviates from the analytical results. The discrepancy between the analytical and finite element simulations can be explained by the fact that the analytical model does not consider the nonuniform distribution of the porosity throughout the cathode, while the finite element simulations takes this effect into consideration. The derivation of the finite element model equations, the numerical implementation, more simulations results, and a detailed discussion of the limitations and accuracy of the analytical and finite element simulations will be presented at the conference.REFERENCES1. M. Mehta, G. Mixon, J. P. Zheng and P. Andrei, J. Electrochem. Soc., 160, A2033 (2013). 2. D. Franceschetti, J. R. Macdonald and R. P. Buck, J. Electrochem. Soc., 138, 1368 (1991). 3. R. P. Buck and C. Mundt, Electrochim. Acta, 44, 1999 (1999)4. P. Andrei, J. P. Zheng, M. Hendrickson and E. J. Plichta, J. Electrochem. Soc., 157, A1287 (2010).

A Multi-Scale Electrochemical and Thermal Model of a LiFePO4 Battery

Modeling and simulation of lithium batteries is becoming of in-creasing importance both for improving the fundamental under-standing of electrochemical processes and for developing battery management systems for practical applications. We present a 1D+1D+1D multi-scale electrochemical and thermal model of a lithium-ion battery with lithium iron phosphate (LiFePO4, LFP) positive electrode material. The model uses a hierarchical repre-sentation of spatial scales: On the nanoscopic level, diffusive transport takes place in the active material particles. On the micro-scopic level, multi-component mass and charge transport as well as heat production is described in a single repeat unit (anode, separa-tor, cathode, current collectors). On the macroscopic scale, the model describes heat transport in the radial direction of a cylindri-cal cell. Molar enthalpies and entropies are incorporated as func-tion of state of charge (SOC) for reliable simulation of heat pro-duction. The model is validated using experimentally-determined discharge curves over a wide range of discharge currents.

Approximate Solution Methods for Solid-State Diffusion in Phase-Change Electrodes

Approximate solution methods have been developed for solid-state diffusion in phase-change electrodes, which, when incorporated into a porous electrode battery model, would provide a computational advantage over the exact rigorous solution without sacrificing accuracy. A shrinking core approach has been used to describe the phase boundary movement in a spherical electrode particle, where a shell of one phase covers a core of the other phase. Two approximations have been developed in this work, which account for diffusion in the shell while taking into account the moving phase interface. It is assumed that there are no concentration gradients in the core at any time. The validity of the approximations has been tested by solving the governing diffusion equation with the moving boundary using the arbitrary Lagrangian–Eulerian application mode in COMSOL Multiphysics. One of the approximations (order-2) is seen to perform very well over a wide range of discharge currents in predicting the positions of the moving interface, surface concentrations, concentration profiles, etc. The order-2 approximation is also seen to capture scenarios where diffusion continues in a single-phase particle after the core has been completely consumed.

Experimental and theoretical study of the transition between diffuse and contracted forms of the glow discharge in argon

The constriction of the positive column of a glow discharge in argon was studied both experimentally and theoretically. In experiments the direct current discharge was maintained in a cylindrical glass tube of 3 cm internal diameter and 75 cm length. The voltage–current U(I) characteristics of the discharge were measured at a gas pressure P from 1 to 120 Torr in a wide range of discharge currents. At P > 20 Torr the measured U(I) characteristics display the classical hysteresis effect: the transition from the diffuse to the contracted discharge form (with increasing current) occurs at a current higher than that for reverse transition (with decreasing current). It was also found that in some cases the so-called partially contracted form of the discharge is realized, when the diffuse and contracted forms coexist in the discharge tube.To calculate the plasma parameters under experimental conditions a 1D axial-symmetric discharge model for pure argon was developed. The details of the model are described and the results of simulations are presented. In particular, the electric field strength E in the positive column was calculated as a function of the discharge current. Theoretical E(I) characteristics are compared with those derived from the experiment. For the first time, the detailed kinetic model without the usage of fit parameters predicts the hysteresis effect in pure Ar with parameters of diffuse and constricted forms of the discharge in good agreement with the experiment.

Two-dimensional, hybrid model of low-pressure glow discharges.

A self-consistent, two-dimensional hybrid fluid-particle model is presented and used to describe the electrical behavior of dc low-pressure discharges in the current range ${10}^{\mathrm{\ensuremath{-}}7}$--${10}^{\mathrm{\ensuremath{-}}2}$ A in argon for products of the gas pressure and the gap spacing (pd) from 1 to 3.3 cm torr. The two-dimensional distributions of the potential, charged particle densities and ionization source term at steady state are shown to illustrate the discharge behavior during the transition from the normal to the abnormal regimes. For the larger values of pd, a positive column region as well as the cathode regions are clearly apparent. The model used here consists of Poisson's equation for the electric field coupled to the continuity equations for the electrons and ions with the important feature that the ionization source term appearing in the continuity equations is nonlocal and determined from a Monte Carlo simulation. This description yields a unified physical picture of discharge behavior in the cathode fall, negative glow, and positive column regions over a wide range of discharge currents.

Sensitivity of a helium-neon laser utilizing 3s2–2p4and 3s2–3p4coupled transitions in neon to discharge current fluctuations

A calculation is reported of the sensitivity of the intensity of λ = 0.63μ, radiation, emitted from a helium-neon laser excited by a dc discharge, to discharge current fluctuations. The influence of simultaneous emission of λ = 3.39μ. radiation (due to a coupled transition) on this sensitivity if found for a wide range of discharge currents and current-fluctuation frequencies. It is shown that fluctuations of the intensity of the λ = 0.63μ radiation, resulting from the corresponding fluctuations of the discharge current, can be reduced considerably not only at low (0–1 kHz) fluctuation frequencies but also at a resonance'' frequency which amounts to a few tens of kilohertz. The results of these calculations are confirmed by experimental results obtained in the frequency range 0–100 kHz.

Positive column of the Cs–Ar low-pressure discharge

A model for the positive columns of dc low-pressure discharges in Cs–Ar mixtures, which includes radial depletion of Cs ground-state atoms and radiation trapping, is presented. The available experimental data for this discharge system were extended in such a way that comparison could be made between model calculations and experiments for a wide range of discharge currents and Cs vapor densities (0.2 × 1019−3.0 × 1019 m−3) and a reasonable agreement is obtained.

Gas Fed Multichannel Hollow Cathode Arcs

A new type of gas fed hollow cathode (multichannel cathode) has been studied. Such cathodes operate with a lower voltage drop and higher maximum current density than conventional one-channel cathodes and have a longer lifetime as well. Moreover, multichannel cathodes allow operation within a wide range of discharge currents and with extremely low gas flow rates. A theoretical analysis clarifies the principles of the operation of this type of cathode. The experimental results are explained and some general rules are presented for the cathode design.