Study of the surface discharge on ferroelectrics

The discharge parameters in Hall thrusters depend strongly on the yield of secondary electron emission from channel walls. Comparative measurements of the yield of secondary electron emission at low energies of primary electrons were performed for several dielectric materials used in Hall thrusters with segmented electrodes. The measurements showed that at low energies of primary electrons the actual energetic dependencies of the total yield of secondary electron emission could differ from fits, which are usually used in theoretical models. The observed differences might be caused by electron backscattering, which is dominant at lower energies and depends strongly on surface properties. Fits based on power or linear laws are relevant at higher energies of primary electrons, where the bulk material properties play a decisive role.

Ferroelectric Plasma Sources (FEPSs) can generate plasma that provides effective space-charge neutralization of intense high-perveance ion beams, as has been demonstrated on the Neutralized Drift Compression Experiment NDCX-I and NDCX-II. This article presents experimental results on charge neutralization of a high-perveance 38 keV Ar+ beam by a plasma produced in a FEPS discharge. By comparing the measured beam radius with the envelope model for space-charge expansion, it is shown that a charge neutralization fraction of 98% is attainable with sufficiently dense FEPS plasma. The transverse electrostatic potential of the ion beam is reduced from 15 V before neutralization to 0.3 V, implying that the energy of the neutralizing electrons is below 0.3 eV. Measurements of the time-evolution of beam radius show that near-complete charge neutralization is established ∼5 μs after the driving pulse is applied to the FEPS and can last for 35 μs. It is argued that the duration of neutralization is much longer than a reasonable lifetime of the plasma produced in the sub-μs surface discharge. Measurements of current flow in the driving circuit of the FEPS show the existence of electron emission into vacuum, which lasts for tens of μs after the high voltage pulse is applied. It is argued that the beam is neutralized by the plasma produced by this process and not by a surface discharge plasma that is produced at the instant the high-voltage pulse is applied.

Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.

The dependence of the discharge current on both the secondary electron emission at the discharge tube wall and the tube diameter was studied. Results of the study are the following: (1) a decrease of the discharge current with the decrease of the coefficient of secondary electron emission; (2) the secondary electron emission occurs with a high intensity at the tube wall limiting the first dark space and with a lower intensity on the tube wall limiting the negative glow end; (3) the discharge current depends on the diameter of the discharge tube through the elementary processes occurring in the region of the cathode fall.

Physics and applications of secondary electron emission: Bruining, H.; Pergamon Press Ltd. London, and McGraw-Hill Co., New York, xii + 178 pp. 130 figures. 25s. net or $5

Analytical model of secondary electron emission yield in electron beam irradiated insulators

Heat removal in metal structures is often the limiting factor in high power, high frequency microwave tube devices. As higher critical heat flux cooling techniques are found, the operating power of the microwave device may be increased if other limiting factors are not of importance. Larger output powers may be realized for brief periods in time at the expense of operating in a pulsed power and/or pulsed cooling mode. During these brief periods, temporary temperature fluctuations on the walls of the microwave tube will be realized. It is hypothesized that the temperature fluctuations and to a much lesser extent temperature gradients may influence secondary electron emission resulting from primary electrons colliding with the microwave wall. Low energy primary electrons, that penetrate the microwave wall, experience the collective effects of the electrons (plasmons) in the conduction band of the metal. It is anticipated that the surface and bulk temperature of the metal wall influences the ability for plasmons to interact with the primary electron in the scattering process. Further, aging effects resulting from heat stress cycling of the metal cavity may alter the state of the cavity walls leading to changes in the secondary electron emission properties. Pulsed, low energy (1 keV), low beam currents are used to examine collectively the change in the spatial distribution profile of secondary electrons as the sample cools without significantly modifying or stressing the surface of the sample under test. SEE distribution profiles will be presented for a metal target at room temperature and in transient cooling mode with peak temperatures on the order of 390° K.

On the emission of secondary electrons from solid deuterium

Monte Carlo study of secondary electron emission from SiO2 induced by focused gallium ion beams

Neutral beam injectors (NBI) used for JT-60 are required to generate negative ions of 500 keV energies. To produce such high-energy ions, the electrostatic accelerators consisting of 3-stage of electrodes and three insulator rings are applied. The insulators are made of Fiberglass Reinforced Plastic (FRP) which is composed of epoxy resin and glass fibers. The surface discharges along the insulators are one of the most serious problems in the development of NBI. To increase the hold-off voltage against surface flashover events, it is necessary to investigate the FRP and epoxy resin insulator properties related to surface discharges in vacuum. This paper describes surface flashover characteristics for epoxy resin, FRP and Alumina samples under vacuum condition. In addition, the measurements of secondary electron emission (SEE) characteristics are also reported. These are important parameters to analyze surface discharge characteristics of insulators in vacuum.

The time varying impedance was measured for pulsed planar surface discharges over a PTFE substrate in a xenon cover gas. Surface discharges were driven by a 2 /spl Omega/, 5 /spl mu/s type C Guillemin PFN, which allowed the discharge to be driven in a controlled manner for 5 /spl mu/s and allowed the calculation of the discharge impedance directly from a measurement of the voltage across the surface discharge. Empirical expressions were found describing the impedance of surface discharges for a constant impedance driving source for various charging voltages and cover gas pressures.

Recent and not so recent experimental data are analyzed to show that the reason for strong electron emission from dielectric cathodes is the incomplete discharge occurring on the dielectric surface due to the electric field there being tangentially nonzero. The places of origin of such discharges are the metal–dielectric–vacuum triple junctions (TJs). As the discharge plasma moves over the surface of the dielectric electrode, the bias current arises, and an electric microexplosion occurs at a TJ. If the number of TJs is large, as it is for a metal grid held tightly to a ferroelectric, electron currents of up to 104 A with densities of more than 102 A cm−2 can be achieved. A surface discharge is initiated by applying a triggering pulse to the metal substrate deposited beforehand onto the opposite side of the ferroelectric. If this pulse leads the accelerating voltage pulse, the electron current is many times the Child–Langmuir current. The reason for the ferroelectric effect is the large permittivity (ε > 103) of the materials used (BaTiO3, PLZT, PZT). Although these devices have come to be known as ferroelectric cathodes, we believe ferroelectric plasma cathodes would be a better term to use to emphasize the key role of plasma effects.

The electronic structure of regular and irregular MgO (111) surface was studied using first‐principles. We conclude that the introduction of surface roughness can reduce the surface bandgap value down to 5.1 eV, i.e. below the critical value of 5.6 eV for xenon ions, thereby increasing the γ and efficacy values.

Authors

We report on a square super-lattice pattern by interaction of surface discharge with the applied voltage increasing in a dielectric barrier discharge system. The pattern consists of bright spots, bright lines, and dim spots. The spatio-temporal dynamics of the pattern is studied by photomultiplier (PMT), high speed video camera and normal camera. Results show that the bright spots discharge several times during each half cycle and they are formed by volume discharge (VD), bright line was formed by superposition of the surface discharges (SDs) over a long period of time, and the dim spots are consist of four small spots. The dim spots are formed by the repulsion of SDs. The light intensity of the bright and dim spots of the patch was compared and it was found that the ratio of the light intensity of the bright spot to the dark spot was about 10. The measurement of the emission spectroscopy show the parameters of the bright spots are different from that of the bright lines and dim spots, the discharge type of bright spots an dim spots is different, and reflects that the dim spots are formed by the interaction of SDs. The formation of the pattern is explained by the wall charge theory.