Anomalous Electron Emission Research Articles

Strongly temperature-dependent, anomalous secondary electron emission of liquid lithium surfaces exposed to a plasma

• First time reported SEE of a CPS wetted with liquid lithium exposed to a plasma. • Formation and dissolution of a strongly temperature dependent surface oxide film. • SEE values from ~0.5 for clean surfaces to ~2 for heavily oxidized samples. • Exposure to 3 × 10 −7 Torr during one hour increases the SEE to values of about 2. • Annealing at increasing temperatures decreases the SEE to lower values. Electron-induced secondary electron emission (SEE) on a liquid lithium surface confined in a capillary porous system (CPS) and exposed to a plasma is reported for the first time. The liquid lithium surface is exposed to the bombardment of a suprathermal electron flux with energies up 150 eV created in a low pressure He Dc-Glow discharge. Various methods of surface oxidation have been used to reproduce realistic conditions in fusion plasma experiments. Exposure to very low residual gas pressures (around 3 × 10 −7 Torr) is enough to increase the maximum of the SEE to values of about 2. The formation and dissolution of a surface oxide film is strongly temperature dependent and plays an important role in the resulting SEE yield and its time evolution: values have been found to range from ~0.8 for clean surfaces to ~2 for samples that have been heavily oxidized. In the case of O 2 molecule exposure, a clear difference with temperature was observed. The molecular exposure at 330 °C had a much stronger effect on the increase in SEE than the molecular exposure at 220 °C. These results have a direct impact on the development of lithium-based divertor targets in fusion as well as in the understanding of the SEE characteristics of contaminated liquid surfaces, which has never been reported before.

Anomalous secondary electron emission of metallic surfaces exposed to a Glow Discharge plasma

Secondary electron emission (SEE) yields, γ, of Li, stainless steel (SS) and W surfaces immersed in a He Direct Current Glow Discharge (dc-GD) Plasma have been calculated from the experimental I–V curves as a function of electron mean energy. The data obtained showed that γLi > γSS > γW. Line emission ratios 728/706 of excited He and Langmuir probe measurements provide a clear evidence of the presence of a suprathermal electron tail responsible for the observed SEE. The results show that SEE is well correlated with the anomalous extra current component found in the I–V curves. The resulting value of γLi is significantly higher than its theoretical value suggesting a possible synergetic effect of the ion bombardment in the SEE of lithium. The effect of Li surface oxidation has also been addressed, leading to a substantial decrease of both, sputtering yield and SEE yield of Li with higher oxygen content.

Research of Electrophysical Properties of Superficial Layers and Interfaces in Polar Dielectric Materials on the Anomalous Electron Emission Spectra

Electron emission spectra excited by soft X-rays were collected from the surfaces of prepoled porous ceramic samples whose porosities varied from 7% to 64.5%. A mathematical treatment of the experimental spectra enabled isolation of the components (elementary spectra) corresponding to extrema in the potential distribution at the “negative” surface of a poled ceramic sample. It may be concluded from comparison of the theoretical and experimental results that the isolated components corresponding to the minima in the potential distribution are mainly formed by the emission from the grain boundaries, while those due to the maxima are produced by the emission from the pores.

Effect of polarization on the intensity of anomalous electron emission in the (1 − x)PbMg1/3Nb2/3O3 + xPbTiO3 system

Spectra of anomalous electron emission from the surface of solid solutions in the (1 − x)PbMg1/3Nb2/3O3 + xPbTiO3 system with x = 0.06, 0.10, 0.13, 0.20, and 0.25 have been studied. While the energy characteristics of the anomalous electron emission spectra vary monotonically with x, the behavior of integrated intensity was found to be nonmonotonic with a maximum occurring at x = 0.1. According to the proposed interpretation, the results obtained are explained by the effect of the potential barrier on the electron emission; the potential barrier depends, in turn, on the permittivity e of the samples.

Charge distribution in surface layers of polarized electret ceramics according to electron spectroscopy measurements

The anomalous electron emission (AEE) from the negative surface of a polarized SrTiO3 ceramic sample is studied both experimentally and theoretically. The form of the low-energy edge and the fine structure of AEE spectra are interpreted within the “brick wall” model. The distribution of electric potential over the charged surface is calculated using Green’s functions under the assumption of p-type conduction of intergrain boundaries. It is demonstrated that the AEE spectra of SrTiO3 ceramic surface mainly reflect the charge distribution in the polarized electret SrTiO3 sample.

Potential relief on the surface of polarized ferroelectric electrets from an analysis of the anomalous electron emission spectra

A technique is proposed for determining the specific features in the potential distribution over a free surface of polarized ferroelectric electrets from analyzing the anomalous electron emission spectra. This technique is applied to examine the potential distribution over the surface of a lead magnoniobate single crystal and ferroelectric ceramics. Reasoning from the results obtained for the lead magnoniobate crystal, the inference is drawn that the degree of perfection of surface layers of ferroelectric single crystals can be controlled using the anomalous electron emission spectra.

Soft X-ray radiation-induced electron emission from the surface of polarized ferroelectrics

Additional electron emission that reflects specific properties of ferroelectrics was detected after the negatively charged surface of these preliminarily polarized materials was exposed to soft X-radiation (hv) ≤3 kV). The properties of this emission, which is referred to as anomalous electron emission (AEE) and is excited in single-crystal and ceramic samples, are studied with a standard X-ray photoelectron spectrometer. It is shown that the parameters of AEE spectra correlate with the characteristics of the materials. The conclusions of the phenomenological theory of the phenomenon are in satisfactory agreement with the experimental results.

Luminescence of metals excited by fast nondestructive loading

The paper reports a study of the luminescence excited on the back side of metal targets irradiated with laser pulses of energy substantially below the plasma-flare formation threshold, and calculation of the temporal and spatial distributions of temperature, thermal stresses, and rate of thermal-stress variation in a sample. The evolution of the luminescence pulse is compared with that of the laser pulse, sample temperature, thermal stresses, and rate of thermal-stress variation. It has been established that the luminescence is excited as soon as the stresses at the sample surface become approximately equal to the yield point of the sample material, its intensity grows as long as the rate of stress rise increases, after which the process decays. The temporal and spatial distributions of temperature, thermal stresses, and rate of thermal-stress variation have also been calculated for the experiments, in which anomalous electron emission from the back side of laser-pulse irradiated metal targets was detected, and which were described in the literature but not appropriately explained. The dynamics of experimental and calculated relations have been compared. A correlation closely similar to that found for mechanoluminescence has been established.

Characteristics of anomalous electron emission from the surface of PbTiO3 and Pb(Zr, Ti)O3 ferroelectric films

The possibility of obtaining anomalous electron emission from thin ferroelectric films is investigated and the emission characteristics are compared with those of bulk samples. The results presented indicate that anomalous electron emission may only be observed in nonlinear dielectrics (i.e., those exhibiting ferroelectric instability). The sensitivity of the anomalous electron emission to perovskite structural elements in thin films with a pyrochlore structure is discussed.

Anomalous electron emission spectra and polarization phenomena in a lead magnesium niobate single crystal

The time dependences of the intensity and the energy positions of fine-structure of the spectra of anomalous electron emission from a polarized lead magnesium niobate (PMN) single crystal irradiated with soft x-rays are investigated experimentally. It is shown that the relaxation time of the electret charge can be determined from the graphs of these dependences, and the depth distribution of the potential in the surface layer of the sample can be determined from the profile of the anomalous electron emission spectrum. Estimates of the fields in the surface layers of a polarized PMN single crystal, obtained from the characteristics of the anomalous electron emission spectra, agree with data obtained by electrophysical research techniques.

Anomalous electron emission from lithium niobate and lithium tantalate single crystals

Anomalous electron emission from the $$(10\overline {14} )$$ surface of LiNbO3 and LiTaO3 single crystals excited by soft x radiation has been discovered and is investigated. The absence of anomalous emission from the $$(10\overline {10} )$$ face of these single crystals is established. The experimental results confirm the theoretical conclusions that the anomalous emission is caused by the presence of a maximum in the distribution of the potential in the near-surface layer. The dependence of the lifetime τ of the anomalous emission on the dielectric constant ɛ of the ferroelectric is calculated.

On the observation of anomalous electron emission from a thin film under the influence of an outside electric field

It is established that accelerated secondary electrons with very low energies emitted from a thin surface layer where Mossbauer absorption and normal photoeffect are taking place show two distinctly separated energy lines. The maximum of the first line corresponds to the applied accelerating voltage while that of the second one corresponds to about 30% higher energy. The investigations are performed with a magnetic beta-spectrometer and with an electrostatic analyser. It is found that the intensity of the second, «anomalous» line depends on the nature of the emitting layer. A number of experiments confirming the reality of the observed effects are reported.

The investigation of controllable secondary electron emission from single particles

This paper deals with the controllable secondary electron emission from a low-density dielectric layer at the passage of single 0.7–2 MeV electrons. The absence of space charge in the dielectric is shown to cause in this case the fundamental difference between the secondary emission observed and the anomalous secondary emission. The secondary emission coefficient as well as the particle detection efficiency exceed by far those for anomalous secondary electron emission.

526. Anomalous field electron emission of SiO 2 + C system. (USSR) : A G Zhdan and M I Elinson, Radiotekh Elektron, 12 (10), Oct 1967, 1788–1793 ( in Russian)

526. Anomalous field electron emission of SiO 2 + C system. (USSR) : A G Zhdan and M I Elinson, Radiotekh Elektron, 12 (10), Oct 1967, 1788–1793 ( in Russian)

Heterogeneous reaction processes in metal combustion

Metals burn predominantly, and in some cases exclusively, by heterogeneous reactions, since both the fuel and the products are usually in the condensed state. In metal combustion, transport processes exert at least a partially controlling influence, and the usual techniques of metal-combustion research are therefore not well suited for obtaining information on reaction kinetics. Current knowledge of rates and mechanisms of these reactions is therefore extremely limited. However, assuming a collision, efficiency of 0.1, one can show that surface reaction rates becomes rate-controlling for characteristic dimensions less than roughly 40 mean free paths. A knowledge of reaction kinetics is therefore of interest if one is concerned with processes at the surface of small particles. This paper deals primarily with the case of vapor-phase burning, in view of its importance in practical applications. In this case, heterogeneous reaction of metal vapor and oxygen (and possibly of volatile oxide species) may take place at the surface of growing oxide smoke particles, in competition with homogeneous gas-phase reaction followed by condensation. The relative importance of the heterogeneous and homogeneous reaction paths is currently the subject of some speculation and controversy. In this connection, a knowledge of the collision efficiency of the heterogeneous reaction would be of considerable interest. For the Mg-O2 reaction, the author previously derived an average collision efficiency of 0.075, but this value cannot be regarded as very reliable. Results obtained by a recently developed method indicate larger values, probably exceeding 0.3. The mechanism of the reaction of metal vapor and oxygen at the oxide surface is unknown. However, from analogy with chemisorption and catalysis at oxide surfaces, it is suggested that electron transfer plays an important role, and, under some conditions, may become ratecontrolling. Observations of luminescence and of anomalous electron emission during reaction of Mg vapor and oxygen at the oxide surface have provided evidence for electronic excitation in the oxide layer.

Anomalous electron emission from metallic surfaces

Low-melting metals and alloys were heated in a Geiger-Müller free-flow tube and the electron emission from their surfaces was observed in order to confirm earlier observations that an electron shower may occur upon solidification and to extend the number of materials so investigated. The solidification process in tin, bismuth, and cadmium is associated with an anomalously high emission rate. No emission peaks were observed during the solidification of lead, lead-tin alloys, and a cadmium-tin alloy. In the cases of lead and a cadmium-tin alloy, however, emission peaks were observed during the heating cycle at temperatures below the melting point. Earlier work of the emission effect (Kramer effect) is reviewed and possible mechanisms are discussed.