Space Charge Of Emitted Electrons Research Articles

Effect of electrode temperature on radiofrequency vacuum breakdown characteristics

The results of a numerical simulation of the heating of a microprotrusion on the surface of a copper electrode under the influence of radiofrequency electromagnetic fields, performed for different temperatures of the electrode, are presented. The calculation model describes self-consistently the thermal processes in the electron and phonon subsystems, the electron emission, and the effect of the space charge of emitted electrons. It has been shown that when the initial electrode temperature is increased to 1000 K, the time of heating of the surface microprotrusion to a critical temperature decreases by a factor of 1.5–1.9, namely, to values ranging from a few to tens of nanoseconds. This may significantly increase the probability of radiofrequency vacuum breakdown on the surface of the accelerating structure of an electron-positron collider.

The dimensional effect of the space charge on the self-consistent electric field at the cathode surface

This work is devoted to a theoretical study of the effect of the space charge of emitted electrons on the electric field strength near the surface of field emitter and a point microprotrusion on a field emission cathode by using a two-dimensional axisymmetric problem statement. Based on the particle-in-cell method, a model has been developed and self-consistent calculations of the electric field strength and field emission characteristics has been performed for the range of emitter tip radius from ~10-4 to ~10-7 cm. For the geometry under investigation it has been shown that the space-charge screening of the external field is substantially less pronounced for emitters whose tip radii are comparable to the size of the region where the space charge is mostly localized. As for emitters having nanometer tip radii, their current-voltage characteristics remain linear in the Fowler-Nordheim coordinates up to field emission current densities of ~109 A/cm2.

Change in the emitter surface during high-current-density field emission from the 〈111〉 trihedral angle of a reconstructed tungsten tip

Changes in the shape and emission characteristics of the atomically sharp trihedral 〈111〉 angle of a tungsten tip reconstructed in an electric field are studied by continuous-mode field desorption microscopy during high-current-density field emission. The main changes in the tip shape and the slope of the Fowler-Nordheim characteristic occur at an emission current of 1–5 μA. At a current of 50–100 μA taken from the angle, the tip shape and emission characteristics stabilize and remain unchanged in the range 0–150 μA. The new tip shape is characterized by the widening of the angle edges; the appearance of {112} and {001} plane steps in them; a decrease in the sizes of the {011} planes forming the angle faces; and the appearance of steplike transition regions between {011}, {001}, and {112} faces. These changes in the tip shape are related to the fact that the emission field is weaker than the electric field used for preliminary tip reconstruction, the weakening of the field by the space charge of emitted electrons, and a nonuniform temperature distribution in the tip.

Effect of the space charge of emitted electrons on field electron emission

It is shown that the conventional technique of substituting the field strength at the space charge-emitter interface that is calculated with the Poisson equation into the Fowler-Nordheim formula considerably overestimates the effect of space charge on field electron emission. In this work, the space-charge-induced field attenuation as a function of the emission current density and radius of curvature of the emitter surface is derived using the model of a planar space-charge layer. It is argued that field electron emission cannot be studied in terms of the spherical diode model, since it assumes the presence of a space charge on the back (nonemitting) emitter surface, which is in fact absent. It is stated that one should consider the discrete character of the charges when investigating the space charge in field electron emission, because the mean spacing between the electrons emitted far exceeds the emission barrier width.

On the screening of the electric field at the cathode surface by an electron space charge at intense field emission

The paper considers the of the space charge of emitted electrons on the electric field at the tip of a field emitter. To analyze this phenomenon, the Poisson equation was solved numerically for a spherical vacuum diode with a field emission cathode. The dependence of the electric field on the geometric field has been compared with the known approximate dependencies, Strong screening of the electric field at the cathode by the space charge of emitted electrons at fields >5/spl times/10/sup 7/ V/cm is noted. Based on the obtained dependencies of the actual field on the geometric one and using the SuperSAM code, the field emission occurring in a Muller projector has been simulated. It has been shown that the ring effect appearing in field emission patterns can well be explained by field emission from the peripheral regions of the field emitter. The current density distributions over the surface and in the bulk of the emitter have been analyzed.

Heating and failure of niobium tip cathodes due to a high-density pulsed field electron emission currenta)

This article presents the investigative results of pulsed threshold current densities of niobium tip cathodes at temperatures ranging from 4.2 to 300 K. It has been shown that the superconducting-to-normal phase state transition of the cathode has no effect on pulsed threshold current densities. Estimations have been made for the factors limiting the current from a superconducting emitter. The experiments have shown that the cooling of the cathode from 300 to 4.2 K changes the dynamics of cathode heating considerably and leads to a steady increase in the threshold current densities with the delay time range of 10−9–10−6 s. Numerical analysis of thermal processes in the emitter has shown that the above effects are not due to the difference between the electron and the phonon temperatures (the electron–phonon relaxation time does not exceed 10−9 s), but to the more intense heat transfer processes and the reduction of Joule heating, with the cooling of the cathode. It has been shown that the space charge of emitted electrons produces a considerable effect on the temperature and current density distribution in the tip apex. Investigations have shown that the mechanism of the tip cathode failure depends on the delay time range. The simple relationship for estimation of average pulsed threshold emission current density of the tip cathode for a delay time range of 10−9–10−7 s is presented.