Microscopic Field Enhancement Research Articles

Lifetime of Metallic Explosive Emission Cathodes and Microscopic Explanations

Metallic cathodes can be used to generate intense relativistic electron beams for high-power microwave devices. Many previous papers have demonstrated that metallic cathodes usually have a limited lifetime. In this paper, three metallic cathodes, namely, stainless steel cathode, copper cathode, and aluminum cathode, are researched with a repetitive-operation accelerator and an X-band relativistic backward-wave oscillator. The cathode surface morphologies are observed with a scanning electron microscope before and after lifetime test. The experimental results demonstrate that the stainless steel cathode has a very limited lifetime ( $ pulses), the copper cathode has a moderate lifetime ( $\sim 10^{4}$ pulses), and the aluminum cathode has a surprisingly long lifetime (at least $10^{5}$ pulses). The different lifetimes of the three cathodes may originate from the different situations of regeneration of microprotrusions on the cathode surface. The large number of regenerated microprotrusions which possess large microscopic field enhancement factors supports the long lifetime of the aluminum cathode.

A new metal-dielectric cathode with long lifetime

Explosive emission cathodes are widely used in high power microwave generation. Conventional metallic cathodes have the disadvantages of bad emission uniformity and short lifetime. In order to improve the emission property of metallic cathodes, a new metal-dielectric cathode is fabricated with the plasma spraying technology. Unlike previous metal-dielectric cathodes, our metal-dielectric cathode adopts a ferroelectric ceramic which possesses a large permittivity. The results of lifetime experiments show the metal-dielectric cathode presents just slight performance deterioration within 2.5 × 105 pulses and thus has a much longer lifetime than the normal copper cathode. The morphology observation demonstrates that the good emission property of the metal-dielectric cathode may benefit from the appearance of irregularities on the dielectric surface, which will have large microscopic field enhancement factors with the help of large permittivity of the ferroelectric material.

Micro-effects of surface polishing treatment on microscopic field enhancement and long vacuum gap breakdown

In this paper, three surface polishing treatments were employed to treat plate titanium electrodes, and microscopic surfaces of the electrodes after polishing were presented. Through comparing the breakdown strength of the 2.5 cm vacuum gap formed by plate titanium electrodes after the three treatments, experimental results showed that the breakdown strength was enhanced by 35% while the micro-surface roughness dropped from 3.5μm to 0.35μm. In view of that, effects of microstructural parameters after polishing on the microscopic field enhancement factor were investigated. The field-uniformity mechanism and the shield effect between micro-protrusions on the rough electrode surface were put forward and demonstrated. Based on the idea that electric field can be shield in a pit, a theoretical model was established to evaluate the maximum field enhancement factor βEmax on the micro-surface. It revealed that 1 ≤ βEmax ≤ 3.96, and βEmax had the maximum decrements of 1.96 and 2.1 both from 3.96 after the mirror polishing and the chemical polishing, respectively. When the surface roughness decreased to the scale from nm to μm, the effort on βEmax reduction through surface polishing was not effective to enhance the vacuum gap breakdown strength any more.

Research on the Emission Uniformity of Explosive Emission Cathodes in Foilless Diodes

Some factors that influence the emission uniformity of explosive emission cathodes (EECs) in foilless diodes such as the guiding magnetic field strength and the rise rate of electric field have been researched in former literatures. This paper is concentrated on another factor that has been overlooked previously, i.e., the defects with dimensions of tens of micrometers on cathode surfaces, especially at blade edges. It is shown that these defects will significantly worsen the emission uniformity of EECs by introducing extraordinarily large microscopic field enhancement factors, and thus they should be eliminated. The micrometer-scale irregularities, however, should be maintained to provide effective emission micropoints. Meanwhile, the outer edge of an annular cathode blade should be rounded off to improve the emission uniformity. After being disposed like this, the emission uniformity of annular graphite cathodes in a foilless diode is obviously improved, which leads to an increase of energy efficiency by more than 20% by broadening the microwave pulse duration under a power level of 2.8 GW for an X-band relativistic backward wave oscillator.

Electron emission contributions to dark current and its relation to microscopic field enhancement and heating in accelerator structures

Analytically tractable models of thermal-field emission, field enhancement, and heating mechanisms (Nottingham and resistive) are developed and combined to make estimates of the fields and temperatures that accompany the development and growth of asperities. The relation of asperity dimensions to dark current is discussed in two experimentally motivated examples. The hypothetical relation of microscopic sources of dark current and heating to breakdown is discussed in the context of Nottingham and resistive heating. The latter are estimated using a general thermal-field methodology. A point-charge model is used to find field enhancement factors. Last, a thermal model is used to estimate the temperature dependence of the resistivity and thermal conductivity. Together, these models suggest that conditions can arise in which the temperature at the apex of an asperity can experience growth and contribute to melting or fracture (or both), and that Nottingham heating generally dominates the resistive heating term.

Fabrication of ultra-clean copper surface to minimize field emission dark currents

The current from copper electrodes treated by four different types of surface cleaning procedures was measured under DC high field gradient condition. The best results were obtained by using the electrode rinsed with ultra-pure water after diamond turning. A field gradient of 47 MV/m was achieved with dark current at the level of 1 nA, and the microscopic field enhancement factor was estimated to be a very low value of 56. The dark current from this electrode was dependent only on the field gradient at the cathode and not affected by the total voltage applied to the gap. In this case the surface would be covered by a Cu 2O layer, which creates few secondary ions by the electron bombardment. On the other hand, a large total voltage effect and a large vacuum increase were observed for the electro-polished electrode. Cu(OH) 2, which would be formed at the copper surface during the electro-polishing process, would emit H 2O molecules during the electron bombardment.

Instabilities of prebreakdown currents in vacuum II: the nature of the emission sites

Prebreakdown electron emission between extended copper electrodes is experimentally studied with a time resolution <100 ns. When taking the current-voltage characteristics sufficiently quickly it is found that after a voltage increment the emission can relax to a lower level with time constants <1 ms. As a consequence, Fowler-Nordheim plots become flat, yielding values of the microscopic field enhancement that are too high. During the deposition of a metal-vapour plasma that originates from neighbouring arc gaps, a steady growth of the emission is observed, which leading to a considerable reduction of the voltage, down to 50% of the initial value. The results are discussed in terms of metallic-field electron emission from atomic structures that are subjected to surface migration under the influence of the electric field. The migration processes can be activated by mechanical shocks. This explains the shock effects found in the preceding paper in this issue.

Field emission in superconducting RF cavities

Broad area rf field emission from superconducting Niobium surfaces was investigated. A specially designed superconducting resonator (reentrant cavity) allows direct measurement of the field emission current. Experiments were performed at four cavity modes between 500 MHz and 3.5 GHz in order to study the frequency dependence of rf field emission. Details for the cavity design, cavity treatment, experimental set-up, and measurement technique are given. Field emission currents in the range 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> A to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> A were detected at macroscopic electric surface fields of 10 to 50 MV/m at the emitter. The data were analyzed with the Fowler-Nord-heim theory, modified for rf fields. The microscopic field enhancement factor B was in the range of 40 to 150 for different surface conditions and showed no significant frequency dependence. These observations are in agreement with the predictions of dc field emission, but in contradiction to the not yet understood field emission phenomena in superconducting accelerating cavities. The hypothesis of field emission from hot emitters, probably consisting of dust particles heated by the rf fields, can qualitatively explain this discrepancy. Experimental observations on superconducting accelerating cavities support this model.

Breakdown and field-emission behaviour of differently prestressed vacuum interrupter copper contacts

To obtain more information about the dielectric strength of open vacuum interrupters under practical conditions, the breakdown behaviour of differently prestressed contacts was determined with test voltages (alternating and pulse voltages with various rates of rise). Also, the corresponding surface micro- structure was examined by recording the field-emission current as a function of the applied voltage; the Fowler-Nordheim equation was used to calculate the microscopic field-enhancement factor and the emission area. A large number of tests were performed using a computer-controlled test and registration system to obtain statistical reliability. Additionally, the influence of long-time voltage application (∼– min) and the influence of surface gas layers were examined. The results of the investigation show clearly that high-voltage testing of vacuum interrupters yields results not representative for the behaviour of the interrupter at service conditions, if the influence of switching operations on the dielectric strength is neglected. The lowest breakdown voltages are caused by no-load opening of welded contacts, not by high-current interruption