Secondary electron emission and surface charging evaluation of alumina ceramics and sapphire

The breakdown of alumina rf windows is mostly caused by multipactor, as well as by material defects and contamination. Since multipator induces localized surface heating, leading to surface melting, it is necessary to observe secondary electron emission (SEE) coefficients of alumina ceramics under high temperature conditions. The SEE coefficients of commercial alumina ceramics and sapphire were measured by a scanning electron microscopy (SEM) with a single short-pulsed electron beam (100 pA, 1 ms) at room temperature and at 650degC. Additive materials used for sintering alumina, such as SiO ₂ and MgO, were also investigated. Surface charging evaluations have also become important because the accumulated charges are discharged at the threshold Held, resulting in surface discharge. The surface charging evaluations were carried out by multi-pulse measurements with the injection of successive pulses on the sample. As a result, reductions in the SEE coefficients with temperature were confirmed, except for sapphire. The multi-pulse measurement results indicated that surface charging of the sapphire was higher than that of other samples. This may be one of the factors that causes sapphire not to be durable for rf window applications, compared with alumina ceramics. Although there are few exceptions, it was found that the SEE coefficients of alumina ceramics increased with the purity and the average grain size     

Effect of mechanical finishes on secondary electron emission of alumina ceramics

The effect of surface roughness of insulator on the secondary electron emission (SEE) coefficients was investigated with changing the incident angle of primary electrons. The SEE coefficients were measured using a scanning electron microscope with a single-pulse electron beam (100 pA, 1 ms). As a result, the SEE coefficients increased with the incident angle for smooth surface, while those of rough surface almost did not change with the incident angle. The SEE coefficients of commercial alumina ceramics with three different surface finishes, i.e. as-sintered, as-ground and mirror-finished samples, were also measured before and after annealing treatments. This treatment was carried out in air at 1400 degC for 1 h. After annealing, the SEE coefficients were higher than those of unannealed samples. This increase is proposed to be due to the defect recoveries as well as neutralizations of charging, which significantly influence the SEE. For relative low purity (95% or less), the SEE coefficients of annealed alumina became lower after mechanical grinding operations and lower again after mirror-finished operations. The SEE coefficients of 99.7% purity alumina were almost unaffected by the mechanical finishes.     

Surface breakdown phenomena in alumina rf windows

The multipactor and flashover phenomena of alumina rf windows used in a high-power klystron have been investigated. Multipactoring due to the high yield of secondary electron emission takes place during rf operation. A spectrum analysis of luminescence due to multipactoring shows that multipactor electron bombardment causes F centers in alumina, thus leading to surface melting. From the results of a high-power examination of rf windows with several kinds of alumina ceramic, it was found that alumina with a crystallized grain boundary and without any voids between the boundaries, thus having a low loss tangent value, is not liable for F centers, even under multipactoring. Flashover in a tree-like pattern of alumina luminescence occasionally takes place on a TiN-coated surface where multipactoring is suppressed. A sapphire window, whose surface was polished and having pre-existing F centers, shows a lower flashover threshold. The annealing effect of polished window surfaces was also investigated     

Light-induced electron emission from polymethylphenyl siloxane oils

Light-induced electron emission from tetramethyl tetraphenyl trisiloxane, (Dow-Corning silicone oil DC704) and trimethyl pentaphenyl siloxane (Wacker Chemie silicone oil AN175) has been measured as a function of photon energy. Absolute quantum yield and threshold energy were determined. The threshold energy of 7.5 eV for both siloxanes is comparable to the thresholds of other polymethylphenyl siloxanes with phenyl/methyl ratios of 1/3 and 1/18, which have been investigated by the authors. The quantum yield as a function of photon energy shows a systematic change with variation of the phenyl/methyl ratio. This dependence on the molecular structure is ascribed to a change of the initial ionization quantum yield     

A self-consistent model of electron emission from metals

We present a model of electron emission from metals, which takes into account deviation of the electron distribution function from the Fermi equilibrium form inside the cathode under the emission effect. Using the Grad method electron distribution function, the emission boundary is expressed consecutively in terms of its hydrodynamic moments: temperature, average velocity, heat flux density. These hydrodynamic moments are caused by the emission itself and thus need a self-consistent determination. New easy-to-use equations for the emission current density, emission heat flux, and total energy distribution function of the emitted electrons are proposed. Consideration was made for a wide range of temperatures (⩽3500 K), field strengths (⩽1.5×10⁸ V/cm) and work function (3.0 to 4.5 eV) of the emitter. A considerable deviation from the Fowler-Nordheim (FN) theory was found at high current density (>10 ⁹ A/cm²)     

Thermal and electrical analysis of alumina and beryllia coaxhigh-power windows under irradiation

Characteristics of dielectric insulators in vacuum windows of coaxial 10 to 100 MHz transmission lines in high-power steady-state use under irradiation are simulated with respect to electric, nuclear, mechanical, and thermal properties. Neutron fluence ~5×10¹⁸ n/cm² at the window is obtained to be sufficiently small to allow beryllia, but not alumina, to be used as dielectric. In beryllia (10^-3 displacements per atom (dpa) due to irradiation) or in un-irradiated alumina (97.5% purity), the temperature is found to rise by not more than 125°C with maximum stress <140 MPa for 50 kV peak voltage at 60 MHz, provided niobium, titanium or materials with similar thermal expansion coefficients are used in water cooled conductors. The tangential electric field is kept well below the surface discharge limit 2 MV/m by using potential rings together with a sufficiently large inclination angle of the conical ceramic with respect to the radial coaxial direction, but high normal fields exceeding the vacuum breakdown limit are obtained near the potential rings. Abandoning the potential rings and deforming the equipotential lines by shaping the ceramic-conductor joint can reduce both tangential and normal field components below the breakdown limit, which appears to be in agreement with recent voltage test experiments     

Observation of the field emission from the melting zone occurringjust before explosive electron emission

Emission patterns corresponding to the transition of intense field emission to explosive electron emission, not observed earlier, have been obtained with the use of a Muller projector and HV pulses of duration 2 to 5 ns. The brightness of these patterns is much higher than that of conventional field emission patterns. Moreover, the images obtained have a specific structure of varied brightness, such that regions of different brightness are “framed” in rings. Analysis of experimental data suggests that these patterns are field-emission in nature. The appearance of emission patterns of this type is accounted for by the electron emission from the liquid phase, occurring immediately prior to the explosion of a microemitter     

High field emission efficiency surface conduction electron emitters

Two different surface conduction electron-emitter (SCE) structures with the nanogap of 90 nm wide fabricated by hydrogen embrittlement (HE) and focused ion beam techniques are simulated for the first time. We employ a three-dimensional particle-in-cell method coupling with finite-difference time-domain scheme to simulate the property of electron emission in these SCEs. Our calibrated simulation predicts high emission efficiency of the SCE structure which is fabricated by HE. Compared with the other SCE structure, it is observed that the proposed structure possesses low power consumption at the fixed emission current when the width of nanogap becomes narrower. The current-voltage characteristics including conducting mechanisms are investigated and explained.     

Pyroelectric electron emission from −Z face polar surface of lithium niobate monodomain single crystal

Pyroelectric electron emission current measurements and spatial electron current distribution collections were investigated from –Z face polar surface of lithium niobate monodomain crystals. The electron emission was detected during cooling due to a field ionization effect. The gap distance variation between the crystal surface and the electron collector significantly influenced the emission behavior. For small gaps (<2 mm) the emission was controlled by intermittent runway ionizations (screening of charge species (electrons and +ions) from plasma breakdown). Whereas, for large gaps (>2 mm) the pyroelectric electron emission was supplied from a soft and maintained plasma medium formation.     

Simple algorithm to calculate T-F electron emission current density

The Gauss-Chebyshev quadrature formula is suggested to calculate the transparency of the potential barrier at the metal-vacuum boundary. It is shown that in the most interesting cases ONE node is enough to obtain accuracy of a few percent when calculating thermo-field electron emission current density.     

Numerical simulation of secondary electron emission charging atinsulator surfaces

The charging process of insulator surfaces in vacuum at HV due to secondary electron emission has been calculated using numerical simulation. The systems considered consist of two electrodes at a HV difference, separated by a glass plate or a hollow glass channel. The simulations give insight into the critical HV positions and into the influence of the geometry, the initial voltage distribution and the material properties on the charging profiles and the leakage current paths. A HV performance increase of these systems can be obtained by decreasing the impact of free electrons, adding a coating on all insulating surfaces which have a high value of E₁ (i.e. the first crossover point of the secondary electron yield curve) and/or are weakly conductive, and breaking up the total voltage difference in smaller parts by adding electrodes to all surfaces at intermediate voltages     

Flashover from surface charge distribution on alumina insulators invacuum

Surface charge distributions of disk type aluminas held between a backside electrode (alumina holder) and a needle electrode to be excited by an impulse voltage (rise time 64 μs, wave tail 700 μs) were measured. The measurement of surface charge distribution on the whole surface area of alumina YSA998 and UHA99 after impulse voltage application revealed that the surface charging can be initiated either from the anode or from the cathode triple junction. The charging initiated from the anode triple junction (for positive polarity) produced positive charge at the anode region and the density is dependent on the applied voltage, while the charging initiated from the cathode triple junction region (for negative polarity) produced negative charge around the cathode region. For positive polarity, the critical values of charge density to the flashover for alumina YSA998 and UHA98 are 5.70 and 17.2 μC/m², respectively     

Noise figure definition valid from RF to optical frequencies

Two noise figure definitions are developed that are consistent with the IEEE Standard and shown to cover the entire frequency range from RF to optical frequencies. In the classical regime, the two definitions merge. In the optical domain, they differ in value, but provide the same information. A simple way of computing the noise figure of a distributed amplifier, such as a Raman amplifier, is presented     

High-field single electron emission rates and their influence onthe Paschen characteristics in compressed gases

By means of a newly developed statistical avalanche counting technique which enables high-field, single electron emission rates (SEERS) to be measured from unsparked freshly prepared surfaces having various thicknesses of oxide film, it is established for the first time that SEERS increases, for a given electric field, with an increase in the bulk modulus (hardness) of the material cathodes; also remarkably the thicker the oxide layer, the higher the SEERS. These trends are reflected in the Paschen sparking characteristics which show that the gaseous electric strength decreases with increase in the bulk modulus and increase of oxide layer thickness of the cathode material; and thus the increase in SEERS. This former trend is opposite to that established more than forty years ago. Other hitherto unobserved phenomena like a gas pressure effect, switching from a low to a high electron emission mode and vice versa, the influence of deliberate sparking and the total absence of spark conditioning are reported. Analysis of SEERS by the more successful Richardson-Schottky rather than the Fowler-Nordheim equation shows that the low electric field intensification factors derived increase with the bulk modulus of the cathode material and ultimately are determined by the polishing procedure     

A new method detaching porous anodic alumina films from aluminum substrates

Recently, a particular focus on the research of porous anodic alumina (PAA) films is taken because of the application for growth of nanostructure materials. However, during detaching the PAA films from aluminum substrate and opening the holes of PAA films, it is quite difficult to keep intact through-hole structure of PAA films using traditional chemical etching method. Furthermore, the traditional method is time-consuming and has contamination because of the use of heavy metal ion solution. Usually, the preparation time is over 20 h. In our work, a new electric- chemical method was proposed for detaching PAA films and opening the holes of PAA films in an environmental friendly solution of HClO₄–CH₃OH in one step. The preparation process can be finished within 5–15 s. Compared with traditional etching method in which there are two-step processes, the electric–chemical method is simple, rapid and contamination-free. A large size PAA films with intact through-hole structure can be obtained.     

Classical interpretation of single electron emission rates (SEERS) at the origin, from oxide covered cathodes in compressed gas and vacuo insulation

In order to quantitatively interpret the previous experimental single electron emission rates (SEERS) obtained by Guile et al (1997), the Richardson/DushmanSchottky (1914) expression is extended by invoking the concept of multiple images of the emitted electron in the naturally occurring or grown oxide layer covering the metal cathode, first proposed by Silvester (1968) in another application. This leads to a new expression for the Schottky barrier lowering term being obtained, involving a converging infinite series of terms, solvable on a computer, and enables the SEERS to be computed for cathodes covered with varying thicknesses of oxide. Numerical agreement between applied theory and experiment is excellent and requires the postulated presence of electron multiplication-avalanching-in the oxide layer. This method can be further applied to the presence of liquid and/or solid dielectric insulation between oxide covered metal electrodes and can take into account the presence of the anode for small electrode spacings.     

Effects of secondary electron emission coefficients on Townsend's second ionization coefficient in argon dielectric barrier discharges

One‐dimensional fluid model simulations are performed to investigate how the secondary electron emission arising from ions (γ_i), metastable atoms (γ_m), and photons (γ_p) affect Townsend's second ionization coefficient (γ′) in argon dielectric barrier discharges. Since Townsend's breakdown criterion determines the value of γ′, it is naturally affected by the discharge condition such as the reduced electric field (E/p) as well as the combination of γ_i, γ_m, and γ_p. It also depends on the other contributions from the indirect ionization processes such as cumulative ionizations or metastable–metastable collision ionizations. When the nonionic electron‐yielding processes such as the effects of γ_m, γ_p, and the indirect ionizations are considered, the calculated γ′ increases largely under the higher pd (pressure times distance) branch of the Paschen curve. Since the experimental characteristics of γ′, which decreases with E/p, can hardly be explained without such nonionic processes, the contributions of those processes can be quite important in Townsend discharges especially under high pd conditions. These processes tend not to appear and γ′ tends to converge to γ_i under the lower pd branch of the Paschen curve. The value of γ_i can possibly be estimated by using γ′ under the lower pd condition. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.