Experimental investigation of emission inhomogeneity of gyrotron cathodes basing on their current-voltage characteristics

A scheme for automatic measurement of current-voltage characteristic of gyroresonance devices has been developed. Efficient emission inhomogeneity of cathode in powerful gyrotrons has been studied basing on analysis of current-voltage characteristics. Experimental data on the parameter of efficient emission inhomogeneity for different regimes are presented, as well as the estimate for admissible efficient inhomogeneity of the cathode for powerful gyrotrons and the dependence of electron beam parameters on efficient inhomogeneity of the cathode. A method of complex measurements is proposed, which makes it possible to judge about the contribution of different physical mechanisms into emission inhomogeneity of the cathode.     

The volt-ampere characteristics of the leakage and emission current of sandwich cathodes

The study of the properties of a metal-dielectric-metal system in a strong electric field has so far led to heterogeneous results. Their agreement with theory is unsatisfactory or lacking entirely. The theoretical conceptions are based on the assumption of the field emission of electrons from the surface of metals to the conduction band of the dielectric and from there by passage through the second metal electrode to vacuum. A perfect dielectric is assumed here. This assumption is probably not fulfilled when realizing the cathodes, as can be estimated from a comparison of the technology of producing a dielectric, for example, with the preparation of intrinsic semi-conductors. The presence of defects in the dielectric layer may have a decisive influence on the properties of the cathodes. In this way we can obtain different results even when using the same materials for making the cathodes.The present paper deals with the influence of the thickness of the dielectric on the electrical properties of cathodes. Two different dielectrics were used, produced in different ways: a dielectric dependent on the metal electrode (oxidizing cathodes) and independent (evaporated cathodes).In conclusion, the author thanks L. Sodomka and K. Frainic for assistance in the experiments.     

Experimental study of a gyrotron operated at the second gyrofrequency harmonic with the single-stage energy recovery

We obtain an output efficiency of 60%, which is record-breaking for gyrotrons operated at the second gyrofrequency harmonic, for an output power of 6 kW in the continuous-wave regime. The increase in the efficiency is achieved by using single-stage energy recovery in a gyrotron with an electrically insulated collector. Experimental results confirm the validity of calculation of the electron-beam energy spectra in the gyrotron collector region. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 850–854, October 2008.     

Measurements of the spread in the initial electron energy in a gyrotron

An analyzer intended for measuring electron energy in gyrotrons is described. Electron energy spectra are measured in various operating modes of an experimental gyrotron. It is shown that, even when no microwave field is generated, the spread in electron energy due to the space-charge effect can be as high as several percent.     

Analysis of current-voltage characteristics of inhomogeneous Schottky diodes at low temperatures

Two approaches of Gaussian distribution of barrier heights in inhomogeneous Schottky diodes have been analyzed by comparing the results for consistency between the two. For this the current-voltage characteristics of inhomogeneous Schottky diodes have been generated by using analytically solved thermionic-emission diffusion equation incorporating Gaussian distribution of barrier heights and by direct numerical integration over a barrier height range. The differences in the results obtained in two approaches are discussed and it is shown that the two approaches yield current-voltage characteristics with slightly different features. The discrepancies in the results obtained in two approaches are attributed to the same series resistance assumed for all elementary barriers of the distribution. It is shown that assigning same series resistance to all barrier of the distribution in numerical integration approach causes current saturation at low bias and inhibits intersection of current-voltage curves from being observable which otherwise occurs in the curves obtained using analytical equation. The paper deals with these aspects in details.     

碳纳米管冷阴极脉冲发射特性及仿真模型研究

针对碳纳米管场致发射冷阴极在微波、毫米波电真空辐射源器件中的应用需求,采用2μs,20 kV的脉冲高压对碳纳米管场致发射冷阴极的脉冲发射特性进行了实验研究.通过改变阴阳极间距,对碳纳米管冷阴极发射电流特性及发生脉冲高压打火后的碳纳米管冷阴极发射特性进行了测试研究.在直径为4 mm的圆形平面碳纳米管冷阴极上获得最大发射电流16 mA,电流密度为127 mA/cm~2.以实验测试数据为基础,结合粒子模拟软件建立碳纳米管冷阴极场致发射仿真模型,给出了该仿真模型的相关参数,为下一步设计研制碳纳米管冷阴极电子光学系统及相关辐射源器件奠定基础.     

爆炸发射阴极特性的研究进展

文章对爆炸发射阴极的物理机制进行了阐述，着重对爆炸发射阴极的各种特性进行了介绍，包括阈值电压、发射均匀性、等离子体闭合速率、阴极寿命等．并且对研究爆炸发射阴极特性所采用的理论和测量方法进行了介绍，给出了目前阴极材料使用中存在的问题．     

Leakage and emission characteristics of condenser cathodes with thin layer of Al2O3

The paper gives the leakage and emission characteristics of sandwich cathodes of the type Al-Al₂O₃-Au prepared on polycrystalline Al and on a layer of Al deposited in vacuum with a layer of Al₂O₃ 100–500 Å thick. The samples with a monotonous leakage characteristic have an exponential emission characteristic and the samples with anomalous leakage characteristic showing a maximum have an emission characteristic beginning at anomally small voltages on the sample and in the energy spectrum they exhibit excess energy. The relationship between these two types and the method of preparing the layers is given. Measurements are described which bear witness to the slow processes taking place in the cathode and the paper gives hypotheses on the mechanism of emission from such cathodes.Thanks are due to I. Emmer who prepared a number of samples and performed several measurements (Figs. 3, 10) for paper [14] which the present author has used here.     

Field emission and current-voltage properties of boron nitride nanotubes

We have measured electrical transport properties of boron nitride nanotubes using an in situ manipulation stage inside a transmission electron microscope. Stable currents were measured in a field emission geometry, but in contact the nanotubes are insulating at low bias. At high bias, the nanotubes show stable, reversible breakdown current.     

Experimental studies of gyrotron electron beam systems

In order to provide electron beams of powerful gyrotrons, magnetron-injection guns operating in the regime of the temperature limited current are used. The electron beam quality and gyrotron performances are defined both by the cathode emission processes and the processes occurring in the electron beam during its formation and transportation. The results of measurements of the energy spectrum and velocity spread of the gyrotron electron beam in different regimes are given. Experimental data on the parameter of efficient emission inhomogeneity for different regimes are presented, as well as the dependencies of electron beam parameters on efficient inhomogeneity of the cathode     

Experimental study and interpretation of the emission band situated at 1.65 eV in cinnabar

In this paper we analyse the cathodoluminescence of cinnabar powder which emission appears as a broad band peaked at 1.65 eV at liquid helium temperature.In a previous paper we showed by kinetics measurements that this luminescence was due to a donor-acceptor pair emission mechanism.However the donor-acceptor model was not sufficient to explain the Gaussian shape of the emission band as well as its important width.We show here the influence of a strong phonon coupling by measurements in terms of temperature. Application of both donor-acceptor and vibronic models allows us to interpret this luminescence satisfactorily. We calculate the fundamental parameters of the emission (Bohr half-radius R_B, maximum radiative probability W_MAX, emission energy of the pairs at infinite separation hv_∞).At the end a deconvolution method which takes into account the vibronic influence permits us to calculate the time-resolved spectra and to compare them with the experimental ones.     

Dynamic current-voltage characteristics of chalcogenide-glass thin-film diodes

The dynamic current-voltage characteristics are analyzed for a metal/chalcogenide glass/metal system in which the metal of one or both electrodes interacts with the glass.     

Static current-voltage characteristics of point-contact crystal rectifiers

An investigation of the current-voltage characteristics of a point-contact rectifier, taking account of the surface states, is described. Consideration is given to the influence of the forming of the rectifier point upon the current-voltage characteristic due to the change in the charge density in the surface state, caused by the redistribution of the charges on the contact periphery and in the bulk material.     

Investigation of the current-voltage characteristics of superconducting film structures

The current-voltage characteristics (CVC) of sandwich structures consisting of superconducting Pb(S) and normalconducting Ag(N) in the combination SNS, NSN and SN have been investigated with the current flow parallel to the phase boundaries. We observed step structures in the CVCs and their dependence on an external magnetic field and on the layer thickness of Pb and Ag in the temperature range between 1.5 K and 4.2 K. The experimental results are compared qualitatively with the behaviour of the CVCs calculated by Hansack and Kümmel¹⁰.     

Possible operation of the KfK gyrotron at harmonics

Expected efficiencies for the possible operation of the KfK gyrotron at the first, second and third harmonics are estimated. The results obtained in the cold cavity approximation and in self-consistent calculations are compared. Mode competition between the modes operating at the fundamental and at the second harmonic is investigated. Regions of stable single-mode generation are established. Effect of the the magnetic field tapering on the mode competition is illustrated.