Study of Multipactor Effect with Applications to Superconductive Radiofrequency Cavities

In this paper a one-dimensional Particle-in-Cell/Monte Carlo collision code has been used in order to study characteristics of multipactors. For multipactor to occur each electron striking the surface must generate more than one secondary on average. The ratio of primary to secondary electrons is given by the secondary emission yield. For this study, calculations were carried out by using Sternglass model that includes energy dependence of the secondary emission yield. The obtained simulation results for the pressure dependence of the breakdown time follow the scaling law. Number of electrons increases in time, while their mean energy decreases. Since secondary electron emission at the cavity surface plays an important role, simulation results, presented here, can help cavity designers predict multipacting issues before fabrication

Three-Dimensional Simulations of the Surface Topography Evolution of Niobium Superconducting Radio Frequency Cavities

This paper contains results of the three-dimensional simulations of the surface topography evolution of the niobium superconducting radio frequency cavities during isotropic and anisotropic etching modes. The initial rough surface is determined from the experimental power spectral density. The simulation results based on the level set method reveal that the time dependence of the root mean square roughness obeys Family-Viscek scaling law. The growth exponential factors beta are determined for both etching modes. Exponential factor for the isotropic etching is 100 times lower than that for the anisotropic etching mode reviling that the isotropic etching is very useful mechanism of the smoothing

Monitoring of respiratory volumes by an long period grating sensor of bending

Here, we present a method of respiratory volumes monitoring using a single fiber-grating sensor of bending. Measurements are conducted using simple monochromatic interrogation scheme that relies on a photodiode measurement of the power transmitted through a long period grating (LPG) sensor at fixed wavelength. Good sensor accuracy in measurements of tidal and minute respiratory volumes for different types of breathing is achieved.Conference on Light in Nanoscience and Nanotechnology (LNN), Oct 20-22, 2015, Hissar, Bulgari

The effect of the enhanced field emission on the characteristics of the superconducting radio frequency cavities

Electron field emission limiting the accelerating gradient in superconducting cavities remains the dominant setback in cavity production. The need to understand and control the field emission has become increasingly important because of the prospect of using high-gradient structures in linear colliders. Since building an accelerator structure is a complicated and costly process, elimination of unnecessary steps has priority. In this paper an analysis of the influence of the enhanced field emission in superconducting radio frequency cavity together with modal field calculations by using COMSOL finite elements package has been presented. The obtained results reveal that the electric field required for the field emission is generated in the cavity irises. The imperfection of the cavity surface leading to very high fields is modelled by a simple cone. The estimated value of the enhancement factor for the cone tip of around 4 is in a good agreement with the data found in the literature. In addition, from the slopes and the intercepts of the Fowler-Nordheim plots, a dependence of the enhancement factor and the effective area on the work function has been estimated

3D finite element eigenmode analysis of coupling mode induced resonance frequency shift in coupled microring resonators

Microring resonators are a rapidly-developing area of research in photonic devices with a wide range of applications including signal processing, filters, sensors, lasers, modulators, switches, memory and slow-light elements [1]. Generally speaking, microring resonators represent frequency selective elements that can perform a variety of functions such as add-drop filtering, switching, and modulating in wavelength-division systems. In coupled-resonator structures, one of the most critical issues is the precise control of the resonance frequency, which depends on both the resonator and cladding material and the resonator geometric parameters (radius, width, height). Also, it has been shown that when single microring is coupled to access waveguides or another rings, the resonance frequency will deviate from its original isolated resonator value. This effect, known as coupling-induced resonance frequency shift (CIFS), which is recently investigated more systematically in [2], causes resonance frequency mismatches between individual resonators and thus significantly impacts the performance of the coupled-resonator systems. By the nature of the problem this effect is most obviously manifested in system eigenspectra, although it is shown [2], [3], that CIFS can be related to the phase responses of the coupling region in the resonator coupling structure. Several methods are used for calculating the response of a microring resonator such as the prominent FDTD or modeling in terms of semi-analytic coupled-mode theory, usually in two space dimensions (2D) and rarely in 3D. Although 2D calculations are sufficient to explain some concepts and phenomena, the rigorous 3D simulations are necessary to determine the parameters of the devices intended to be used in real WDM systems, especially when the dimensions of the system are comparable with the light wavelength. For many reasons, finite element method (FEM) is the method of choice for accurate and fast simulations of photonic systems. It enables rigorous treatment of full Maxwell's equations in complicated geometries and inhomogeneous domains. Arbitrary high-order methods for faster convergence and the error control through automatic adaptive mesh refinement are available in many commercial and academic FEM packages. In our previos paper [4] we analysed eigenspectra and CIFS in finite length microring resonator arrays (systems without access waveguides) using 2D FEM method. Here we present a detailed investigation of CIFS effects in coupled microring resonators system configured as the high order serial filter based on eigenspectra analysis using full 3D vectorial FEM method. Such calculations are computationaly much more demanding, and require careful devising of adaptive mesh refinement strategy, in order to make it feasible, even on the most powerful workstation.VI International School and Conference on Photonics and COST actions: MP1406 and MP1402 : PHOTONICA2017 : August 23 - September 1, 2017; Belgrade

Numerical model of proton beam transport through electric scanner

In this work, using the numerical simulation method, the vertical and horizontal displacement of the proton beam is analyzed, by changing the voltage of the plan parallel electrodes of the electric scanner. The initial proton beam was modeled by centered Gaussian distribution. The electrostatic field of the scanner was obtained using the finite element method. The dependence of the beam center trensverse shift, as well as the beam losses at the output of the scanner, on the applied electrode voltage, has been determined. Comparison between proton trajectories calculated using analytical/numerical electric field has been performed. Based on this, the effective length of the scanner electrodes was determined. The results of this paper are applied in a technical solution that provides homogeneous fluence distribution in sample irradiation by 3 - 30 keV proton beams, obtained from a light ion source

Focusing properties of a square electrostatic rainbow lens doublet

This is a study on the properties of a square electrostatic rainbow lens doublet. The said optical element consists of two square electrostatic rainbow lenses with the second lens axially rotated for 45 degrees with respect to the first one. The propagation of a proton beam with a kinetic energy of 10 keV through the doublet is in the focus of our analysis. The potential of the electrodes of both lenses is 2 kV. The electrostatic potential and the electric field components of the lens doublet are calculated using a 3-D computer code based on the method of moments. Spatial and angular distributions of protons propagating through the lens doublet, as well as the parameters defining beam quality, are investigated. As in the case of the single square electrostatic rainbow lens, the evolution of these distributions is determined by the evolution of corresponding rainbow lines, generated by the use of the theory of crystal rainbows. Our study shows that a beam core in the shape of a cusped square is formed by the spatial rainbow line that appears first. This rainbow line occurs during proton propagation through the first lens. The beam core retains the cusped square shape during the propagation through the second lens. The electrostatic field of the second lens causes the appearance of an additional spatial rainbow line, which encompasses the beam core and defines the outer border of the beam. This rainbow line constitutes the main difference between the cases of the lens doublet and a single lens. [Projekat Ministarstva nauke Republike Srbije, br. III45006