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 cosily 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 lip of around 4 is in agood 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. © 2018; Vinca Inst Nuclear Sci. All rights reserved

Experimental Studies of The Evaporation of Pure Liquid Droplets in A Single-Axis Non-Resonant Levitator

Though a simple daily observation, evaporation of drops is still poorly understood due to the complex nature that involves hydrodynamic effects in the bulk fluids and transport phenomena at the liquid-vapor interface. This paper reports on the evaporation of single component droplets (water, ethanol, acetone, and glycerol) levitated in a single-axis non-resonant levitator. It was observed that the acetone and ethanol evaporated faster than water, although the acetone is the most volatile. The estimated lifetime of acetone is less than 5min, which is much shorter as compared to 56min for ethanol or about 90min for water droplets. On the other hand, glycerol showed no tendency to evaporate. With increasing the evaporation time, the ratio of large and small semi-axis decreases and tends to 1 corresponding to changes in drops shape from oblate ellipsoid to a sphere. Based on the classical D2-law, the surface regression rates have been estimated. © 2020 All Rights Reserved

Softver za simulaciju nagrizanja silicijuma LSPro

U zapreminskom mikromašinstvu procesom vlažnog hemijskog nagrizanja odstranjuje se nepotrebni silicijum u otvoru maskirajućeg materijala i formiraju se kompleksni trodimenzionalni oblici. Dobijeni trodimenzionalni oblici predstavljaju neku vrstu projekcije dvodimenzionalnog, planarnog lika sa maske u treću dimenziju, koja je ovom slučaju debljina silicijumske pločice i prostiru se do dubine nagrizanja. Za vreme anizotropnog vlažnog hemijskog nagrizanja silicijuma u vodenom rastvoru TMAH koncentracije 25 tež. % TMAH na temperaturi od 80 °C različite kristalografske ravni se nagrizaju različitim brzinama. Zbog različitih brzina nagrizanja neke ravni će se pojavljivati tokom nagrizanja dok će druge nestajati. Tokom nagrizanja javlja se problem podgrizanja konveksnih uglova i pojave određenih kristalografski ravni koje deformišu trodimenzionalne silicijumske strukture. Problem podgrizanja se može rešiti direktnim uračunavanjem ove pojave u projektovanju i kompenzacijom dodavanjem viška materijala na mestima koja se brže nagrizaju (strukture za kompenzaciju). Projektovanje željenog oblika u zapreminskom mikromašinstvu je najčešće veoma komplikovano i zahteva puno eksperimentalnog rada da bi se predvidelo kako se ponaša odgovarajuće sredstvo za nagrizanje. Eskperimentalni rad zahteva potrošnju silicijumskih pločica i sredstva za nagrizanje, kao i primenu procesa termičke oksidacije i hemijskih pranja u pripremi za eksperiment. Razvoj softvera za simulaciju nagrizanja silicijuma u anizotropnom vodenom rastvori smanjuje troškove razvoja i proizvodnje u IHTM-CMT jer se pre izrade silicijumske strukture mogu simulirati različite maske i izabrati onaj dizajn koji je najbolji

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

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

Rise time of silicon p-i-n photodiodes

Exact analytical expressions are derived for the short circuit photodiode currents excited by light pulses, under the assumption that the drift carrier velocity linearly depends on the electric field in the depletion layer. Reflection from the back surface of the photodiode is taken into account. Using the obtained expressions it is possible to establish a connection between the rise time t,~e and the product aWeff of the absorption coefficient a(A) and effective depletion layer width Wen(W) at various ratios of the diode thickness and the effective depletion layer width. The influence of the RC-constant (where C is the photodiode effective capacity and R is the sum of the diode series and loading resistances) on the rise time is also analyzed. One of the most important conclusions is that generally the rise time is larger for p-n-n + photodiode configurations than for n-p-p + configurations at the same substrate resistivity