The Advanced Electron Microscopy team at CERN uses advanced material characterization techniques, such as Scanning Electron Microscope – Focused Ion Beam (SEM-FIB), Electron Backscattered Diffraction (EBSD), or 3D FIB tomography in order to evaluate production techniques, help to optimize fabrication processes, and to analyse materials used in accelerator facility (e.g. as collimators,...
In order to determine if beam loss damage can induce breakdown, tests have been done in the pulsed DC system at CERN. This system consists of 2 large surface area high precision machined electrodes that are placed in parallel to each other with a gap between 40um and 100um, under high vacuum. The systems are dedicated to studying electrical breakdown phenomena and conditioning processes. A...
Recent tests have been carried out in a cryogenic pulsed high-voltage system in the FREIA Laboratory to investigate the aptness of heat-treated (soft) copper and non-treated (hard) copper as a candidate for CLIC accelerating structures. The surface of the soft copper electrodes was exposed to high electric fields in the range from room temperature to cryogenic temperatures. We calculate the...
One of the major topics in the vacuum arcing community during the recent years has been the balance between the bulk and surface effects on the breakdown generation.
In our recent study, argon and oxygen plasma were used to clean the surfaces of copper electrodes prior exposing them to high-voltace DC pulses and breakdowns. This was done without the need to open the vacuum and expose the...
Vacuum breakdown in the pulsed DC experiment at CERN was investigated by means of electrical measurements, SEM and STEM imaging of the surface and cross sections of the copper electrodes, and numerical modeling. The breakdown sites comprise one or more craters. There is plastic deformation beneath the craters.
The crater(s) represent the epicenter of the breakdown, where electron emission...
Cu electrodes are widely used in accelerators, for example in CLIC [1]. One of the problems arising in the accelerator is the breakdown phenomenon [2] causing damage to the accelerating structures and disturbances in the accelerated beam. The cause of the vacuum breakdowns is still under investigation and the electrodes regularly investigated for clues.
In this study a Cu cathode with its...
The possibility that the flow of electrons emitted by a cathode at high dc voltage is essentially due to local emission of a covering dielectric layer is at the basis of the BIRD (Breakdown Induced by Rupture of Dielectric) model. This model assumes that, in presence of sufficient electric field, the electrons trapped in polarization structures of the dielectric layer are extracted by quantum...
This work compares three different procedures for the application of the Lightning Impulse Voltage Waveform (LIVW) with 1.2/50 μs front/half value times: Up & Down (UD), Always Breakdown (AwBD) and Multilevel (ML) to obtain the voltage breakdown distribution for Vacuum Interrupters (VIs).
The voltage pulses were applied to several VIs for medium voltage application with both fixed and...
Considering quantum effects of field emission in a nanogap, we explore the impact of space charge on the field emission current density by using one dimension quantum model which includes exchange-correlation effects and space-charge effects. It is found that the quantum effect is significant within a certain range of applied voltage and gap. Therefore, the quantum regime, high current regime,...
Electron field emission plays an essential role in a wide range of applications, such as, electron microscopes, X-ray sources, high power microwave sources and amplifiers, vacuum micro-electronics, and emerging nano-electronics [1-5]. Ultra-thin coatings are fabricated onto metallic cathodes to provide chemical and mechanical protection, and longer current stability, smaller turn-on electric...
Spatially resolving electron emission from field emission cathode surface is a crucial step to characterize and quantify emission uniformity and emission area, and therefore, beam brightness and current density. It is especially important for mm-wave/terahertz sources for which narrow beam is a must, or for miniaturized field emission devices where high current density is required. It is also...
High voltage insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER Neutral Beam Injector (NBI).
The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the...
In a quest to identify low density materials for anode wires to be used in electrostatic septa for accelerator particle beam extraction, a basic test set-up was constructed at CERN. The set-up re-uses many components of the former LEAR electrostatic injection septum to limit the cost. As such, it inherits design features from the former septum, such as a displacement system for the anode and...
As field emission effects play critical role in the physics of breakdown, modelling these effects is a critical aspect to better understand and mitigate breakdown. Proposed here is an initial particle distribution generator called FEgen version 1 (based on the time dependent Fowler-Nordheim equation), a part of the FEmaster platform which is a comprehensive toolkit for field emission...
Vacuum arcs (breakdowns) limit operation in high-gradient accelerating structures and other high-voltage vacuum devices. To investigate the phenomenon a collaboration has been established at CERN to explore the application of machine learning in the CLIC (Compact Linear Collider) high-gradient test stands.
Currently, a machine learning framework has been developed to analyse the test stand...
As the ubiquity of field emission sources becomes more prevalent in that a variety of cathode materials and geometries are being tested or used, an easy-to-use data processing pipeline FEbeam was developed, and it is a part of the FEmaster platform [1-3]. This algorithm processes and converts raw data to the standard format enabling further physics interpretation: combining 17 different...
The advantages of the vacuum technology such as environmental compatibility and emission-free solutions make the vacuum interrupters attractive for switching applications in power grids. The lifetime of the contact system is mainly limited by accumulated thermal load of the electrode surface. Therefore, various measures are applied for the reduction of this load. Two types of electrode...
Analysis of deviations from the similarity law, observed at high and very high pressures in experiments on discharge ignition and breakdown in corona-like configurations, can serve as a useful, albeit inevitably indirect, source of information about microprotrusions on the surface of electrodes.
Current-voltage characteristics of field electron emission from cold cathodes in vacuum follow...
Recently a novel fabrication method of silicon field emission arrays (FEAs) was introduced by Edler et al. [1], [2]. By combining wafer saw dicing with anisotropic wet chemical etching a versatile, inexpensive and easy reproducible manufacturing process is being presented. In accordance to literature the formation of the tips is explained by the differing etching rates of silicon crystal...
In this talk, I will present a new experimental method of beta estimation that allows for a field-dependent estimation of beta for the first time to our knowledge. I will present the theory behind the method as well as how to use the code supplied for its implementation.
Atomic diffusion on metallic surfaces under electric field is known to be biased towards higher field values. In the vicinity of a field-enhancing protrusion, stochastically hopping surface atoms are hypothesized to contribute to the growth of the protrusion by this bias mechanism.
In the context of the Compact Linear Collider (CLIC), field-enhancer growth by diffusion is one of the...
The quantum-mechanical problem of the motion of electrons through a potential barrier with an additional near-surface dipole layer is considered. An analytical generalization of the Fowler-Nordheim formula, for such potential barriers, was made. Using such a model is a way to consider the influence of vacancies and pores existing in the surface layer of a metal on the field emission current...
An electron within a piece of condensed matter is held into the material by a surface barrier. In
simple basic models, the force preventing electron escape is attributed to an image attraction between
the electron and the material surface. This gives rise to an energy barrier of zero-field height H that
prevents classical escape. The application of a classical electrostatic field of...
We present a model for stochastic, micron-scale field emission for use in Particle-In-Cell Direct Simulation Monte Carlo (PIC-DSMC) simulations of vacuum discharge. PIC-DSMC simulations of mm-sized electrodes cannot resolve atomic-scale (nm) surface features and therefore we generate micron-scale probability density distributions for an effective “local” work function, field enhancement...
This Poster forms part of a quiet campaign by the author to bring the theory of field electron
emission (FE), as used in technological contexts, into a common form based on: (a) the "new" rules
for writing scientific equations introduced as part of the 1970s reforms associated with the
introduction of SI units; and (b) modern formulations [1,2] of the 1956 FE theory of Murphy and
Good (MG)...
In this work, we continue looking into polarization characteristics of surfaces in the presence of adatoms by means of density functional theory. We follow the changes of these characteristics during possible transitions of an adatom as an event of self-diffusion on the W{110} surface. We analyze the dependencies of these characteristics on the environment of the adatom when it jumps towards...
When metal surfaces are exposed to the hydrogen ion irradiation, light ions are expected to penetrate deep into the material and dissolve in the matrix. However, these atoms are seen to cause significant modification of surfaces indicating that they accumulate in vicinity of the surface. The process known as blistering may reduces the vacuum dielectric strength above the metal surface, which...
Plasma-wall interaction is one of the most critical factors determining plasma parameters in fusion devices. Plasma parameters, material properties, and morphology of plasma-facing components (PFCs) determine this interaction. PFCs must satisfy the needed requirements, such as operation under high thermal and particle irradiation. In the case of tungsten (W) PFC, its surface morphology may...
Feasibility of the production with ionized sputtering of Mo/Cu multilayers with nanometer periodicity inside compact X-band radio-frequency cavities is studied. High gradients require high electromagnetic fields and power flows, which pose serious issues with the materials. Pulsed stress at surface could be responsible of the surface breakup by cyclic fatigue. Nanometer periodicity metallic...
As breakdowns are a major concern in the use of vacuum electronics systems and DC/RF accelerators, there is a need then to be able to characterize these breakdowns and measure the probability rate during conditioning of these devices in real time [1,2]. This results in the need for a software that can automate the conditioning process. Such automation could allow for high accuracy calculations...
High-voltage accelerating structures (and other vacuum devices) are limited by the occurrence of vacuum arcs. To operate reliably such components must be subjected to a conditioning process in which the operating voltage is slowly increased over time while monitoring for arcs. This process is performed at great temporal and monetary expense, typically requiring millions of RF pulses or several...
The Voltage Holding Prediction Model (VHPM) is a design technique formulated at Consorzio RFX that combines numerical modelling and experimental data in order to calculate a breakdown probability curve. The cuve is based on the two parameters Weibull distribution for any multi-electrode multi-voltage vacuum insulated system under dc voltage. The progresses in employing the VHPM to predict the...
Almost 120 years after the isolation of vacuum arcs, the exact mechanisms of the arcs and the damage they produce are still being debated. We describe our simple and general model of the vacuum arc that can incorporate all active mechanisms and aims to explain all relevant data. Our four stage model, is based on experiments done at 805 MHz with a variety of cavity geometries, magnetic fields,...