This Poster reports a small part of a multi-stage project to improve the interpretation of measured field electron emission (FE) current-voltage [I_m(V_m)] data. The first stage of the project involves five or so proposals that aim to improve data interpretation within the framework of the almost universally used (but not physically realistic) planar emitter approximation. This models...
We are modeling the effects of a field enhancement factor (β) on the generated Fowler-Nordheim current using the Particle-in-Cell Direct Simulation Monte Carlo (PIC-DSMC) method for a vacuum environment. In the present work, we vary the DC voltage, and hence electric field, between two parallel Pt plates in which the modeled electrode surface elements are given a local work function and β by...
An ideal field electron emission (FE) device/system is one in which: (a) the measured current I_m equals the emission current I_e; (b) the characteristic local barrier field F_C is related to the measured voltage V_m by the formula F_C = V_m/ζ_C where the voltage conversion length ζ_C is effectively constant; and (c) the work-function is constant. An orthodox FE...
Highly-alloyed chrome-nickel stainless steel is the most important material in vacuum technology. Especially in high and ultra-high vacuum technology it is used for vacuum chambers and components. One of the main research focuses for 2005 [1] was the study of parasitic field emission of specially treated stainless-steel surfaces for applications in accelerator technology. The measured...
The development of a negative ion beam source has been in progress in KOREA as part of ITER technology R&D programme. The RF negative ion beam source is one of the key parts of ITER neutral beam injection (NBI) system. In particular, the use of Cs leads to a remarkable enhancement of negative ion production and thus enables the extraction of high current negative ion beams. The final...
The dynamics of dislocation activity under intense electric fields has been proposed as a potential nucleation mechanism of breakdowns. Results of searches for high-frequency fluctuations in field-emitted current in the pulsed DC system at CERN, that could arise as a consequence of changes in the surface geometry as a result of dislocation motion.
The IFIC RF laboratory will perform the conditioning and high power tests of S-band accelerating structures designed for hadron-therapy with the purpose of investigate their high gradient behaviour. The facility is based on the scheme of the Xbox-3, a X-band test facility at CERN, adapted to operate in S-band. Two medium peak-power (7.5 MW) and high repetition rate (400 Hz) S-band klystrons...
Field emitted electrons have important consequences in the operation of high-gradient RF accelerating structures both by generating so-called dark currents and initiating RF breakdown. The latter is an important limitation of the performance in such devices. Another kind of vacuum discharge that primarily affects the operation of lower-field RF components, for example those used in space...
An underestimated phenomenon in the operation of field electron emission is the smooth transition to glow discharge as a significant pre-stage of vacuum arcs, which usually occur more frequently at higher operation vacuum pressures and higher emission currents in the range of normal glow discharge (> 0.1 mA). This phenomenon can be understood by investigating the transition from field electron...
ArcPIC is a 2D electrostatic particle-in-cell code for simulating vacuum arc discharge plasmas.
It is meant to be flexible, making it easy to modify with new surface boundary conditions, external circuits, and more.
ArcPIC was open-sourced 2014, and has since then gone through a technical development and started to be used by a larger community.
Especially, the build system has been...
SPIDER is the full-size prototype of ITER Neutral Beam Injector ion source, in operation since June 2018
as part of the ITER Neutral Beam Test Facility located in Padova, Italy. The ion source of SPIDER is
composed of eight Radio Frequency drivers operated in low hydrogen gas pressure, 0.3 Pa, driven at 1
MHz with a total power of 800 kW, an extraction and acceleration system is the used to...
The work concerns relativistic effects and the influence of an external magnetic field on the
transmission coefficient. The Fowler-Nordheim equation has been relativistically generalized and effect of
the Lorentz contraction of a potential barrier at the metal-vacuum interface has been found. Influence of
the magnetic field parallel to a metal surface on the transmission coefficient is taken into...
The commonest method of characterising a cold field electron emitter is to measure its current-voltage characteristics, and the commonest method of analysing these characteristics is by means of a Fowler-Nordheim (FN) plot [1]. For example, the field enhancement factor can be calculated. But for novel investigations of the transition from field emission to plasma discharges an extended use of...
Direct field ionisation of neutral atoms may happen when they are immersed in an external electric field of magnitude comparable to the atomic electric field seen by the outer electron shells. The resulting ionisation probability may have a magnitude comparable to the usual electron impact ionisation probability, and thus this process should not be neglected in plasma simulations codes. In...
This research is on the COMSOL simulation of flashover occurring in a MEMS (Micro-Electro-Mechanical System) insulator and electrode unit, both of the electric field at triple point and of electron trajectories in several practical geometries. In some applications of MEMS technology, electrostatic field up to 5-20kV/mm is of interest. One of these applications is using MEMS components for...
Dark current is particularly relevant during operation in high-gradient linear accelerators. Resulting from the capture of field emitted electrons, dark current produces additional radiation that needs to be accounted for in experiments. In this paper, an analysis of dark current is presented for four accelerating structures that were tested and conditioned in CERN's X-band test facility for...
A design technique with empirical scaling has been developed in order to design a robust vacuum insulation on the 5-stage electrostatic accelerator with large surface area and locally concentrated electric field, aiming for the acceleration of 1 MeV, 40 A deuterium beams for fusion application. So far, there was no practical design technique and no applicable database of the voltage holding...
In RFX-mod2, an experiment for the magnetic confinement of fusion relevant plasmas presently under construction, the in-vessel conductive plasma facing components are expected to be subjected during transient plasma current phases (start-up and termination) to significant electric fields, in the kV/mm range. While such electric fields are of no concern for components in vacuum, the presence of...
Breakdowns generated with pulsed DC systems between large and fixed Cu electrodes are typically highly localized near the edge of the electrodes. The effect is especially strong on BDs which are generated after the electrode has been conditioned, i.e. when the electric field is near the maximum level. Understanding the reason for this phenomenon can be a key factor in hindering the breakdown...
The laser-collision induced fluorescence (LCIF) diagnostic allows quantitative and multidimensional interrogation of electron densities and temperatures. We use this diagnostic to interrogate a cathodic arc expanding in 65 mTorr of Helium. Two-dimensional spatial and time resolved images of electron densities and temperatures within the plasma are shown. Optimization methods were implemented...
The high-Q operation of the ITER tokamak will require two Neutral Beam Injectors (NBIs) for plasma heating and current drive. Each beam will be generated by a 40A current of Deuterium negative ions, accelerated up to the specific energy of 1MeV and then neutralized. The power delivered to the plasma by each NBI shall reach 16 MW with duration up to 1h. The beam source will be constituted by an...
In the framework of the Physics Beyond Colliders (PBC) study at CERN, the Electric Dipole Moment (EDM) working group is investigating the feasibility of building a storage ring to precisely measure the permanent electric dipole moment of the proton. As a preparation for this main ring, a prototype ring (PTR) is proposed to demonstrate the feasibility of technologies that are not yet...
The characterization of the X-ray energy spectrum during the high voltage conditioning of a multi electrode vacuum insulated system (such as the MITICA electrostatic accelerator) could be useful to understand which stage is causing the micro discharge onset. This technique seems particularly promising for measuring small dark currents (I< 10μA), which are generally difficult to be measured...
LES (Large Electrode System) is the DC system at CERN, used for observations of the gap between electrodes. This system has view ports that allow for different imaging devices to be set up in parallel to observe the breakdown process. Vacuum arcs can often be seen by eye due to relatively high light intensity, but there are other possible sources of light that may exist before a breakdown....
Pulsed DC systems dedicated to the study of electrical breakdown phenomenon and the conditioning process are part of the CLIC (Compact Linear Collider) project. There are two pulsed DC systems operational at CERN, as well as similar systems at Helsinki and Uppsala Universities. In the systems, two plane electrodes with large surface areas are placed parallel to each other with a separation of...
Useful models of arcing capable of predicting operating gradients in limited environments are beginning to be produced, but generalizing these models to many applications is coming very slowly. We will show how each stage of the arcing process can be explained by many possible mechanisms, each with its own parameter space and peculiarities, and outline how a general model, applicable to many...
Combining classical electrodynamics and density functional theory (DFT) calculations, we develop a general and rigorous theoretical framework that describes the energetics of metal surfaces under high electric fields. We show that the behavior of a surface atom in the presence of an electric field can be described by the polarization characteristics of the permanent and field-induced charges...
Kinetic Monte Carlo (KMC) is among the most efficient methods for modelling diffusion. In the context of CLIC, we're interested in the diffusion processes on the Cu surface - especially under high electric fields that are present prior to electric breakdown events.
The accuracy of a KMC model relies on the comprehensiveness of the catalogue of different migration events that are available...
The High Voltage Padova Test Facility (HVPTF) is an experimental device for investigating HV insulation in vacuum, in support of the realization of MITICA, the prototype of a neutral beam injector for ITER. The facility investigates the physical phenomena underlying voltage holding in vacuum, such as the mechanisms causing breakdowns and the electrode conditioning process, along with testing...
The CLIC project aims to establish an accelerating gradient of 100MV/m within novel high gradient X-band accelerating structures, this leads to surface electric fields on the order of 220MV/m. In this context, CERN have developed high power X-band RF test stands (Xboxes) to test the prototype structures for CLIC at high power. A summary and analysis of recent RF results from the test stands...
High voltage platform are of widespread use in nuclear physics accelerator in order to provide the initial acceleration of ion. The ion source environment is particular challeging for high voltage holding, since discharge can be energized from source radiation or secondary particle generated by the ion beam.
Moreover spectrometers for charge to mass ratio of exotic nuclei requires a well...
The prevaling way of analyzing dark currents is by looking only at the mean current. While there have been some work to extract usefull information from that, there's a lot of data being ignored by neglecting the inherently statistical nature of the current.
In this talk, I will present a novel method for field estimation and its advantages as opposed to the current method of beta estimation...
Vacuum arc initiation is sensitive to the initial state of the metal electrodes. Surface roughness, oxide thickness, and other surface contaminants impact the arc initiation. In this poster we present results on the impact on arc initiation of dosing metal electrodes with a variety of common gases, such as CO, H2O, O2, Ar, methane, and others. We show results of how the current and voltage...
The neutral beam injection (NBI) system in KSTAR (Korea Superconductor Tokamak Advanced Research) provide heating and current drive to KSTAR tokamak plasma. It is composed of a plasma generator, an accelerator, and neutralizers. The plasma generator and the neutralizer decide the plasma chemistry and finally the beam fraction while the accelerator decides the beam optics. In general, the...
