In the vacuum transmission lines of terawatt pulsed-power accelerators such as Sandia National Laboratories’ Z machine, high electric fields (MV/m) result in field emission of electrons from cathodes. Left unchecked, these electrons are lost to and heat anode surfaces, leading to the emission of positive ions, and the generation of expanding electrode plasmas ($10^{16}-10^{18}$ cm${}^{-3}$)....
High gradient RF challenges in the muon collider will be presented and discussed.
The behavior of plasma in the anode region of a DC discharge is complex due to its dependence on the geometry of the anode, gas type and pressure, and discharge current. Understanding this behavior is an ongoing effort due to its importance in developing a general theory for gas discharges and designs for charged particle sources. The anode plasma sheath displays interesting phenomena such as...
The role of the magnetic field in vacuum arcing has been neglected in many experimental and computational studies, while it is present in many applications. Future accelerators such as muon colliders and technologies such as vacuum interrupters involve significant magnetic fields that potentially influence the plasma initiation dynamics of vacuum arcing, as they can significantly focus the...
A relativistic one-dimensional particle-in-cell (PIC) Monte Carlo Collision (MCC) model is developed to study velvet cathode plasma formation and expansion in high-voltage diodes. Velvet cathodes are used in high-power vacuum diodes for pulsed power systems such as magnetically insulated line oscillators (MILOs) for generating high-power microwaves (HPMs) and magnetically insulated...
The influence of pores filled with a dielectric, in particular hydrogen, on the current of field electron emission from the structural materials of accelerator structures was studied. A review of the literature shows that pores can form in the near-surface layer in which dielectric, in particular hydrogen, can accumulate. Therefore, the study of the influence of dielectric inclusions in the...
Predicting the occurrence of unintended gas breakdown in narrow gaps within plasma processing chambers is essential for the development of future plasma sources in the semiconductor industry. This study[1] conducted experimental and theoretical analyses focusing on the unexpected discharge events in narrow gaps. We observed a notable drop in the gas breakdown voltage when exposed to an...
Breakdown in DC gas discharges is primarily described by Paschen’s law. While Paschen’s law accounts for the flux balance correctly, the relation between the reduced electric field and the ionization coefficient is given empirically. In this study, we investigate DC breakdown using a full-fluid moment (FFM) model, which is benchmarked against a particle-in-cell/Monte-Carlo collision (PIC-MCC)...
While arcing formation mechanism has been widely studied, the influence of arcing on background plasma has remained underexplored. In this study, we investigated the effect of arcing on capacitively coupled plasma by employing arcing induced probe (AIP), which plays a role in localizing arcing on probe tip edge. We analyzed behavior of capacitively coupled plasma by analyzing various...
In electrostatic fusion reactors like the Orbitron, maintaining reliable high-voltage levels is crucial for the confinement of charged particles. Avalanche Energy's ultra-compact bushing, MAKO, plays the key role of transferring voltage from ambient pressure to an ultra-high vacuum environment with pressures below 1e-8 Torr. It features a coaxial configuration with a 1.8-5 cm gap distance and...
Nuclear physics experiments at Jefferson Lab (JLab) require spin-polarized electron beams generated from delicate semiconductor photocathodes in a photoemission electron gun (photogun). The JLab photogun operates at 130 kV using an inverted geometry alumina insulator (feedthrough) as a holding structure to the highly polished stainless steel cathode electrode inside an ultra-high vacuum...
C-band accelerators have been of particular interest in recent years due to their ability to provide high gradients and transport high charge beams for applications such as colliders and medical technologies. These technologies are made possible by new advancements in high gradient technologies that can suppress the breakdown rate in a particular structure by using distributed coupling,...
A cryogenic, HV conditioning system integrated in a stand-alone cryocooler is operated at FREIA laboratory in Uppsala in order to investigate the fundamental mechanisms of field emission and breakdown nucleation. A series of high-field measurements has been carried out with pairs of metal electrodes (copper, niobium and titanium) at temperatures ranging from ambient down to 4K.
The cryogenic...
In order to investigate the mechanisms behind vacuum arc formation, it is beneficial to use as many diagnostic tools as possible at a wide temperature range for different metals. The cryogenic HV pulsing system in FREIA laboratory is experimentally following this line of research. We study vacuum arc breakdowns and surface conditioning using high-repetition rate DC pulses at a wide range of...
Vacuum arcing involves the coupling of multiple physical mechanisms starting from electron emission and leading to plasma formation. The importance of different physical interactions for arc occurrence is an important aspect which is still largely not understood. Here we use particle-in-cell simulations with Monte Carlo collisions concurrently coupled with electron emission and heating...
The High Voltage Padova Test Facility (HVPTF) is an experiment set in Padova, Italy, operating in the framework of the Neutral Beam Test Facility project of the ITER tokamak. The purpose of HVPTF is to study the phenomenology of discharge events occurring between electrodes at high voltage differences over long vacuum gaps, which is crucial in the development of the Neutral Beam Injector (NBI)...
The High Voltage Padova Test Facility (HVPTF) is an experimental device for investigating High Voltage Direct Current insulation in vacuum, in support of the realization of MITICA, the prototype of a neutral beam injector for ITER. Inside a high vacuum chamber, two stainless steel electrodes, separated by a few centimeters gap, can achieve a voltage difference up to 800 kV. During the...
The Large Electrode System (LES) is a pulsed high-voltage DC test stand located at CERN where its main objective is investigating the origin behind breakdowns. Breakdowns are an important limiting factor in high electric field applications and ongoing studies are performed to better understand the origin behind this phenomenon. Our experimental setup requires two electrodes, having a gap...
This talk presents theoretical work on the nano-protrusion hypothesis. We review and expand the theorical work on the study of the electron beam shape, as a mean to determine the dimensions of the hypothetical nano-protrusion where the electrons come from. State-of-the-art field emission theory and computational tools have been used to calculate the kinetic energy of the electrons as a...
Two chamfered electrodes were tested in the Large Electrode System (LES) at CERN. Having a flat-shaped cathode and a frustum shaped anode gives a higher electric field in the centre of the electrodes, while it linearly decays towards the edge.
Normal breakdown rates are observed in the centre of the electrodes. However, lower breakdown rates with regards to the electric field was found for...
Various theoretical efforts explored the link between breakdown nucleation and pre-breakdown plastic activity in surfaces exposed to high electric fields. However, identifying such a mechanism is challenging as there are no clear indications of what structural evolution evidence should be identifiable in ex-situ post-mortem samples. The current state of research efforts to identify and measure...
Vacuum arcs, also known as breakdowns (VBD), are a major limiting factor for various applications such as particle accelerators, fusion reactors, vacuum interrupters, X-ray sources, and space applications. However, the physical mechanisms underlying the very initiation of the phenomenon still remain unclear. Recent experimental evidence indicates that the distribution of electromagnetic power...
The electrothermal instability (ETI), ubiquitous in materials which carry large currents, is driven by the dependence of the material resistivity on temperature. The filamentation mode of the instability occurs in plasmas, where the gradient of the Spitzer resistivity with temperature is negative, and results in non-uniform filaments of hot, current-dense plasma. The ETI is potentially...
In high power vacuum arcs, the physics of the plasma and the surrounding surfaces can be strongly coupled both thermally and materially by energy deposition from the plasma to the materials and by gas-phase species emitted from the materials into the plasma, respectively. The former can also produce surface geometry modifications which then feed back to the electric field. The first step in...
The Large Electrode System (LES) is a high voltage pulsed DC test stand to investigate breakdowns. Many materials have undergone testing by conditioning, as well as field emission measurements and light spectroscopy investigation.
The influence of pores filled with a dielectric, in particular hydrogen, on the current of field electron emission from the structural materials of accelerator structures was studied. A review of the literature shows that pores can form in the near-surface layer in which dielectric, in particular hydrogen, can accumulate. Therefore, the study of the influence of dielectric inclusions in the...
The influence of pores filled with a dielectric, in particular hydrogen, on the current of field electron emission from the structural materials of accelerator structures was studied. A review of the literature shows that pores can form in the near-surface layer in which dielectric, in particular hydrogen, can accumulate. Therefore, the study of the influence of dielectric inclusions in the...
