Humans have been fascinated by lightning and thunder since antiquity, assigning them divine properties and origin. Ancient Greeks and Romans considered them a manifestation of the presence of Zeus/Jupiter, while similar tales were told by the Vikings about Thor. Nowadays we understand this phenomenon much better. We know that although air is normally an electrical insulator, if a sufficiently strong electric field is applied, it causes an ionisation avalanche of the air molecules, which eventually converts the air from the gas phase to the plasma phase, creating an arc that conducts electricity very well and causes a violent discharge of the electric charges that induce the electric field in the first place.
Our interest in arcs extends into many areas of modern technology, in particular in high-gradient particle accelerators like the Compact LInear Collider (CLIC) where arcs represent an important performance limitation. Although such discharges are very well understood in an insulating material or gas, their occurrence in a vacuum has still several aspects that remain poorly understood. The problem is that in a vacuum there are no gas particles to ionise. On the contrary, it is the metal electrodes themselves that locally convert into plasma and cause the arc, which also causes them significant damage. Still, it is not yet quite well understood how such an ionisation is initiated on a carefully cleaned and smoothened metal surface. Understanding such a phenomenon poses a significant challenge, as the physical processes before or during a vacuum arc are extremely complex and they occur at an extremely small (close to atomic) level, within an extremely limited (typically sub-ns) time interval. Such processes are extremely hard to study experimentally, while simulations also face significant challenges.
In order to create a multi-disciplinary and multi-application community able to address vacuum arcs, a workshop series dedicated to vacuum arcs MeVArc (MEchanisms of Vacuum ARcs) was initiated eleven years ago by the CLIC collaboration. This international workshop takes place every one and a half years, attracting many researchers from various countries and disciplines, including material science, surface physics, electromagnetics, and plasma physics. It focuses on gaining insights on the physical processes involved in the onset and evolution of vacuum arcs, covering theory, simulation, and experiments, as well as developing new mitigating techniques.
The most recent edition of MeVArc was held from 8 to 12 March and was held online, organised mutually by the universities of Helsinki, Tartu, and Uppsala. The workshop had 137 registered participants and a peak participation of over 80. There were 22 presentations and two lively poster sessions which allowed open discussion and recreated to some extent the ambiance of an in-person workshop.
The next MeVArc will take place in Crete, Greece, in September 2022, organised by the University of Tartu. Hopefully in person…
Some of the subjects that were discussed
The workshop covered reports from various applications where vacuum arcs are important (e.g. particle accelerators, vacuum interrupters, vacuum nanoelectronic components and space systems), as well as dedicated experimental and theoretical studies of various physical processes involved in vacuum arcing. Several experimental, post-mortem diagnostics, and simulation studies where presented on the breakdown dynamics in DC arc testing systems, with special focus on the dependence on the hydrogen irradiation of the electrodes (of high interest for the next-generation proton bream source of the LHC), their cryogenic temperatures, as well as their cleaning preparation procedures. Several studies were also presented on plasma evolution dynamics, field emission theory and experiments, and breakdown prediction systems.
Finally, vacuum arcs of high interest for linear particle accelerators, since their accelerating gradient is largely limited by vacuum breakdown. When the accelerating electric fields reach a limit of the order of a few hundreds of MV/m, vacuum breakdown occurs at unacceptable rates, despite significant efforts to prepare and condition the accelerating structures at their best performance state. In order to overcome this harsh limitation, significant advances are needed both in understanding the physics behind vacuum arcs and engineering good techniques to mitigate them. For this reason, the accelerator community has shown a high interest in vacuum arcs, with scientists from various accelerator facilities, such as CLIC, SLAC, KEK, XFEL, MITICA etc, participating actively in MeVArc and more generally, the vacuum arc research community.