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Breakdowns may occur in high-voltage applications even in ultrahigh vacuum conditions. Previously, we showed that it is important to pay attention to the post-breakdown voltage recovery in order to limit the appearance of secondary breakdowns associated with the primary ones. This can improve the overall efficiency of the high-voltage device. In this study, we focus on the optimization of the linear post-breakdown voltage recovery, with the principle aim of alleviating the problem of the secondary breakdowns. We investigate voltage recovery scenarios with different starting voltages and slopes of linear voltage increase by using a pulsed dc system. We find that a higher number of pulses during the voltage recovery produces fewer secondary BDs and a lower overall breakdown rate. Lowering the number of pulses led to more dramatic voltage recovery resulting in higher breakdown rates. A steeper voltage increase rate lead to a more localized occurrence of the secondary breakdowns near the end of the voltage recovery period. It was also found that the peak BD probability is regularly observed around 1 s after the end of the ramping period and that its value decreases exponentially with the amount of energy put into the system during the ramping. The value also decays exponentially with a half-life of (1.4$\pm$0.3) ms if the voltage only increased between the voltage recovery steps., Comment: 10 pages, 6 figures

Vacuum electrical breakdowns, also known as vacuum arcs, are a limiting factor in many devices that are based on application of high electric fields near their component surfaces. Understanding of processes that lead to breakdown events may help mitigating their appearance and suggest ways for improving operational efficiency of power-consuming devices. Stability of surface performance at a given value of the electric field is affected by the conditioning state, i.e. how long the surface was exposed to this field. Hence, optimization of the surface conditioning procedure can significantly speed up the preparatory steps for high-voltage applications. In this article, we use pulsed dc systems to optimize the surface conditioning procedure of copper electrodes, focusing on the effects of voltage recovery after breakdowns, variable repetition rates as well as long waiting times between pulsing runs. Despite the differences in the experimental scales, ranging from $10^{-4}$ s between pulses, up to pulsing breaks of $10^5$ s, the experiments show that the longer the idle time between the pulses, the more probable it is that the next pulse produces a breakdown. We also notice that secondary breakdowns, i.e. those which correlate with the previous ones, take place mainly during the voltage recovery stage. We link these events with deposition of residual atoms from vacuum on the electrode surfaces. Minimizing the number of pauses during the voltage recovery stage reduces power losses due to secondary breakdown events improving efficiency of the surface conditioning., 14 pages, 11 figures. Submitted to Physical Review Accelerators and Beams

High electric fields are present in a rapidly growing number of applications, which include elementary particle accelerators, vacuum interrupters, miniature x-ray sources, and satellites. Many of these applications are limited by the breakdown strength of the materials exposed to electric fields. Different methods have been developed to improve the quality of metal electrode surfaces, aiming to increase their breakdown strength. Not many systematical studies have been performed to provide a proper understanding of what contributes to the correlation between the breakdown strength and the quality of the surface. In this work, we apply a novel method for reducing vacuum arc breakdowns by cleaning the electrode surfaces with O and Ar plasma. The method can be used to alter the surfaces of the Cu electrodes in situ, i.e., without exposing them to air between the measurements. This plasma cleaning treatment is shown to reduce the number of surface impurities and to speed up the conditioning process of the samples under high-voltage pulses. Specifically, the first breakdown field was observed to increase by more than 90% after the plasma cleaning.

Understanding the microscopic phenomena behind vacuum arc ignition and generation is crucial for being able to control the breakdown rate, thus improving the effectiveness of many high-voltage applications where frequent breakdowns limit the operation. In this work, statistical properties of various aspects of breakdown, such as the number of pulses between breakdowns, breakdown locations and crater sizes are studied independently with almost identical Pulsed DC Systems at the University of Helsinki and in CERN. In high-gradient experiments, copper electrodes with parallel plate capacitor geometry, undergo thousands of breakdowns. The results support the classification of the events into primary and secondary breakdowns, based on the distance and number of pulses between two breakdowns. Primary events follow a power law on the log--log scale with the slope $\alpha \approx 1.33$, while the secondaries are highly dependent on the pulsing parameters., Comment: 11 pages, 13 figures

Electric discharge is present in various aspects of our everyday lives. Internal combustion engines rely on spark plugs for the running of the motor, fluorescent lighting functions by gas discharge and a lightning bolt strikes somewhere on earth every second. An electrical breakdown is an event where a voltage across two conductive electrodes, separated by an electrically insulating medium, becomes high enough for the insulating properties of the medium to be weakened, subsequently allowing an electric current to pass through the medium. A special type of such an event is a vacuum arc breakdown, where the electrodes are separated by a gap of void, which acts as a good insulator, but will still be breached under sufficiently high voltage. When controlled, the electric arcing can be used as a powerful tool to focus energy to a specific location. However, several applications are also hindered by the occurrence of breakdowns, including particle accelerators, vacuum interrupters and solar panels. A common factor in these applications is the aim to maximize the electric field strength to optimize the operational efficiency and ecological footprint of such a device. The breakdown phenomenon is at the crossroads of many fields of science, including plasma, materials and surface physics. Effort to explain the breakdown origin has been ongoing for more than a hundred years, and, despite of the constant progress, there are only hypotheses on the exact nature of the process. This work presents an experimental approach for studying the breakdown phenomenon between Cu electrodes, separated by a vacuum gap. The breakdowns are generated as a consequence of repeatedly applying high-voltage pulses across the gap. As a result, statistics, such as breakdown frequency, of the events are investigated and any effects on the surface analyzed. It was shown that cleaning the electrode surface, either by the electric pulsing or plasma treatment, improves the breakdown resistance of the system, whereas any idle time between the high-voltage pulses increases the breakdown probability. Furthermore, it was found that the breakdown events can be attributed to distinct classes, suggesting separate processes responsible for the breakdown generation. One set of processes were labeled extrinsic, as they are driven by the external factors responsible of the surface contamination of the electrode surface. The other processes were characterized as intrinsic, as they were defined by inherent material properties and continued affecting the breakdown frequency even when the effect of extrinsic processes was minimized by plasma cleaning of the surface. Understanding the formation mechanisms of a vacuum arc breakdown allows designing applications that can sustain higher electric fields without breakdown events. The results of this work provide insight on how improving the surface state of an electrode can increase its breakdown resistance. Additionally, an algorithm is presented for recovering the pulsing voltage after a previous breakdown to a high level in an optimal way, with a minimal probability of follow-up breakdowns. Sähköisiä purkauksia esiintyy lukuisissa arkipäiväisissäkin tilanteissa. Polttomoottorit käyttävät sytytystulppia moottorin käynnistämiseen, loisteputkivalaistuksen toiminta perustuu sähköpurkauksiin kaasussa ja salama iskee jossain päin maapalloa joka sekunti. Sähköinen läpilyönti on tapahtuma, jossa kahden eristeellä erotetun, sähköä johtavan elektrodin välinen sähköjännite kasvaa liian suureksi, eristeen kestokyvylle. Tämän seurauksena sähkövirta pääsee kulkemaan elektrodien välillä. Tyhjiövalokaariläpilyönti on tämän ilmiön erikoistapaus, jossa elektrodeja erottaa tyhjiö, joka toimii hyvänä eristeenä, mutta on silti murrettavissa, kun jännite nousee riittävän suureksi. Hallituissa olosuhteissa valokaaria voi käyttää tehokkaana työkaluna energian keskittämiseksi määrättyyn paikkaan. Läpilyönnit kuitenkin myös rajoittavat useiden sovelluksien toimintaa. Näihin kuuluvat muun muassa hiukkaskiihdyttimet, tyhjiökatkaisijat ja aurinkokennot. Kyseisiä sovelluksia yhdistää tavoite sähkökentän voimakkuuden maksimoimiseksi, mikä mahdollistaa laitteen käytön maksimitehokkuudella, ekologinen jalanjälki minimoiden. Läpilyöntien tutkiminen on monien tieteenalojen, kuten plasma-, materiaali- ja pintafysiikan risteyksessä. Läpilyönteihin liittyviä ilmiöitä on yritetty selittää jo yli sadan vuoden ajan. Tasaisesta edistyksestä huolimatta tapahtumaketjun tarkasta luonteesta tunnetaan kuitenkin edelleen vain erilaisia hypoteeseja. Tässä työssä tutkitaan tyhjiövalokaariläpilyönti-ilmiötä kuparielektrodien välissä kokeellisin menetelmin. Läpilyöntejä synnytetään tuottamalla korkeajännitepulsseja elektrodien välillä ja niitä tutkitaan analysoimalla tapahtumien tilastollisia ominaisuuksia, kuten taajutta, sekä niiden vaikutuksia elektrodien pintoihin. Tutkimuksissa osoitettiin, että elektrodien pinnan puhdistaminen - joko sähköpulsseilla tai plasmakäsittelyllä - parantaa läpilyöntikestävyyttä, kun taas minkä tahansa pituinen aika pulssien välillä kasvattaa läpilyöntien todennäköisyyttä. Lisäksi huomattiin, että läpilyöntejä voi luokitella eri kategorioihin, mikä puolestaan vihjaa erilaisista prosesseista, jotka vaikuttavat läpilyöntien syntyyn. Yksi prosessityyppi tunnistettiin ulkoiseksi, sillä siihen vaikuttavat erityisesti ulkoiset tekijät, kuten elektrodipinnan kontaminaatio. Toinen prosessityyppi luokiteltiin sisäiseksi, koska sen nähtiin liittyvän materiaalin luontaisiin ominaisuuksiin ja sen vaikutus läpilyöntitaajuuteen säilyi, vaikka ulkoisten prosessien vaikutus minimoitiin pinnan plasmapuhdistuksella. Läpilyöntien syntymekanismien ymmärtäminen mahdollistaa sovellukset, jotka kestävät entistä korkeampia sähkökenttiä ilman läpilyöntejä. Tämän työn tulokset kertovat, kuinka elektrodin pinnan puhtaustilan parantaminen voi nostaa sen läpilyöntikestävyyttä. Lisäksi työssä esitellään optimaalinen algoritmi pulssitusjännitteen nostamiseksi aiemman läpilyönnin jälkeen niin, että välittömien jatkoläpilyöntien todennäköisyys voidaan minimoida.