Real-time visualization of parallel simulations in CERN material design

This work presents the implementation of the in situ visualization module for multiscale-multiphysics simulation code FEMOCS and demonstrates its behavior in the simulation of vacuum breakdown. The visualization module makes it possible to observe in real-time the course of the simulation in FEMOCS and makes it more straightforward to set up a new simulation or develop additional features into the code. The first and second chapters briefly introduce the vacuum breakdown phenomenon and describe general aspects of numerical simulations. The third chapter describes the in situ method as a way of improving FEMOCS. The fourth and fifth chapters present the final solution and the impact of the solution on the overall running time of the simulation

Structural evolution and thermal runaway of refractory W and Mo nanotips in the vacuum under high electric field from PIC-ED-MD simulations

We performed multiscale-multiphysics simulations for W, Mo and Cu nanotips under high electric field to investigate their structural evolution and thermal runaway process. The critical electric field values for the electric prebreakdown condition are predicted to be 311 MV m(-1), 570 MV m(-1) and 675 MV m(-1) for Cu, Mo and W nanotips respectively (R (0) = 1 nm, H (0) = 100 nm). The boiling point of the metal is found to be a good predictor of the critical electric field strength for the initiation of thermal runaway. For metal nanotips made of refractory metals such as W and Mo, the structural thermal runaway process is determined by the rapid growth of small protrusions and their subsequent sharpening and thinning under the high electric stress on the apex region. On the other hand, the more intense atomic evaporation of Cu metal nanotips is caused by the ejection of large droplets generated by recrystallization and necking of the molten region at the apex of the nanotip. The differences in the observed structural evolutions of nanotips between refractory metals and the Cu during the thermal runaway event clearly show the strong influence of melting and boiling points on the electric prebreakdown process in nanoscale.Peer reviewe

Computer simulations of swift heavy ion effects in graphene and amorphous bulk materials

Ion irradiation is capable of modifying the structure of materials. By changing the structure in a controlled fashion, it is possible to tune the material properties to create new devices. One of the most exciting applications of ion irradiation can be found in the semiconductor industry, where this technique is used to manufacture electronic components. Swift Heavy Ions (SHI) are a specific type of ion irradiation characterized by their high mass, and elevated energies (> 1 MeV/amu). These ions interact mainly with the electrons in the material, and often leave traces of cylindrical defected regions known as ion tracks. These defects are few nanometers wide but can be up to several microns long. SHI irradiation can be used to produce ion tracks of tunable size, making it a suitable technique for several promising applications. By analyzing the abundance of tracks one can estimate the age of geological and archaeological samples. The etched ion tracks in silica (a-SiO2) serve as excellent templates to grow arrays of metallic nanowires. On the other hand, SHI irradiation of graphene allows also the engineering of high efficiency water desallinators. Experimental measurements have shown that SHIs can produce defects in graphene, and that their size grows with the ion energies. Previous reports have identified a core-shell structure within the ion tracks of several amorphous materials: a-SiO2, a-Si3N4 and a-Si. Nevertheless, current experimental techniques are not able to resolve the structure of these defects, nor the mechanism of their formation. In this study, we use Molecular Dynamics, augmented by the two-temperature model and a Monte Carlo-based electronic cascade model to study the formation mechanism and structure of defects at the atomic level, as well as to investigate the early electron dynamics after the ion impact in these materials. Our simulations show that SHI irradiation can create pore-like defects in graphene; even when a considerable amount of the energy initially deposited by the ion is removed via electron emission during the development of the electron cascade. Moreover, we attribute the formation of the core-shell structure in a-SiO2, a-Si3N4 and a-Si to the different transient pressure levels at the time when the core and the shell of the track solidify; higher (lower) pressures leading to higher (lower) densities, respectively. This work constitutes a step forward at modeling the interaction of SHI in 2D and amorphous materials, and sheds light on the mechanisms how defects form in these type of materials under extreme conditions of high-energy ion irradiation.Ionisäteily voi aiheuttaa muutoksia materiaalien rakenteessa. Muuttamalla rakennetta hallitusti pystyy säätämään materiaalin ominaisuuksia uusien laitteiden luomiseksi. Yksi jännittävimmistä ionisäteilyn sovelluksista löytyy puolijohdeteollisuudessa, jossa tätä tekniikkaa käytetään elektronisten komponenttien valmistukseen. Ripeät raskas-ionit ovat tietyntyyppinen ionisäteily, jolle on ominaista niiden iso massa ja suuret energiat (> 1 MeV per nukleoni). Nämä ionit vuorovaikuttavat pääasiassa materiaalin elektronien kanssa ja indusoivat usein sylinterinmuotoisen defektiä, joita kutsutaan ionin jäljeksi. Nämä jäljet ovat muutaman nanometrin levyisiä, mutta voivat olla jopa useita mikroneja pitkiä. Ripeät raskas-ionit voidaan käyttää tuottamaan erikokoisia ionin jälkiä, mikä tekee siitä sopivan tekniikan useisiin lupaaviin sovelluksiin. Analysoimalla jälkien määrä voidaan arvioida geologisten ja arkeologisten näytteiden ikä. Piidioksidin (a-SiO2) etsattyt ionin jäljet toimivat erinomaisina malleina metallisten nanolankojen tuottamiseen. Toisaalta, ripeät raskas-ionisäteilytys voi käyttää valmistamaan grafeenista tehokkaan veden desallinaattorin. Kokeelliset mittaukset ovat osoittaneet, tämä ionisäteilytys voi aiheuttaa defektiä grafeenissa ja että niiden koko kasvaa ionienergioiden myötä. Aiemmissa raporteissa on tunnistettu ydin-kuori rakenne useiden amorfisten materiaalien: a-SiO2: n, a-Si3N4: n ja a-Si: n ionin jäljissä. Nykyiset kokeelliset tekniikat eivät kuitenkaan pysty ratkaisemaan näiden vikojen rakennetta eikä niiden muodostumismekanismia. Tässä tutkimuksessa käytämme molecular dynamicsia, kahden lämpötilan malli ja Monte Carlo -pohjainen elektroninen kaskadimalli, tutkiaksemme defekti muodostumismekanismia ja rakennetta atomitasolla sekä tutkiaksemme varhaista elektronidynamiikkaa näiden materiaalien ionivaikutuksen jälkeen. Simulaatiomme osoittavat, että ripeät raskas-ionisäteilytys voi aiheuttaa huokosmaisia defektiä grafeenissa; silloinkin, kun huomattava määrä ionin alun perin sijoittamaa energiaa poistetaan elektronipäästöjen kautta elektronikaskadin kehittämisen aikana. Lisäksi annamme ydin-kuoren rakenteen muodostumisen a-SiO2: ssa, a-Si3N4: ssä ja a-Si: ssä eri ohimenevistä painetasoista silloin, kun ionin jäljen ydin ja kuori kiinteytyvät; suuremmat (alemmat) paineet, jotka johtavat suurempaan (pienempään) tiheyteen. Tämä työ on askel eteenpäin ripeät raskas-ionisäteilytyksen vuorovaikutuksen mallintamisessa 2D- ja amorfisissa materiaaleissa ja paljastaa mekanismeja, miten defektiä muodostuvat tämäntyyppisissä materiaaleissa äärimmäisessä korkean energian ionisäteilyssä

Dynamic coupling between particle-in-cell and atomistic simulations

We propose a method to directly couple molecular dynamics, the finite element method, and particle-in-cell techniques to simulate metal surface response to high electric fields. We use this method to simulate the evolution of a field-emitting tip under thermal runaway by fully including the three-dimensional space-charge effects. We also present a comparison of the runaway process between two tip geometries of different widths. The results show with high statistical significance that in the case of sufficiently narrow field emitters, the thermal runaway occurs in cycles where intensive neutral evaporation alternates with cooling periods. The comparison with previous works shows that the evaporation rate in the regime of intensive evaporation is sufficient to ignite a plasma arc above the simulated field emitters.Peer reviewe