PUT 57 - Multiscale simulations of dislocation generation in rf electric fields in the linear accelerator design (01.01.2013 - 31.12.2016)

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Short overview

The Compact Linear Collider (CLIC) is a new accelerator, developed in CERN, utilizing electron-positron beam collisions to reach energies from 0.5 TeV to 5 TeV. To achieve this kind of energies, very high accelerating gradients, reaching over 100 MV/m, are needed. However, in such kind of high electrical field one of the key problems, arising during the operations are the repeated electrical breakdowns in the accelerating structures. The electrical breakdowns are caused by the vacuum arching, but currently, the exact mechanism leading to vacuum arching is not known. Generally, it is assumed, that the vacuum arch is triggered by nanoscale field emitters (needle-like structures), appearing on the surface of the accelerating structures. In current work, different computational methods, like molecular dynamics and Kinetic Monte-Carlo are utilized to identify the physical phenomena leading to the growth of these field emitting tips.

Outcomes

Compact Linear Collider (CLIC) is a new electron positron accelerator developed in CERN, capable of colision energies in range of 0.5 TeV-5 TeV. To reach such energies, extreme electric field, in magnitude of 100 MV/m are needed. Under such conditions, repeated electrical breakdowns emerge as critical issues, limiting the performance of the device. The electrical breakdowns are caused by vacuum arching. While it is generally assumed that the vacuum aching is initiated by needle like electric field enhancing defects, the mechanism of generating such surface features is unknown. To understand the mechanisms responsible for generating such emitters, multiscale simulations are needed – the time between breakdowns can be measured in seconds or minutes; the emitters are in size of nanometers and breakdown damage is in micrometer scale. In addition, the time to melt single emitter can be measured in nanoseconds. Based on these facts, the aim of current study was to develop multiscale simulation methodologies and apply them to the breakdown problem to understand mechanisms leading to the initiation of field emitters. Developed methodology makes it possible to conduct studies electro-mechanical and thermal response of material in several length and time scales. Application of the approach provided several mechanisms, like subsurface material defects, possibly responsible of initiating the field emitters or memory effect of emitters that can possibly explain experimentally observed surface electric field fluctuations. As one of the most important result, appearance of field emitter was observed in case of high electric field experiment.