arXiv subjects

T. Silva

Publications and source records attributed to T. Silva.

5 recordsLinked to original sources

arXiv2026

LoKI-GM: a global model framework for plasma chemistry studies

Global (zero-dimensional or spatially averaged) models are widely employed to study complex chemistries in low-temperature plasmas (LTPs). By adopting a spatially average description of the plasma, they substantially reduce the computational cost while still providing reliable and detailed insight into the key processes taking place in the plasma. In their most general formulation, global models involve the coupled solution of a Boltzmann solver (to describe the electron kinetics) and a Chemistry solver (to describe the heavy-species kinetics). This paper presents a tutorial on the LisbOn Kinetics Global Model (LoKI-GM) framework, developed in MATLAB and available as open-source code. The framework couples the Boltzmann solver LoKI-B, which solves the space-independent form of the two-term electron Boltzmann equation for non-magnetised non-equilibrium LTPs, excited by DC/HF electric fields or time-dependent (non-oscillatory) electric fields, and the Chemistry solver LoKI-C, which solves the system of zero-dimensional rate balance equations for the main charged and neutral species in the plasma and at the surface, receiving as input the kinetic schemes for the gas/plasma/surface system under study. The inclusion of several transport models together with support for surface kinetics models are distinguishing features of LoKI-GM compared with other global chemistry models. We briefly present the formulation of LoKI-GM, including its numerical solution strategy, input/output parameters, and calculation workflow for both active discharges and afterglow plasmas. The main differences with respect to existing global models are highlighted, and the flexibility of the code for plasma chemistry studies is demonstrated through representative simulation results obtained across a range of gas discharge configurations and operating conditions.

physics.plasm-ph↗

arXiv2023

Validation of non-equilibrium kinetics in CO2-N2 plasmas

This work explores the effect of N2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent kinetic model, previously validated for pure CO2 and CO2-O2 discharges, is further extended by adding the kinetics of N2. The vibrational kinetics considered include levels up to v = 10 for CO, v = 59 for N2 and up to v1 = 2 and v2 = v3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and account for electron-impact excitation and de-excitation (e-V), vibration-to-translation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge operating in pure CO2 and in CO2-N2 mixtures, at pressures in the range 0.6 - 4 Torr (80.00 - 533.33 Pa) and a current of 50 mA. The experimental results show a higher vibrational temperature of the different modes of CO2 and CO and an increased dissociation fraction of CO2, that can reach values as high as 70%, when N2 is added to the plasma. On the one hand, the simulations suggest that the former effect is the result of the CO2-N2 and CO-N2 V-V transfers and the reduction of quenching due to the decrease of atomic oxygen concentration; on the other hand, the dilution of CO2 and dissociation products, CO and O2, reduces the importance of back reactions and contributes to the higher CO2 dissociation fraction with increased N2 content in the mixture, while the N2(B3Pg) electronically excited state further enhances the CO2 dissociation.

physics.plasm-ph↗

arXiv2022

Study of vibrational kinetics of CO2 and CO in CO2-O2 plasmas under non-equilibrium conditions

This work explores the effect of O2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent model, previously validated for pure CO2 discharges, is further extended by adding the vibrational kinetics of CO, including electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The vibrational kinetics considered include levels up to v = 10 for CO and up to v1=2 and v2=v3=5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and accounts for e-V, V-T in collisions between CO, CO2 and O2 molecules and O atoms and V-V processes involving all possible transfers involving CO2 and CO molecules. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge, operating at pressures in the range 0.4 - 5 Torr (53.33 - 666.66 Pa). The experimental results show a lower vibrational temperature of the different modes of CO2 and a decreased dissociation fraction of CO2 when O2 is added to the plasma but an increase of the vibrational temperature of CO. On the one hand, the simulations suggest that the former effect is the result of the stronger V-T energy-transfer collisions with O atoms which leads to an increase of the relaxation of the CO2 vibrational modes; On the other hand, the back reactions with O2 contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture.

physics.plasm-ph↗

arXiv2021

Stable Positron Acceleration in Thin, Warm, Hollow Plasma Channels

Hollow plasma channels are attractive for lepton acceleration because they provide intrinsic emittance preservation regimes. However, beam breakup instabilities dominate the dynamics. Here, we show that thin, warm hollow channels can sustain large-amplitude plasma waves ready for high-quality positron acceleration. We verify that the combination of warm electrons and thin hollow channel enables positron focusing structures. Such focusing wakefields unlock beam breakup damping mechanisms. We demonstrate that such channels emerge self-consistently during the long-term plasma dynamics in the blowout's regime aftermath, allowing for experimental demonstration.

physics.plasm-ph↗

arXiv2020

Dissipation of electron-beam-driven plasma wakes

Metre-scale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nano-Coulomb electron bunches. To reach useful average currents, however, the enormous energy density that the driver deposits into the wake must be removed efficiently between shots. Yet mechanisms by which wakes dissipate their energy into surrounding plasma remain poorly understood. Here, we report ps-time-resolved, grazing-angle optical shadowgraphic measurements and large-scale particle-in-cell simulations of ion channels emerging from broken wakes that electron bunches from the SLAC linac generate in tenuous lithium plasma. Measurements show the channel boundary expands radially at 1 million metres-per-second for over a nanosecond. Simulations show that ions and electrons that the original wake propels outward, carrying 90 percent of its energy, drive this expansion by impact-ionizing surrounding neutral lithium. The results provide a basis for understanding global thermodynamics of multi-GeV plasma accelerators, which underlie their viability for applications demanding high average beam current.

physics.plasm-ph↗