arXiv subjects

Vasco Guerra

Publications and source records attributed to Vasco Guerra.

13 recordsLinked to original sources

arXiv2026

Stochastic hierarchical data-driven optimization: application to plasma-surface kinetics

This work introduces a stochastic hierarchical optimization framework inspired by Sloppy Model theory for the efficient calibration of physical models. Central to this method is the use of a reduced Hessian approximation, which identifies and targets the stiff parameter subspace using minimal simulation queries. This strategy enables efficient navigation of highly anisotropic landscapes, avoiding the computational burden of exhaustive sampling. To ensure rigorous inference, we integrate this approach with a probabilistic formulation that derives a principled objective loss function directly from observed data. We validate the framework by applying it to the problem of plasma-surface interactions, where accurate modelling is strictly limited by uncertainties in surface reactivity parameters and the computational cost of kinetic simulations. Comparative analysis demonstrates that our method consistently outperforms baseline optimization techniques in sample efficiency. This approach offers a general and scalable tool for optimizing models of complex reaction systems, ranging from plasma chemistry to biochemical networks.

cs.LG↗

arXiv2026

Conditional Denoising Model as a Physical Surrogate Model

Surrogate modeling for complex physical systems typically faces a trade-off between data-fitting accuracy and physical consistency. Physics-consistent approaches typically treat physical laws as soft constraints within the loss function, a strategy that frequently fails to guarantee strict adherence to the governing equations, or rely on post-processing corrections that do not intrinsically learn the underlying solution geometry. To address these limitations, we introduce the {Conditional Denoising Model (CDM)}, a generative model designed to learn the geometry of the physical manifold itself. By training the network to restore clean states from noisy ones, the model learns a vector field that points continuously towards the valid solution subspace. We introduce a time-independent formulation that transforms inference into a deterministic fixed-point iteration, effectively projecting noisy approximations onto the equilibrium manifold. Validated on a low-temperature plasma physics and chemistry benchmark, the CDM achieves higher parameter and data efficiency than physics-consistent baselines. Crucially, we demonstrate that the denoising objective acts as a powerful implicit regularizer: despite never seeing the governing equations during training, the model adheres to physical constraints more strictly than baselines trained with explicit physics losses.

cs.LG↗

arXiv2025

Boosting the NOx production in microwave air plasma: A synergy of chemistry and vibrational kinetics

This study employs a quasi-1.5D multi-temperature model to investigate the mechanisms governing NOx production and energy costs in microwave plasma reactors operating at 80 mbar, focusing on the interplay of vibrational, chemical and electron kinetics, thermodynamics, and transport processes across the discharge and afterglow. In the plasma discharge zone, non-thermal processes enhance NOx production as electrons transfer energy effectively to the vibrational mode of N2. However, the non-thermal enhancement is found to diminish rapidly within the central-afterglow region. The simulation results show good agreement with experimental data for both the temperature profile and energy cost. Turbulent effects facilitate radial NO diffusion into cooler regions while simultaneously enhancing cooling of the axial region. These findings highlight the potential to improve NOx synthesis efficiency by optimizing turbulence and maintaining non-thermal conditions, offering new opportunities for the advancement of plasma-based chemical processes.

physics.plasm-ph↗

arXiv2025

Physics-consistent machine learning: output projection onto physical manifolds

Data-driven machine learning models often require extensive datasets, which can be costly or inaccessible, and their predictions may fail to comply with established physical laws. Current approaches for incorporating physical priors mitigate these issues by penalizing deviations from known physical laws, as in physics-informed neural networks, or by designing architectures that automatically satisfy specific invariants. However, penalization approaches do not guarantee compliance with physical constraints for unseen inputs, and invariant-based methods lack flexibility and generality. We propose a novel physics-consistent machine learning method that directly enforces compliance with physical principles by projecting model outputs onto the manifold defined by these laws. This procedure ensures that predictions inherently adhere to the chosen physical constraints, improving reliability and interpretability. Our method is demonstrated on two systems: a spring-mass system and a low-temperature reactive plasma. Compared to purely data-driven models, our approach significantly reduces errors in physical law compliance, enhances predictive accuracy of physical quantities, and outperforms alternatives when working with simpler models or limited datasets. The proposed projection-based technique is versatile and can function independently or in conjunction with existing physics-informed neural networks, offering a powerful, general, and scalable solution for developing fast and reliable surrogate models of complex physical systems, particularly in resource-constrained scenarios.

cs.LG↗

arXiv2023

A reaction mechanism for oxygen plasmas

This work presents a reaction mechanism for oxygen plasmas, i.e. a set of reactions and corresponding rate coefficients that are validated against benchmark experiments. The kinetic scheme is validated in a DC glow discharge for gas pressures of 0.2-10 Torr and currents of 10-40 mA, using the 0D LisbOn KInetics (LoKI) simulation tool and available experimental data. The comparison comprises not only the densities of the main species in the discharge - $\mathrm{O_2(X^3Σ_g^-)}$, $\mathrm{O_2(a^1Δ_g)}$, $\mathrm{O_2(b^1Σ_g^+)}$ and $\mathrm{O(^3P)}$ - but also the self-consistent calculation of the reduced electric field and the gas temperature. The main processes involved in the creation and destruction of these species are identified. Moreover, the results show that the oxygen atoms play a dominant role in gas heating, via recombination at the wall and quenching of $\mathrm{O_2(X^3Σ_g^-,v)}$ vibrations and $\mathrm{O_2}$ electronically-excited states. It is argued that the development and validation of kinetic schemes for plasma chemistry should adopt a paradigm based on the comparison against standard validation tests, as it is done in electron swarm validation of cross sections.

physics.plasm-ph↗

arXiv2022

Benchmarking between fluid and global models for low-pressure oxygen DC glow discharges

This work focuses on the benchmarking between a zero-dimensional (0D) global model (LoKI) and a one-dimensional (1D) radial fluid model for the positive column of oxygen DC glow discharges in a tube of 1 cm inner radius at pressures between 0.5 Torr and 10 Torr. The data used in the two models are the same, so that the difference between the models is reduced to dimensionality. A good agreement is found between the two models on the main discharge parameters, with relative differences below 5%. The agreement on species average number densities, charged and neutral, is slightly worse, with relative differences increasing with pressure from 11% at 0.5 Torr to 57% at 10 Torr. The success of the 0D global model in describing these plasmas through volume averaged quantities decreases with pressure, due to pressure-driven narrowing of radial profiles. Hence, in the studied conditions, we recommend the use of volume-averaged models only in the pressure range up to 10 Torr.

physics.plasm-ph↗

arXiv2021

Zero-dimensional and pseudo-one-dimensional models of atmospheric-pressure plasma jet in binary and ternary mixtures of oxygen and nitrogen with helium background

A zero-dimensional (volume-averaged) and a pseudo-one-dimensional (plug-flow) model are developed to investigate atmospheric-pressure plasma jet devices operated with He, He/O$_2$, He/N$_2$ and He/N$_2$/O$_2$ mixtures. The models are coupled with the Boltzmann equation under the two-term approximation to self-consistently calculate the electron energy distribution function (EEDF). The simulation results are verified against spatially resolved model calculations and validated against a wide variety of measurement data. The nitric oxide (NO) concentration is thoroughly characterized for a variation of the gas mixture ratio, helium flow rate and absorbed power. The concentration measurements at low power are better captured by the simulation with a larger hypothetical "effective" rate coefficient value for the reactive quenching N$_2$(A$^3Σ$,B$^3Π$) + O($^3$P) $\to$ NO + N($^2$D). This suggests that the NO production at low power is also covered by the species N$_2$(A$^3Σ$,B$^3Π$;v>0) and multiple higher N$_2$ electronically excited states instead of only N$_2$(A$^3Σ$,B$^3Π$;v=0) in this quenching. Furthermore, the O($^3$P) density measurements under the same operation conditions are also better predicted by the simulations with a consideration of the aforementioned hypothetical rate coefficient value. It is found that the contribution of the vibrationally excited nitrogen molecules N$_2$(v$\geqslant$13) to the net NO formation rate gains more significance at higher power. The vibrational distribution functions (VDFs) of O$_2$(v<41) and N$_2$(v<58) are investigated. The sensitivity of the zero-dimensional model with respect to a variation of the VDF resolutions, wall reaction probabilities and synthetic air impurity levels is presented. The simulated plasma properties are sensitive to the variation especially for a feeding gas mixture containing nitrogen.

physics.plasm-ph↗

arXiv2012

The concepts of work and heat and the first and second laws of thermodynamics

A simple and effective approach to thermodynamics is suggested, which solves the major difficulties in the traditional presentation of the subject. The internal energy is introduced from the behavior of deformable bodies, whereas the importance of keeping in mind the microscopic picture is emphasized. A straightforward model is used to show that the internal energy depends on the volume and entropy, from where the relationship between mechanics and thermodynamics is immediate, mechanics corresponding to isentropic thermodynamics. The questions of evolution to equilibrium and irreversibility are studied under the light of the action of the "dynamic force", which has a dissipative character. The present formulation leads to a discussion and clarification of the physical meaning of various thermodynamic quantities, such as pressure, temperature, work and heat. The adiabatic piston problem is analyzed, as a paradigmatic case where the notions of "adiabatic" and "heat transfer" are often ill-defined.

physics.gen-ph↗

arXiv2010

Comment on 'A close examination of the motion of an adiabatic piston' by Eric A. Gislason [Am. J. Phys. 78, 995-1001 (2010)]

A recent paper by Gislason published in Am. J. Phys. deals with the celebrated example of the so-called "adiabatic piston", a system involving two ideal gases contained in a horizontal cylinder and separated by an insulating piston that moves without friction. While the analysis presented in that paper is rather comprehensive, very interesting and useful as a teaching tool, it can be somewhat misleading if not taken within its appropriate context. As a matter of fact, the evolution to equilibrium involves two phases, a faster one leading to the equalization of pressures, and a slower one bringing the system to identical temperatures. Although Gislason addresses only the first process, we note that the final state after the second phase, the evolution to equal temperatures once the pressures are the same, is described by thermodynamics. Therefore, the discussion of the adiabatic piston given by Gislason can and should be enriched, in order to promote a proper and general view of thermodynamics.

physics.class-ph↗

arXiv2009

Comment on 'A one-way speed of light experiment'

A recent paper published in Am. J. Phys. describes an experiment designed to measure the one-way speed of light. Although the experiment is very interesting, in particular to be used in student laboratories, it is in fact determining the two-way speed of light.

physics.gen-ph↗

arXiv2006

Special Relativity in Absolute Space: from a contradiction in terms to an obviousness

This work deals with the questions of absolute space and relativity. In particular, an alternative derivation of the effects described by special relativity is provided, which is based on a description that assumes a privileged reference frame. The present theory follows the ideas of Lorentz and Poincare, abandoning a strict view of Einstein's "equivalence" of all inertial frames. The meaning of the Principle of Relativity is discussed and elucidated, and it is shown that it is not incompatible with the existence of a preferred, absolute, frame. Most scientists nowadays still consider the basic assumptions of the theory proposed here to be plain wrong. Moreover, they tend to see an irreconcilable conflict between the Lorentz-Poincare and the Einstein-Minkowski formulations. However, as stated by John Bell [Bell1988], although there is a stringent "difference in philosophy" between both views, "the facts of physics do not oblige us to accept one philosophy rather than the other". The validity of Bell's assertion is unambiguously demonstrated, and it is shown how and why both approaches do indeed agree in the description of (most of?) the physical phenomena. Evidently, the physical meaning of the different physical quantities - such as "time", "speed", "simultaneity" and "synchronization" - is quite different in both programmes. And yet, for perplexing it may look at first sight, the present theory, developed under the Lorentz-Poincare assumption of a preferred reference frame, somehow encompasses Einstein's theory. There is no conflict, as there is one theory. It is stressed that reality is not changed by the choices one makes to describe it, so it is not changed by the particular way in which the clocks have been set.

physics.ed-ph↗

arXiv2005

Is the assumption of a special system of reference consistent with Special Relativity?

In a previous work we have shown that the null result of the Michelson-Morley experiment in vacuum is deeply connected with the notion of time. The same is true for the the postulate of constancy of the two-way speed of light in vacuum in all frames independently of the state of motion of the emitting body. The argumentation formerly given is very general and has to be true not only within Special Relativity and its `equivalence' of all inertial frames, but as well as in Lorentz-Poincaré scenario of a preferred reference frame. This paper is the second of a trilogy intending to revisit the foundations of Special Relativity, and addresses the question of the constancy of the one-way speed of light and of the differences and similarities between both scenarios. Although they manifestly differ in philosophy, it is debated why and how the assumption of a "special system of reference experimentally inaccessible" is indeed compatible with Einstein's Special Relativity, as beautifully outlined and discussed by John Bell [1]. This rather trivial statement is still astonishing nowadays to a big majority of scientists. The purpose of this work is to bring such assertion into perspective, widening the somewhat narrow view of Special Relativity often presented in textbooks.

physics.ed-ph↗

arXiv2005

The conceptualization of time and the constancy of the speed of light

In this work we show that the null result of the Michelson-Morley experiment in vacuum is deeply connected with the notion of time. It can be deduced without any mathematics only from the assumption that all good clocks can be used to measure time with the same results, independently of the machinery involved in their manufacturing. A second important assumption, intrinsic to the very notion of time, is that clocks measure time in the same way in different frames, i.e., the notion of time is the same in all inertial frames. Under this assumption, we point out that the "postulate" of constancy of the "two-way" speed of light in vacuum in all frames independently of the state of motion of the emitting body is also strongly related to the concept of time, together with the existence of a limit speed in the "rest frame". This postulate simply results from the construction of clocks where tic-tacs are made by objects traveling with the limit speed.

physics.ed-ph↗