Improved local-constant-field approximation for strong-field QED codes

The local-constant-field approximation (LCFA) is an essential theoretical tool for investigating strong-field QED phenomena in background electromagnetic fields with complex spacetime structure. In our previous work [Phys.~Rev.~A~\textbf{98}, 012134 (2018)] we have analyzed the shortcomings of the LCFA in nonlinear Compton scattering at low emitted photon energies for the case of a background plane-wave field. Here, we generalize that analysis to background fields, which can feature a virtually arbitrary spacetime structure. In addition, we provide an explicit and simple implementation of an improved expression of the nonlinear Compton scattering differential probability that solves the main shortcomings of the standard LCFA in the infrared region, and is suitable for background electromagnetic fields with arbitrary spacetime structure such as those occurring in particle-in-cell simulations. Finally, we carry out a systematic procedure to calculate the probability of nonlinear Compton scattering per unit of emitted photon light-cone energy and of nonlinear Breit-Wheeler pair production per unit of produced positron light-cone energy beyond the LCFA in a plane-wave background field, which allows us to identify the limits of validity of this approximation quantitatively.Comment: 15 pages, 3 figure

Syntrophic LCFA degradation: generating a high-energy carrier from low-energy metabolism

For many years the interest in anaerobic degradation of lipids and long-chain fatty acids (LCFA) focused on technology and process developments. Attention to the microbiology of LCFA conversion was boosted by results showing that high methane yields could be obtained during anaerobic degradation of LCFA. The principle pathway of LCFA degradation is through -oxidation, producing acetate and hydrogen. For this conversion to be thermodynamically feasible, in methanogenic environments, acetogenic LCFA-degrading bacteria have to cooperate in syntrophy with hydrogen-consuming archaea. DGGE fingerprinting and 16S rRNA gene sequencing showed the importance of Syntrophomonas-like bacteria during batch and continuous degradation of unsaturated and saturated LCFA. The 7 species described thus far with the ability to grow on LCFA (with more than 12 carbon atoms), all belong to the families Syntrophomonadaceae and Syntrophaceae and, among these, only 4 species have the capability of utilizing mono- and/or polyunsaturated LCFA. Syntrophomonas zehnderi is able to degrade a wide range of saturated and unsaturated LCFA (C4 to C18) and could be detected in sludge samples from fed-batch and continuous reactors degrading oleate (monounsaturated C18). Batch bioaugmentation assays showed that addition of S. zenhderi could improve methane recovery from LCFA. The molecular mechanisms of anaerobic-LCFA degradation by S. zehnderi are being further studied by proteomics and whole genome sequencing. Identifying the components involved in the conversion of unsaturated LCFA-conversion is of particular interest. Related to this, the microbiology of oleate to palmitate (saturated C16) conversion by mixed cultures is also being studied using stable isotope probing and metaproteomics

Optimization of the anaerobic co-digestion of pasteurized slaughterhouse waste, pig slurry and glycerine

The feasibility of co-digestion of blends of two different animal by-products (pig manure and pasteurized slaughterhouse waste) and recovered glycerine was studied in mesophilic conditions. Experiments were performed in a lab-scale CSTR along 490 days, with a hydraulic retention time of 21–33 days and with a step-wise increased organic loading rate, by adding and/or changing the wastes ratio, from 0.8 to 3.2 kgCOD m-3 d-1. The best methane production rate (0.64 Nm3CH4 m-3 d-1) represented an increment of 2.9-fold the initial one (0.22 Nm3CH4 m-3 d-1 with pig manure solely). It was attained with a ternary mixture composed, in terms of inlet volatile solids, by 35% pig slurry, 47% pasteurized slaughterhouse waste and 18% glycerine. This blend was obtained through a stepwise C/N adjustment: this strategy led to a more balanced biodegradation due to unstressed bacterial populations through the performance, showed by the VFA-related indicators. Besides this, an improved methane yield (+153%) and an organic matter removal efficiency (+83%), regarding the digestion of solely pig slurry, were attained when the C/N ratio was adjusted to 10.3.Postprint (author's final draft

Mineralization of LCFA associated with anaerobic sludge : kinetics, enhancement of methanogenic activity, and effect of VFA

Long-chain fatty acids (LCFA) associated with anaerobic sludge by mechanisms of precipitation, adsorption, or entrapment can be biodegraded to methane. The mineralization kinetics of biomass-associated LCFA were established according to an inhibition model based on Haldane’s enzymatic inhibition kinetics. A value around 1,000 mg COD-LCFA g VSS-1 was obtained for the optimal specific LCFA content that allowed the maximal mineralization rate. For sludge with specific LCFA contents of 2838 ± 63 and 4571 ± 257 mg COD-LCFA g VSS-1, the specific methanogenic activities in the presence of acetate, butyrate, and H2/CO2 were significantly enhanced after the mineralization of the biomass-associated LCFA. For sludge with a specific LCFA content near the optimal value defined by the kinetic model, the effect of adding VFA to the medium was studied during the mineralization of the biomass-associated LCFA. Different patterns were obtained for each individual substrate. Acetate and butyrate were preferentially consumed by the consortium, but in the case of propionate no evidence of a sequential consumption pattern could be withdrawn. It was concluded that LCFA do not exert a bactericidal neither a permanent toxic effect toward the anaerobic consortia. A discussion is addressed to the relative roles of a reversible inhibitory effect and a transport limitation effect imposed by the LCFA surrounding the cells.Fundação para a Ciência e Tecnologia (FCT) - PRAXIS XXI/BD/20326/99, SFRH/BD/8726/2002. Fundo Social Europeu (FSE)

Radiation beaming in the quantum regime

Classical theories of radiation reaction predict that the electron motion is confined to the plane defined by the electron's instantaneous momentum and the force exerted by the external electromagnetic field. However, in the quantum radiation reaction regime, where the recoil exerted by individual quanta becomes significant, the electron can scatter `out-of-plane', as the photon is emitted into a cone with finite opening angle. We show that Monte Carlo implementation of an angularly resolved emission rate leads to substantially improved agreement with exact QED calculations of nonlinear Compton scattering. Furthermore, we show that the transverse recoil caused by this finite beaming, while negligible in many high-intensity scenarios, can be identified in the increase in divergence, in the plane perpendicular to the polarization, of a high-energy electron beam that interacts with a linearly polarized, ultraintense laser.Comment: 8 pages, 5 figures; accepted for publication in Phys Rev

Pair production from the vacuum by a weakly inhomogeneous space-dependent electric potential step

There exists a clear physical motivation for theoretical studies of the vacuum instability related to the production of electron-positron pairs from a vacuum due to strong external electric fields. Various nonperturbative (with respect to the external fields) calculation methods were developed. Some of these methods are based on possible exact solutions of the Dirac equation. Unfortunately, there are only few cases when such solutions are known. Recently, an approximate but still nonperturbative approach to treat the vacuum instability caused by slowly varying $t$-electric potential steps (time dependent external fields that vanish as $|t|\rightarrow\infty$), which does not depend on the existence of the corresponding exact solutions, was formulated in Ref. [S. P. Gavrilov, D. M. Gitman, Phys. Rev. D \textbf{95}, 076013 (2017)]. Here, we present an approximate calculation method to treat nonperturbatively the vacuum instability in arbitrary weakly inhomogeneous $x$-electric potential steps (time-independent electric fields of a constant direction that are concentrated in restricted space areas, which means that the fields vanish as $|x|\rightarrow\infty$) in the absence of the corresponding exact solutions. Defining the weakly inhomogeneous regime in general terms, we demonstrate the universal character of the vacuum instability. This universality is associated with a large density of states excited from the vacuum by the electric field. Such a density appears in our approach as a large parameter. We derive universal representations for the total number and current density of the created particles. Relations of these representations with a locally constant field approximation for Schwinger's effective action are found.Comment: 17 pages; misprints corrected, misprints corrected, the title slightly changed during review process; version accepted for publicatio

Effect of sulfate and iron (III) on LCFA degradation by a methanogenic community

[Excerpt] Under anaerobic conditions long chain fatty acids (LCFA) can be converted to methane by syntrophic bacteria and methanogenic archaea. LCFA degradation was also reported in the presence of alternative hydrogenotrophic partners, such as sulfate-reducing bacteria (SRB) and iron-reducing bacteria (IRB), which generally show higher affinity for H2 than methanogens and are more resistant to LCFA [1,2,3]. Their presence in a microbial culture degrading LCFA can be advantageous to reduce LCFA toxicity towards methanogens, although high concentrations of external electron acceptor (EEA) can lead to outcompetition of methanogens and cease methane production. In this work, we tested the effect of adding sub-stoichiometric concentrations of sulfate and iron(III) to methanogenic communities degrading LCFA. (...

Co-digestion of the mechanically recovered organic fraction of municipal solid waste with slaughterhouse wastes

The current work aimed to resolve some long-standing questions about the potential benefits and limitations of co-digestion of slaughterhouse wastes. To achieve this, a laboratory-scale trial was carried out using the mechanically recovered organic fraction of municipal solid waste mixed with either sheep blood or a mixture of pig intestines with flotation fat. Both of these co-substrates are difficult to digest in isolation because of their high nitrogen and lipid concentrations, and are regulated as Category 3 materials under the Animal By-Products Regulations (EC 1069/2009). The results showed that at an organic loading rate of 2 kg VS m?3 day?1 with the slaughterhouse material making up 20% of the load on a volatile solids basis the process could operate successfully. As the loading was increased to 4 kg VS m?3 day?1 signs of inhibition appeared with both co-substrates, however, and volumetric methane production was reduced to a point where co-digestion gave no process advantage. The main operational problem encountered was an increase in the concentration of volatile fatty acids in the digestate, particularly propionic acid: this was thought to be a result of ammonia toxicity. The concentration of potentially toxic elements in the digestate made it unsuitable for agricultural application for food production, although the increased nitrogen content made it more valuable as a fertiliser for non-food crop use