Ultrarelativistic quasiclassical wave functions in strong laser and atomic fields

The problem of an ultrarelativistic charge in the presence of an atomic and a plane-wave field is investigated in the quasiclassical regime by including exactly the effects of both background fields. Starting from the quasiclassical Green's function obtained in [Phys. Lett. B \textbf{717}, 224 (2012)], the corresponding in- and out-wave functions are derived in the experimentally relevant case of the particle initially counterpropagating with respect to the plane wave. The knowledge of these electron wave functions opens the possibility of investigating a variety of problems in strong-field QED, where both the atomic field and the laser field are strong enough to be taken into account exactly from the beginning in the calculations.Comment: 24 pages, no figure

Investigation of classical radiation reaction with aligned crystals

Classical radiation reaction is the effect of the electromagnetic field emitted by an accelerated electric charge on the motion of the charge itself. The self-consistent underlying classical equation of motion including radiation-reaction effects, the Landau-Lifshitz equation, has never been tested experimentally, in spite of the first theoretical treatments of radiation reaction having been developed more than a century ago. Here we show that classical radiation reaction effects, in particular those due to the near electromagnetic field, as predicted by the Landau-Lifshitz equation, can be measured in principle using presently available facilities, in the energy emission spectrum of $30\text{-}\text{GeV}$ electrons crossing a $0.55$-$\text{mm}$ thick diamond crystal in the axial channeling regime. Our theoretical results indicate the feasibility of the suggested setup, e.g., at the CERN Secondary Beam Areas (SBA) beamlines.Comment: 8 pages, 5 figure

Effect of a strong laser field on e+e−e^+ e^- photoproduction by relativistic nuclei

We study the influence of a strong laser field on the Bethe-Heitler photoproduction process by a relativistic nucleus. The laser field propagates in the same direction as the incoming high-energy photon and it is taken into account exactly in the calculations. Two cases are considered in detail. In the first case, the energy of the incoming photon in the nucleus rest frame is much larger than the electron's rest energy. The presence of the laser field may significantly suppress the photoproduction rate at soon available values of laser parameters. In the second case, the energy of the incoming photon in the rest frame of the nucleus is less than and close to the electron-positron pair production threshold. The presence of the laser field allows for the pair production process and the obtained electron-positron rate is much larger than in the presence of only the laser and the nuclear field. In both cases we have observed a strong dependence of the rate on the mutual polarization of the laser field and of the high-energy photon and the most favorable configuration is with laser field and high-energy photon linearly polarized in the same direction. The effects discussed are in principle measurable with presently available proton accelerators and laser systems.Comment: 21 pages, 4 figure

Steady, periodic, quasi-periodic and chaotic flow regimes in toroidal pipes

Incompressible flow in a toroidal pipe was investigated by direct numerical simulation. The curvature a/c (radius of the cross section / radius of the torus) was 0.3 or 0.1 and the bulk Reynolds number ranged between 3500 and 14 700. The study revealed a rich scenario of transition to turbulence. For the higher curvature a/c = 0.3, a supercritical transition from stationary to periodic flow (Hopf bifurcation) was observed at Re=4600. The periodic flow was characterized by a travelling wave which, in the whole periodic Re range, took the form of a varicose modulation of the twin Dean vortex rings, included 8 wavelengths along the axis of the torus, and exhibited instantaneous anti-symmetry about the equatorial midplane. A further transition to quasi-periodic flow, characterized by two independent fundamental frequencies and their first few harmonics, occurred at Re=5200. The two frequencies were associated with two travelling wave systems, the first consisting of a varicose modulation of the Dean vortex rings, the second of an array of oblique near-wall vortices produced at the edge of the Dean cells, co-rotating with these latter and travelling from the inner towards the outer side, against the secondary circulation. For the lower curvature a/c=0.1, the results suggested the existence of a subcritical Hopf bifurcation at Re=5200 and of a secondary Hopf bifurcation to quasi-periodic flow at a lower Reynolds number of ~4900. Starting from zero-velocity initial conditions, the steady-state flow remained stable up to a Reynolds number of 5139, while a further increase in Re to 5208 yielded an abrupt transition to quasi-periodic flow which remained stable up to Re=6280 or larger. When a quasi-periodic solution (e.g., that obtained for Re=5658) was used as the initial condition and Re was made to decrease, the quasi-periodic regime remained stable down to values of Re well below the subcritical Hopf bifurcation at ~5200. Only a further, substantial decrease of Re to ~4108 led to the smooth disappearance of mode II and to a stable periodic solution. An abrupt transition to stationary flow was obtained when the Reynolds number decreased well below 4000 (e.g., a test case was computed for Re =3490). All periodic and quasi-periodic solutions for a/c=0.1 exhibited instantaneous symmetry about the equatorial midplane. Also the further transition from quasi-periodic to chaotic flow occurred with different mechanisms for the two curvatures. For a/c=0.3, quasi-periodic flow was obtained in the whole Reynolds number range 5270-7850. As Re increased slightly beyond this value (Re=8160), strong fluctuations, associated with random streamwise vortices, arose in the outer region. The ensuing chaotic flow regime was characterized by a broadband, almost continuous, frequency spectrum. A further increase of Re to 13180 did not modify to any appreciable extent the flow regime and the distribution of the velocity fluctuation intensity. For a/c=0.1, the convergence of the results to quasi-periodic flow became impossible to achieve as Re increased beyond ~6280, and was replaced by long and erratic transients. For Re=8160, the solution, albeit stationary in a statistical sense, was chaotic and exhibited a large number of frequencies, but the outer region remained basically stationary. Only when Re increased further, the outer region became unsteady and was characterized by the production of streamwise vortices which were then transported by the secondary flow destroying all remains of regular oscillations

Pair production in a strong slowly varying magnetic field: the effect of a background gravitational field

The production probability of an $e^--e^+$ pair in the presence of a strong, uniform and slowly varying magnetic field is calculated by taking into account the presence of a background gravitational field. The curvature of the spacetime metric induced by the gravitational field not only changes the transition probabilities calculated in the Minkowski spacetime but also primes transitions that are strictly forbidden in absence of the gravitational field.Comment: 56 pages, no figure

Angular distribution of high-energy e+e−e^+e^- photoproduction close to the end of spectrum

We consider the differential cross section of electron-positron pair production by a high-energy photon in a strong Coulomb field close to the end of the electron or positron spectrum. When the momentum transfer largely exceeds the electron mass, the cross section is obtained analytically in a compact form. Coulomb corrections essentially modify the cross section even for moderate values of the nuclear charge number $Z$. In the same kinematical region, the angular distribution for bound-free pair production, bremsstrahlung, and photorecombination is also obtained.Comment: 12 pages, 4 figure

Pre-test CFD simulations of the NACIE-UP BFPS test section

The present paper is focused on the CFD pre-test analysis and design of the new experimental facility Blocked Fuel Pin bundle Simulator (BFPS) that will be installed into the NACIE-UP (NAtural CIrculation Experiment-UPgrade) facility located at the ENEA Brasimone Research Center (Italy). The BFPS test section will carry out suitable experiments to fully investigate different flow blockage regimes in a 19 fuel pin bundle providing experimental data in support of the development of the ALFRED (Advanced Lead-cooled Fast Reactor European Demonstrator) LFR DEMO. The geometrical domain of the fuel pin bundle simulator was designed to reproduce the geometrical features of ALFRED, e.g. the external wrapper in the active region and the spacer grids. Pre-tests calculations were carried out by applying accurate boundary conditions; the conjugate heat transfer in the clad is also considered. The blockages investigated are internal blockages of different extensions and in different locations: central sub-channel blockage, corner sub-channel blockage, edge sub-channel blockage, one sector blockage, and two-sector blockage. RANS simulations were carried out adopting the ANSYS CFX commercial code with the laminar sublayer resolved by the mesh resolution. The loci of the peak temperatures and their width as predicted by the CFD simulations are used for determining the location of the pin bundle instrumentation. The CFD pre-test analysis allowed also investigating the temperature distribution in the clad to operate the test section safely

Nonlinear double Compton scattering in the full quantum regime

A detailed analysis of the process of two photon emission by an electron scattered from a high-intensity laser pulse is presented. The calculations are performed in the framework of strong-field QED and include exactly the presence of the laser field, described as a plane wave. We investigate the full quantum regime of interaction, where photon recoil plays an essential role in the emission process, and substantially alters the emitted photon spectra as compared to those in previously-studied regimes. We provide a semiclassical explanation for such differences, based on the possibility of assigning a trajectory to the electron in the laser field before and after each quantum photon emission. Our numerical results indicate the feasibility of investigating experimentally the full quantum regime of nonlinear double Compton scattering with already available plasma-based electron accelerator and laser technology.Comment: 5 pages, 3 figure

Stimulated vacuum emission and photon absorption in strong electromagnetic fields

According to quantum electrodynamics (QED), a strong external field can make the vacuum state decay producing electron-positron pairs. Here we investigate emission of soft photons which accompanies a nonperturbative process of pair production. Our analysis is carried out within the Furry picture to first order in the fine-structure constant. It is shown that the presence of photons in the initial state gives rise to an additional (stimulated) channel of photon emission besides the pure vacuum one. On the other hand, the number of final (signal) photons includes also a negative contribution due to photon absorption within the pair production process. These contributions are evaluated and compared. To obtain quantitative predictions in the domain of realistic field parameters, we employ the WKB approach. We propose to use an optical probe photon beam, whose intensity changes as it traverses a spatial region where a strong electric component of a background laser field is present. It is demonstrated that relative intensity changes on the level of $1 \%$ can be experimentally observed once the intensity of the strong background field exceeds $10^{27}~\text{W/cm}^2$ within a large laser wavelength interval. This finding is expected to significantly support possible experimental investigations of nonlinear QED phenomena in the nonperturbative regime