Atomic Physics Processes Research Articles

Simulation of plasma loading of high-pressure RF cavities

Muon beam-induced plasma loading of radio-frequency (RF) cavities filled with high pressure hydrogen gas with 1% dry air dopant has been studied via numerical simulations. The electromagnetic code SPACE, that resolves relevant atomic physics processes, including ionization by the muon beam, electron attachment to dopant molecules, and electron-ion and ion-ion recombination, has been used. Simulations studies have been performed in the range of parameters typical for practical muon cooling channels.

High-Energy-Density Physics at the National Ignition Facility

At modern laser facilities, energy densities ranging from 1 Mbar to many hundreds of gigabars can regularly be achieved. These high-energy states of matter last for mere moments, measured in nanoseconds to tens of picoseconds, but during those times numerous high-precision instruments can be employed, revealing remarkable compressed matter physics, radiation–hydrodynamics physics, laser–matter interaction physics, and nuclear physics processes. We review the current progress of high-energy-density physics at the National Ignition Facility and describe the underlying physical principles.

Overview of the future upgrade of the INFN-LNS superconducting cyclotron

The LNS Superconducting Cyclotron, named “Ciclotrone Superconduttore” (CS), has been in operation for more than 20 years. A wide range of ion species from hydrogen to lead, with energy in the range 10 to 80 AMeV, have been delivered to users. The maximum beam power is limited to 100 W due to the beam dissipation on the electrostatic deflectors. To fulfil the demand of users aiming at studying rare processes in nuclear physics, an upgrade of the cyclotron is necessarily intended to increase the intensity of ion beams with mass lower than 40 a.m.u. up to a power 10 kW. This will be achieved by means of extraction by stripping. This solution needs to replace the cryostat including the superconducting coils. The present capability of the cyclotron will be maintained, i.e. all the ion species allowed by the operating diagram will be available, being extracted by electrostatic extraction. In addition to the high power beams for nuclear physics, it will be possible to produce medical radioisotopes like [Formula: see text] using an internal target.

Expanding the role of impurity spectroscopy for investigating the physics of high-Z dissipative divertors

New techniques that attempt to more fully exploit spectroscopic diagnostics in the divertor and pedestal region during highly dissipative scenarios are demonstrated using experimental results from recent low-Z seeding experiments on Alcator C-Mod, JET and ASDEX Upgrade. To exhaust power at high parallel heat flux, q∥ > 1 GW/m2, while minimizing erosion, reactors with solid, high-Z plasma facing components (PFCs) are expected to use extrinsic impurity seeding. Due to transport and atomic physics processes which impact impurity ionization balance, so-called ‘non-coronal’ effects, we do not accurately know and have yet to demonstrate the maximum q∥ which can be mitigated in a tokamak. Radiation enhancement for nitrogen is shown to arise primarily from changes in Li- and Be-like charge states on open field lines, but also through transport-driven enhancement of H- and He-like charge states in the pedestal region. Measurements are presented from nitrogen seeded H-mode and L-mode plasmas where emission from N1+ through N6+ are observed. Active charge exchange spectroscopy of partially ionized low-Z impurities in the plasma edge is explored to measure N5+ and N6+ within the confined plasma, while passive UV and visible spectroscopy is used to measure N1+-N4+ in the boundary. Examples from recent JET and Alcator C-Mod experiments which employ nitrogen seeding highlight how improving spectroscopic coverage can be used to gain empirical insight and provide more data to validate boundary simulations.

Wikipedia mining of hidden links between political leaders

We describe a new method of reduced Google matrix which allows to establish direct and hidden links between a subset of nodes of a large directed network. This approach uses parallels with quantum scattering theory, developed for processes in nuclear and mesoscopic physics and quantum chaos. The method is applied to the Wikipedia networks in different language editions analyzing several groups of political leaders of USA, UK, Germany, France, Russia and G20. We demonstrate that this approach allows to recover reliably direct and hidden links among political leaders. We argue that the reduced Google matrix method can form the mathematical basis for studies in social and political sciences analyzing Leader-Members eXchange (LMX).

Impurity transport in Alcator C-Mod in the presence of poloidal density variation induced by ion cyclotron resonance heating

Impurity particle transport in an ion cyclotron resonance heated Alcator C-Mod discharge is studied with local gyrokinetic simulations and a theoretical model including the effect of poloidal asymmetries and elongation. In spite of the strong minority temperature anisotropy in the deep core region, the poloidal asymmetries are found to have a negligible effect on the turbulent impurity transport due to low magnetic shear in this region, in agreement with the experimental observations. According to the theoretical model, in outer core regions poloidal asymmetries may contribute to the reduction of the impurity peaking, but uncertainties in atomic physics processes prevent quantitative comparison with experiments.

Influence of atomic physics on EDGE2D-EIRENE simulations of JET divertor detachment with carbon and beryllium/tungsten plasma-facing components

The EDGE2D-EIRENE code is applied for simulation of divertor detachment during matched density ramp experiments in high triangularity, L-mode plasmas in both JET-Carbon (JET-C) and JET-ITER-Like Wall (JET-ILW). The code runs without drifts and includes either C or Be as impurity, but not W, assuming that the W targets have been coated with Be via main chamber migration. The simulations reproduce reasonably well the observed particle flux detachment as density is raised in both JET-C and JET-ILW experiments and can better match the experimental in-out divertor target power asymmetry if the heat flux entering the outer divertor is artificially set at around 2–3 times that entering the inner divertor. A careful comparison between different sets of atomic physics processes used in EIRENE shows that the detachment modelled by EDGE2D-EIRENE relies only on an increase of the particle sinks and not on a decrease of the ionization source. For the rollover and the beginning of the partially detached phase, the particle losses by perpendicular transport and the molecular activated recombination processes are mainly involved. For a deeper detachment with significant target ion flux reduction, volume recombination appears to be the main contributor. The elastic molecule-ion collisions are also important to provide good neutral confinement in the divertor and thus stabilize the simulations at low electron temperature (Te), when the sink terms are strong. Comparison between EDGE2D-EIRENE and SOLPS4.3 simulations of the density ramp in C shows similar detachment trends, but the importance of the elastic ion-molecule collisions is reduced in SOLPS4.3. Both codes suggest that any process capable of increasing the neutral confinement in the divertor should help to improve the modelling of the detachment. A further outcome of this work has been to demonstrate that key JET divertor diagnostic signals—Langmuir probe Te and bolometric tomographic reconstructions—are running beyond the limit of validity in high recycling and detached conditions and cannot be reliably used for code validation. The simulations do, however, reproduce the trend of the evolution of the line integrated bolometer chord measurements. The comparison between the code results and high-n Balmer line radiation intensity profiles confirms that a strong volume recombination is present during the experimental detachment and may play a role in this process, as suggested by the code.

Modelling of non-LTE atomic physics processes in hot dense plasmas during the interaction with an intense short pulse laser

The implicit 2D3V particle-in-cell (PIC) code developed to study the interaction of intense lasers with matter (Petrov and Davis 2008 Comput. Phys. Commun. 179 868–80; 2011 Phys. Plasmas 18 073102) has been extended to include atomic physics under extreme energy density conditions. The atomic physics model is applied to aluminium. Each ionization stage contains two levels: one ground and one lumped excited state, for which various atomic physics processes such as optical field ionization, collisional ionization, excitation, de-excitation and radiative decay describe the population density. Two-dimensional PIC simulations have been carried out for laser pulses with peak intensity 1 × 1020 W cm−2, pulse duration 60 fs, spot size 3 µm and energy 0.75 J interacting with ultrathin (0.2 µm) Al foil. Radiation emitted during the laser–target interaction is computed by accounting for both bound–bound transitions and bremsstrahlung radiation. We demonstrate that the radiation signature of laser-produced plasma can be used as a complementary tool to other diagnostic techniques used in laser–plasma interactions. Finally, results from the PIC model are compared to equilibrium calculations (Maxwell–Boltzmann and Saha). In the early stages of laser–plasma interactions (<100 fs) the plasma is far from equilibrium and equilibrium models can not be applied with confidence to model the plasma.

Explicit schemes for time propagating many-body wave functions

Accurate theoretical data on many time-dependent processes in atomic and molecular physics and in chemistry require the direct numerical ab initio solution of the time-dependent Schr\"odinger equation, thereby motivating the development of very efficient time propagators. These usually involve the solution of very large systems of first-order differential equations that are characterized by a high degree of stiffness. In this contribution, we analyze and compare the performance of the explicit one-step algorithms of Fatunla and Arnoldi. Both algorithms have exactly the same stability function, therefore sharing the same stability properties that turn out to be optimum. Their respective accuracy, however, differs significantly and depends on the physical situation involved. In order to test this accuracy, we use a predictor-corrector scheme in which the predictor is either Fatunla's or Arnoldi's algorithm and the corrector, a fully implicit four-stage Radau IIA method of order 7. In this contribution, we consider two physical processes. The first one is the ionization of an atomic system by a short and intense electromagnetic pulse; the atomic systems include a one-dimensional Gaussian model potential as well as atomic hydrogen and helium, both in full dimensionality. The second process is the decoherence of two-electron quantum states when a time-independent perturbation is applied to a planar two-electron quantum dot where both electrons are confined in an anharmonic potential. Even though the Hamiltonian of this system is time independent the corresponding differential equation shows a striking stiffness which makes the time integration extremely difficult. In the case of the one-dimensional Gaussian potential we discuss in detail the possibility of monitoring the time step for both explicit algorithms. In the other physical situations that are much more demanding in term of computations, we show that the accuracy of both algorithms depends strongly on the degree of stiffness of the problem.

Kinetic theory and atomic physics corrections for determination of ion velocities from charge-exchange spectroscopy

Charge-exchange spectroscopy is a powerful diagnostic tool for determining ion temperatures, densities and rotational velocities in tokamak plasmas. This technique depends on detailed understanding of the atomic physics processes that affect the measured apparent velocities with respect to the true ion rotational velocities. These atomic effects are mainly due to energy dependence of the charge-exchange cross-sections, and in the case of poloidal velocities, due to gyro-motion of the ion during the finite lifetime of the excited states. Accurate lifetimes are necessary for correct interpretation of measured poloidal velocities, specially for high density plasma regimes on machines such as ITER, where l-mixing effects must be taken into account. In this work, a full nl-resolved atomic collisional radiative model coupled with a full kinetic calculation that includes the effects of electric and magnetic fields on the ion gyro-motion is presented for the first time. The model directly calculates from atomic physics first principles the excited state lifetimes that are necessary to evaluate the gyro-orbit effects. It is shown that even for low density plasmas where l-mixing effects are unimportant and coronal conditions can be assumed, the nl-resolved model is necessary for an accurate description of the gyro-motion effects to determine poloidal velocities. This solution shows good agreement when compared to three QH-mode shots on DIII-D, which contain a wide range of toroidal velocities and high ion temperatures where greater atomic corrections are needed. The velocities obtained from the model are compared to experimental velocities determined from co- and counter-injection of neutral beams on DIII-D.

Monte Carlo simulations of GeoPET experiments: 3D images of tracer distributions (18F, 124I and 58Co) in Opalinus clay, anhydrite and quartz

Understanding conservative fluid flow and reactive tracer transport in soils and rock formations requires quantitative transport visualization methods in 3D+t. After a decade of research and development we established the GeoPET as a non-destructive method with unrivalled sensitivity and selectivity, with due spatial and temporal resolution by applying Positron Emission Tomography (PET), a nuclear medicine imaging method, to dense rock material. Requirements for reaching the physical limit of image resolution of nearly 1mm are (a) a high-resolution PET-camera, like our ClearPET scanner (Raytest), and (b) appropriate correction methods for scatter and attenuation of 511keV—photons in the dense geological material. The latter are by far more significant in dense geological material than in human and small animal body tissue (water).Here we present data from Monte Carlo simulations (MCS) reflecting selected GeoPET experiments. The MCS consider all involved nuclear physical processes of the measurement with the ClearPET-system and allow us to quantify the sensitivity of the method and the scatter fractions in geological media as function of material (quartz, Opalinus clay and anhydrite compared to water), PET isotope (18F, 58Co and 124I), and geometric system parameters. The synthetic data sets obtained by MCS are the basis for detailed performance assessment studies allowing for image quality improvements. A scatter correction method is applied exemplarily by subtracting projections of simulated scattered coincidences from experimental data sets prior to image reconstruction with an iterative reconstruction process.

Influence of atomic processes on the implosion of plasma liners

The influence of atomic physics processes on the implosion of plasma liners for magneto-inertial nuclear fusion has been investigated numerically by using the method of front tracking in spherically symmetric geometry and equation of state models accounting for dissociation and ionization. Simulation studies of the self-collapse of argon liners to be used in the Los Alamos Plasma Liner Experiment (PLX) program have been performed as well as studies of implosion of deuterium and argon liners on plasma targets. Results show that atomic processes in converging liners reduce the temperature of liners and increase the Mach number that results in the increase of the stagnation pressure and the fusion energy gain. For deuterium and argon liners imploding on plasma targets, dissociation and ionization increased the stagnation pressure and the fusion energy gain by the factor of 1.5 (deuterium) and 2 (argon) correspondingly. Similarly, ionization during the self-collapse of argon liners leads to approximately doubling of the Mach number and the stagnation pressure. The influence of the longitudinal density spread of the liner has also been investigated. The self-collapse stagnation pressure decreased by the factor of 8.7 when the initial position of the liner was shifted from the merging radius (33 cm) to the PLX chamber edge (137.2 cm). Simulations with and without the heat conduction demonstrated that the heat conduction has negligible effect on the self-collapse pressure of argon liners.

Transport of Neutrals in Scrape‐Off Layer Plasmas: Autoregressive Beta Process to Describe Plasma Density and Temperature Fluctuations

AbstractImproving the description of the far scrape‐off layer (SOL) in current edge code suites such as SOLPS is the subject of current efforts, given the importance of main chamber plasma wall interactions (first wall recycling and sputtering) in the design and operation of tokamaks. One of the specific features of the far SOL is the large amplitude of turbulent fluctuations. Neutral particles (atom and molecules) are affected by these fluctuations through the rates of atomic physics processes. In accordance with experimentally measured PDF (probability density function) for both the plasma density and temperature fluctuations, ne and Te are modeled using a Beta PDF. Space and time correlations are accounted for by using an autogressive process. The effects of fluctuations on the neutral atoms penetration depth, as well as on the beryllium sputtering yield are discussed (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Determination of the effective atomic and mass numbers for mixture and compound materials in high energy photon interactions

In consideration the radiological properties of materials and studying the scattering processes in atomic and nuclear physics, the effective atomic and mass numbers is widely employed. These numbers have been calculated for any mixed or composite materials in interaction with high energy photons (Linac in radiation therapy). A pair equation in terms of these numbers is obtained. The first equation has been derived from the conservation of mass energy law and the second by minimizing the binding energy from the semiempirical mass formula (Myers and Swiatecki formula) that gives a relation between atomic and mass numbers for stable nuclei approximately. By these equations one can obtain the effective atomic and mass numbers for any compound or mixed materials uniquely. These numbers are calculated for some materials and compared with the other studies.

Nonregular phenomena in systems of quantum wells in the presence of tunneling processes

The effect of tunneling on inelastic processes in atomic collision physics has been studied using two examples: head-on collision between hydrogen atoms and protons and hydrogen-atom reflection from an impenetrable wall. These phenomena are analyzed using the numerical solution of the nonstationary Schrodinger equation.

Doorway states, the super-radiant mechanism and nuclear reactions

In the 1950s the possibility of forming a ''super-radiant'' (SR) state in a gas of atoms confined to a volume of a size smaller than the wave length of radiation was suggested by Dicke. The atoms, with two levels, are coupled through their common radiation field. This indirect coupling leads to a redistribution of lifetimes among unstable intrinsic states. A strongly decaying SR state is created at the expense of the rest of the states of the system. Recently the connection of this mechanism to the notion of doorway states in low-energy nuclear reactions, was pointed out. The conditions for appearance of such doorways in nuclear physics processes are discussed.

High-energy isolated attosecond pulses generated by above-saturation few-cycle fields

The applications of isolated attosecond pulses reported to date, which have demonstrated the great potential of attosecond technology in the investigation of ultrafast electronic processes, have been limited by the low photon flux of the available attosecond sources. We report on the generation of isolated sub-160-as pulses (at a photon energy of ∼30 eV) with a pulse energy, on target, of a few nanojoules. The efficient generation of isolated attosecond pulses in noble gases is produced by 5-fs driving pulses with controlled electric field and peak intensity beyond the gas saturation intensity. The availability of attosecond sources with high peak intensities has potential in opening new avenues for attosecond-pump/attosecond-probe studies of electronic processes in atomic and molecular physics, with interesting prospects in the field of coherent control of electronic motion in complex systems in the attosecond temporal regime. Researchers report the generation of isolated sub-160-attosecond pulses that have photon energies of 30 eV, resulting in an on-target pulse energy of a few nanojoules. The availability of attosecond sources with high peak intensities may open new avenues for attosecond pump/probe studies of electronic processes in atomic and molecular physics.

Doorway States and the Super-Radiant Mechanism in Nuclear Reactions

In the early 1950s the possibility of forming a super-radiant (SR) state in a gas of atoms confined to a volume of a size smaller than the wave length of radiation was suggested by Dicke. The atoms are coupled through their common radiation field. This indirect interaction through the continuum leads to a redistribution of lifetimes among unstable intrinsic states. A rapidly decaying SR state is created at the expense of the rest of the states of the system that are robbed of their decay probability and become narrow. This mechanism is general and analogous phenomena should appear in many quantum systems when quasi-bound states are strongly coupled through common decay channels. Recently this mechanism was considered for generating doorway states in low-energy nuclear reactions. A discussion of the conditions for appearance of doorways in nuclear physics processes will is presented.

Quantification of mercury in XVIII century books by Energy Dispersive X-Ray Fluorescence (EDXRF)

This work describes the quantitative analysis of mercury present in the ink used to colour some books of XVIII century. The mercury content was determined by Energy Dispersive X-ray Spectrometry. This is a non-destructive technique which allows elemental identification and quantification (Z > 13) by atomic physics processes. The organic pigments cannot be identified by this technique, taking into account that its composition is mainly C, O and H. Levels of 2 wt.% and 4.5 wt.% were measured in 1756 and 1753 books respectively. No significant amount of mercury was observed in other red books, on a total of 11, all from XVIII century: 1720, 1732, 1753, 1756, 1780, 1798, 1800. More than one book for each year were analysed. The studied books belong to a private collection, and were selected taking into account the age and the reddish colour of their external parts. High content on Fe were observed in some of the books. This work highlights the application of a physics technique in a very important aspect for art and cultural heritage conservation and restoration, considering that high levels of toxic elements might be found in ancient documents. It is of great importance that preliminary elemental analyses are performed on ancient documents before handling them, because they might constitute some danger for restorers, conservators and collectors. This work highlights, for the first time, the danger of some ancient books. They might contain a very high concentration of mercury, which is toxic for the organism. This is also a particularly important problem of public health never mentioned in literature before.

PROGRESS IN THE DESCRIPTION OF NUCLEAR PHYSICS FROM LATTICE QCD

It has been a natural desire for a long time to be able to describe nuclear physics in terms of the fundamental strong interaction. Recently some significant progress has been made in this area in terms of lattice QCD calculations of simple nuclear physics processes such as nucleon nucleon scattering. An attempt is made to introduce the progress made in this area, to an audience composed mainly of many-body theorists (non-lattice QCD and even non-particle/nuclear physics) interested in inter-disciplinary approaches.