Impurity Dynamics Research Articles

Investigation of Impurity Dynamics at GOL-3 Facility

Dynamics of light impurities at multimirror trap GOL-3 was studied using imaging spectroscopy diagnostics in VUV and visible spectral range. The results of impurity balance and transport simulation as well as comparison with measurements are presented in this paper. Concentration of impurities in plasma and diffusion coefficient are found out from the measurements. Suitability using VUV and XUV spectroscopy for measuring main plasma parameters is considered.

Hydrogen Release Through Metallic Surface: The Role of Sputtering and of the Impurity Dynamics

A metal surface at elevated temperatures in vacuum is usually covered by a monolayer of nonmetallic impurities. Such a monolayer reduces by many orders of magnitude the rate constant, kr, of the recombinative release of absorbed hydrogen. Due to that, D/T recycling slows down, and D/T retention and permeation rise, by orders of magnitude at the metal interaction with energetic hydrogen particles. In reality, impurity coverage is resulting from a dynamic balance between impurity removal by physical and chemical sputtering and impurity supply both from the metal bulk due to surface segregation and from the outside, e.g., due to reactions with residual gases, etc. The question arises how and in what way does such impurity dynamics affect kr?Nb containing a solute O impurity was taken as an example for our experiments using a combination of ion beam and permeation techniques. Under our ultra-high vacuum conditions, the surface segregation of solute impurity was the only mechanism to restore the monolayer coverage subject to sputtering by energetic hydrogen. It is found that at elevated temperatures, sputtering typically can neither remove the impurity monolayer nor even noticeably increase the concentration of impurity vacancies in the coverage as compared to its equilibrium value. As a result, the rate of hydrogen reemission cannot be noticeably increased by sputtering until the concentration of solute impurity is significantly decreased over a definite zone beneath the surface. An interesting point is that the depth of this zone turns out to be the larger, the greater the concentration of solute impurity; e.g., it may stretch to a few tenths of millimeter in Nb at an O concentration of a few tenths of atomic per cent. Thus it is the metal purity that is expected to be the key factor determining the reemission, retention and permeation at the metal interaction with energetic hydrogen at elevated temperatures.

Comparison of impurity generation and penetration models withspectroscopy for the Tore Supra ergodic divertor

Three-dimensional (3D) simulations of impurity dynamics in the vicinity ofa Tore Supra ergodic divertor neutralizer are presented and compared withspectroscopic observations for a medium density pulse in the high recyclingregime. The numerical tool used for the description of impurity generationand transport is a version of the Monte Carlo code BBQ, while the geometryof the magnetic field lines is calculated by use of the MASTOC code.Substantial quantitative consistency is found between the experimental dataand our simulations based on recent impurity generation and propagationmodels. For the given plasma conditions, physical sputtering induced byimpact from deuterium ions is found to be the dominant mechanism leading tothe carbon contamination of the edge plasma. Chemical sputtering releases acomparable number of carbon impurities from the surface interacting withthe plasma but those particles are rapidly re-deposited or pumped away. Wefind the role of friction of the impurities with the background plasma tobe of crucial importance for the determination of the impurity distributionclose to the neutralizer. We further find that a flow reversal of thebackground plasma takes place at the leading edge of the neutralizer with aflow velocity of a few per cent of the ion sound speed in the region thatis just clear of the edge.

Quantum impurity dynamics in two-dimensional antiferromagnets and superconductors

We present the universal theory of arbitrary, localized impurities in a confining paramagnetic state of two-dimensional antiferromagnets with global SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1 excitations, \ensuremath{\Delta}, is assumed to be significantly smaller than a typical nearest neighbor exchange. In the absence of impurities, it was argued in earlier work [Chubukov et al., Phys. Rev. B 49, 11 919 (1994)] that the low-temperature quantum dynamics is universally and completely determined by the values of \ensuremath{\Delta} and a spin-wave velocity c. Here we establish the remarkable fact that no additional parameters are necessary for an antiferromagnet with a dilute concentration of impurities, ${n}_{\mathrm{imp}}$---each impurity is completely characterized by a integer/half-odd-integer valued spin S which measures the net uncompensated Berry phase due to spin precession in its vicinity. We compute the impurity-induced damping of the spin-1 collective mode of the antiferromagnet: the damping occurs on an energy scale $\ensuremath{\Gamma}{=n}_{\mathrm{imp}}(\ensuremath{\Elzxh}{c)}^{2}/\ensuremath{\Delta},$ and we predict a universal, asymmetric line shape for the collective mode peak. We argue that, under suitable conditions, our results apply unchanged (or in some cases, with minor modifications) to d-wave superconductors, and compare them to recent neutron-scattering experiments on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ by Fong et al. [Phys. Rev. Lett. 82, 1939 (1999)]. We also describe the universal evolution of numerous measurable correlations as the host antiferromagnet undergoes a quantum phase transition to a N\'eel ordered state.

Impurity dynamics in binary van der Waals clusters created by pick-up

We present results from molecular dynamics simulations concerning the creation of binary van der Waals clusters under a very large range of possible experimental pick-up conditions. Special emphasis is put on the dynamical processes occurring during and after the “pick-up” of Ne, SiF4, Kr, and Xe by Arn clusters with n ranging from 53 to 5000 atoms. Both Ne and SiF4 impurities are shown to normally reside in cluster surface states. For certain experimental conditions, however, both dopants may present transient matrix states. Matrix states are found to be the most probable final locations for Kr and Xe dopants under all considered conditions. We show that the dopant penetration depth crucially depends on cluster size, cluster velocity, nature of the dopant, and buffer gas pressure and that the final result is not always predictable from simple equilibrium considerations.

Impurity dynamics in boson quantum films

We calculate the binding energy and dynamic properties of $^{3}\mathrm{He}$ atoms in films of $^{4}\mathrm{He}$ absorbed to graphite and several alkali metal substrates. Without adjustable parameters, we obtain excellent agreement with the experimental binding energies for the ground state of the $^{3}\mathrm{He}$ impurity. We then introduce a time-dependent variational wave function to calculate dynamic properties of the impurity. The motion of the impurity atom parallel to the surface is characterized by an effective mass due to hydrodynamic backflow. We also obtain the spectrum of excitation energies and the lifetime of the excited states. When both lifetime and backflow effects are included, the energy of the first excited state of the impurity and the effective mass of the ground state, agree well with experimental data.

A self-consistent analysis of the impurity dynamics on the FTU tokamak

A self-consistent analysis of the impurity dynamics for bulk and scrape-off layer (SOL) plasmas in the Frascati Tokamak Upgrade (FTU) is presented. The model is based on a O-D description of the plasma transport in the tokamak centre and on a l-D model of the SOL plasma dynamics. The FTU configuration with a poloidal Inconel limiter is considered. Preliminary calculations for the configuration with the molybdenum and tungsten limiter plates are also presented. A comparison with the experimental data shows good agreement with the results of the model

Impurity dynamics of boson liquid films

The dynamic properties of3He impurity atoms in4He films are calculated using a dynamic, microscopic variational theory. Our results for the binding energies, excitations energies, and effective masses on strong substrates agree well with experimental data of the Amherst group. No substrate bound states are found for substrates as weak as Na. The theory also provides an unambiguous description of atom scattering and sticking. The dependence of the sticking and scattering coefficients on the substrate and the excitations in the film have been calculated.

NMR Study of Impurity Electronic Structure and Dynamics

NMR Study of Impurity Electronic Structure and Dynamics

Study of impurity retention in the scrape-off layer of the FTU tokamak

A new approach to obtaining information about the impurity dynamics along the open field lines in the scrape-off layer of a tokamak plasma is described. This technique exploits a movable metallic target that allows the study of the impurity influx from the target and of the impurity content in the main plasma as a function of the plasma parameters at the radial location of the target. A simple 1-D model is presented, which is able to reproduce the observed relationship between those two quantities in these experiments in FTU. The results obtained indicate that the friction and electrostatic forces associated with the plasma flow to the target dominate the impurity dynamics along open field lines. The influence of the connection length on the screening of the impurities, predicted by the model, has been verified experimentally

Nuclear probes for investigating radiation damage

Atomic nuclei or elementary particles, produced by particle accelerators or nuclear reactors, are employed as nuclear probes, or so-called observer atoms, which provide information about their local surroundings on an atomic scale. The study of static and dynamic properties of defects with these radioactive probe atoms is performed via the hyperfine interactions. They occur between the nuclear electromagnetic moments of the probe atoms and the electromagnetic fields in their surroundings. From the interaction parameters the desired information on the solid state with reference to the probe atom is obtained. For example the structural arrangement and chemical nature of the neighboring atoms, the local magnetic and electric fields, the magnetic state of the probe atom, the binding inside the host matrix, the vibrational state and diffusion properties, and, in particular, information on the existence of intrinsic defects, i.e. self-interstitials and lattice vacancies, and extrinsic defects, i.e. impurity atoms at substitutional or interstitial lattice sites. Thereby, in most cases, information on defect complexes is obtained in which the radioactive probe atom is a constituent. The main emphasis is laid on the results of two probe techniques, Mössbauer effect and PAC/PAD. In addition a brief survey of other techniques, such as β-NMR, muon-spin rotation and blocking using radioactive emitter atoms, is given. Microscopic details of defect centres and impurity dynamics are discussed, with attention shifting from metallic systems and their annealing stages to defect centers in semiconductors.

Low-temperature properties of anisotropic superconductors with kondo impurities

We present a self-consistent theory of superconductors in the presence of Kondo impurities, using large-$N$ slave-boson methods to treat the impurity dynamics. The technique is tested on the s-wave case and shown to give good results compared to other methods for $T_K > T_c$. We calculate low temperature thermodynamic and transport properties for various superconducting states, including isotropic s-wave and representative anisotropic model states with line and point nodes on the Fermi surface.

Dynamics of passively advected impurities in simple two-dimensional flow models

The motion of passively advected impurities with density ρp different from the fluid density ρf in simple models of two-dimensional velocity fields is studied. The impurity dynamics strongly depends on the parameter δ=ρf/ρp. For a stationary streamfunction, impurities that are lighter than the fluid undergo regular motions and converge to the centers of the advection cells. Particles denser than the fluid exhibit chaotic trajectories and standard diffusion at large times. For isotropic velocity fields, the diffusion coefficients display a scaling dependence upon the parameter ε=1−δ. For heavy impurities in weakly anisotropic velocity fields, the diffusion coefficients in the x and y directions may be different by several orders of magnitude. These features bear several resemblances to the motion of fluid particles in the presence of additive noise. For a time-periodic streamfunction, the heavy particles undergo chaotic trajectories and standard diffusion; light particles may either display chaotic behavior and diffusion or regular motions ending with periodic trajectories, depending upon the values of the control parameters of the Eulerian flow. At intermediate times, the particle distribution displays complex caustic structures. Except that at very short times, the distribution of advected impurities trace neither the density nor the velocity of the advecting flow, even for δ only slightly different from one.

Interaction between tunneling impurities in metals.

We study the effective interaction between impurities which can tunnel separately between two or more wells and which couple via Coulomb interaction to the conduction electrons of the host metal. The effect of the electronic environment on the impurity dynamics is analyzed by means of a renormalization-group method based on a path-integral formulation of the problem. In the tight-binding limit, the most important paths are those where the system stays most of the time in a given configuration with occasional tunneling processes. The intrinsic asymmetry between configurations arising from the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) --type interactions and the nonquadratic spatial dependence of the long-time retarded coupling creates the need for a generalized scaling scheme which, when applied to the formally similar problem of a spin chain with long-range 1/${r}^{2}$ interactions of arbitrary spin dependence, can account for the presence of symmetry-breaking terms. To understand the interplay between energy bias and hopping energy, an analysis is provided of the time scales for a particle in an asymmetric double well. For the double-impurity case, a crossover is found between a high-temperature or large-separation regime, where impurities behave as independent, and a low-temperature and short-distance regime, where correlated tunneling is established between the most favorable configurations. In this regime the dynamics becomes very sensitive to the geometry and localization effects tend to be enhanced. For many-impurity systems, it is proposed that, at low temperatures and high concentrations, oscillatory long-range interactions and renormalization by low-energy electronic excitations conspire to reduce or even totally suppress the tunneling activity.

Thermally driven convective cells and tokamak edge turbulence

A unified theory for the dynamics of thermally driven convective cell turbulence is presented. The cells are excited by the combined effects of radiative cooling and resistivity gradient drive. The model also includes impurity dynamics. Parallel thermal and impurity flows enhanced by turbulent radial diffusion regulate and saturate overlapping cells, even in regimes dominated by thermal instability. Transport coefficients and fluctuation levels characteristic of the saturated turbulence are calculated. It is found that the impurity radiation increases transport coefficients for high density plasmas, while the parallel conduction damping, elevated by radial diffusion, in turn quenches the thermal instability. The enhancement due to radiative cooling provides a resolution to the dilemma of explaining the experimental observation that potential fluctuations exceed density fluctuations in the edge plasma (eΦ/Te >n/n0).

Role of impurity dynamics in resistivity-gradient-driven turbulence and tokamak edge plasma phenomena

The role of impurity dynamics in resistivity-gradient-driven turbulence is investigated in the context of modeling tokamak edge plasma phenomena. The effects of impurity concentration fluctuations and gradients on the linear behavior of rippling instabilities and on the nonlinear evolution and saturation of resistivity-gradient-driven turbulence are studied both analytically and computationally. At saturation, fluctuation levels and particle and thermal diffusivities are calculated. In particular, the mean-square turbulent radial velocity is given by 〈v̂2r〉 =(E0Ls/Bz)2 (L−1η+L−1z)2. Thus edged peaked impurity concentrations tend to enhance the turbulence, while axially peaked concentrations tend to quench it. The theoretical predictions are in semiquantitative agreement with experimental results from the TEXT [Bull. Am. Phys. Soc. 30, 1443 (1985)], Caltech [Phys. Fluids 29, 309 (1986)], and Tosca [in the Proceedings of the 12th European Conference on Controlled Fusion and Plasma Physics (European Physical Society, Budapest, 1985), Vol. I, p. 311] tokamaks. Finally, a theory of the density clamp observed during CO-NBI on the ISX-B tokamak [in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1981), Vol. I, p. 377] is proposed.

Electron-localisation effects on impurity dynamics. II. Two-level systems

For pt.I see ibid., vol.18, p.3739 (1985). The scattering of electrons on two-level systems (TLS) in a disordered medium is investigated. The perturbative series obtained is summed up in leading logarithmic order. It is found, that localisation effects increase TK, the crossover temperature. Furthermore TK shows a maximum as a function of disorder. For a certain range of parameters a stronger than logarithmic dependence is obtained ( delta rho d=2 approximately T-1, delta rho d=3 approximately T-12/).

Impurity Dynamics of the XY‐Model

The time dependent expectation value of a magnetic perturbation of the XY‐Hamiltonian is computed exactly. Thermalization information is obtained.

Impurity dynamics in a one-dimensional chain

The behavior of an impurity spin in a one-dimensional chain is investigated using the Glauber model. Two different types of impurities are considered and exact expressions for the average spin of the impurity, given that the impurity was initially excited out of equilibrium, are found. The behavior of these models is discussed in detail and their relevance to other physical situations is considered.

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T c and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above T c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.