Presence Of Trapped Particles Research Articles

Quantum vortex reconnections mediated by trapped particles

Reconnections between quantum vortex filaments in presence of trapped particles are investigated using numerical simulations of the Gross--Pitaevskii equation. Particles are described with classical degrees of freedom and modeled as highly repulsive potentials which deplete the superfluid. First, the case of a vortex dipole with a single particle trapped inside one of the vortices is studied. It is shown that the reconnection takes place at the position of the particle as a consequence of the symmetry breaking induced by it. The separation rate between the reconnecting points is compatible with the known dynamics of quantum vortex reconnections and it is independent of the particle mass and size. After the reconnection, the particle is pushed away with a constant velocity and its trajectory is deflected because of the transverse momentum exchange with the vortex filaments. The momentum exchanges between the particle, the vortex, and a density pulse are characterized. Finally, the reconnection of two linked rings, each of them with several initially randomly distributed particles is studied. It is observed that generically, reconnections take place at the location of trapped particles. It is shown that reconnection dynamics is unaffected for light particles.

Influence of Environmental Humidity on the Wear and Friction of a Silica/Silicon Tribopair Lubricated with a Hydrophilic Ionic Liquid.

In this study, the tribological behavior of silica/silicon surfaces lubricated with the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM] EtSO4) was investigated. Tests were carried out in the presence of either humid air (45-55% relative humidity) or in a nitrogen atmosphere, and the results were compared with those obtained using pure water as a lubricant. The cross-sectional analysis of the contact area performed by focused-ion-beam scanning electron microscopy indicated the presence of cracks in the subsurface region, showing that brittle fracture contributed to wear. Sliding promoted the formation of a third body, the presence of which was indicated by optical and secondary electron microscopy. X-ray photoelectron spectroscopy showed that the third body was mostly composed of silicon oxides. The accumulation of the debris was controlled by the presence of water: in the presence of a nitrogen atmosphere, particles were trapped between the sliding surfaces, whereas in the case of humid air, the debris was progressively removed from the contact. Notably, the presence of trapped particles was associated with higher values of wear coefficients of both disks and pins. In addition, a lower roughness was observed along the direction of sliding in the case of water-containing ionic liquid. The observed trends in wear and the combined results of the various techniques, as well as the comparison with tests carried out in the presence of pure water, all point to the characteristic tribochemical reactions of water with silicon-based materials, namely, the formation of a sacrificial layer of hydrated oxide and the dissociative adsorption of water at crack tips of SiO2. In the absence of water, the lack of a tribochemical mechanism forming a sacrificial layer leads to a microfracture-dominated wear mechanism over the entire duration of the test, thus leading to more severe wear. The possible occurrence of stress-induced phase transformation of silicon during sliding is also discussed.

Studies of dust acoustic double layers in the presence of trapped particles

An analytical solution for time stationary weak double layers in an unmagnetized dusty plasma by using Sagdeev’s pseudopotential method is studied considering dusts with constant charge and mass. Here, the plasma consists of electrons, ions and negatively-charged micron-sized dust particles. From the study, expressions for the potential and the dust acoustic velocity of the double layer is determined.

Instabilities of a circularly polarized wave with trapped particles in an isotropic plasma

The structure and stability of a transverse electromagnetic wave propagating with a velocity lower than the speed of light in an unmagnetized plasma are considered. The stationary finite-amplitude wave is described by exact solutions to the Vlasov-Maxwell equations. However, unlike the well-known electrostatic analog, the Bernstein-Greene-Kruskal wave, the wave structure is determined to a large extent by the presence of trapped particles with a shear of transverse velocities, without which the existence of waves with a refraction index larger than unity is impossible. It is shown that the main origin of the wave instability is the longitudinal motion of trapped particles relative to the background plasma. Expressions for the growth rates in the main instability regimes are found under definite restrictions on the wave parameters.

Influence of particle trapping on the penetration of high-frequency electromagnetic field into a semibounded plasma

Penetration of a high-frequency (hf) electromagnetic field into a semibounded plasma in the presence of trapped particles is considered. It is shown that the trapping of particles by the hf field considerably changes the condition for its propagation. In the stationary case, with increasing the hf field amplitude at the boundary, the frequency above which the plasma is transparent continually decreases. But the trapping of particles was found to give a lower limit for this critical frequency, i.e., this lower limit goes to zero when there is no trapping continually as the incident field amplitude increases, meanwhile the length of penetration increases and approaches a finite limit, in contrast to the untrapped case, where it increases monotonically. Moreover, when the hf field amplitude at the boundary exceeds some critical value, the plasma will become transparent and the localized structure of the hf field will change to an oscillatory structure.

Coupling of ηi and trapped electron modes in plasmas with negative magnetic shear

In toroidal collisionless plasmas, the ion temperature gradient (ITG or ηi) and the trapped electron (TE) modes are shown to be weakly (strongly) coupled when both the temperature gradients and the driving mechanism of the trapped electrons are moderate to strong (weak but finite). In the regime of strong coupling, there is a single hybrid mode, unstable for all ηi in plasmas with positive magnetic shear. For the weak coupling case, two independent unstable modes, one in the ion and the other in the electron diamagnetic direction, are found to coexist. In either situation, the negative magnetic shear exerts a strong stabilizing influence; the stabilizing effect is considerably enhanced by the presence of trapped particles. It is predicted that for a given set of plasma parameters, it will be much harder to simultaneously excite the two modes in a plasma with negative shear. The results of this study are significant for tokamak experiments.

Resistive ballooning modes in an axisymmetric toroidal plasma with long mean free path

Tokamak devices normally operate at such high temperatures that the resistive fluid description is inappropriate. In particular, the collision frequency may be low enough for trapped particles to exist. However, on account of the high conductivity of such plasmas, one can identify two separate scale lengths when discussing resistive ballooning modes. By describing plasma motion on one of these, the connection length, in terms of kinetic theory the dynamics of trapped particles can be incorporated. On the resistive scale length, this leads to a description in terms of modified fluid equations in which trapped particle effects appear. The resulting equations are analyzed and the presence of trapped particles is found to modify the stability properties qualitatively.

On the existence of the scattering operator for the linear Boltzmann equation

Existence theorems are proven in a study of the scattering problem for the linear Boltzmann equation (transport equation), describing the motion of a cloud of nonself-interacting particles (neutrons) in phase space. Also Simon's weak coupling result is discussed, and a meaningful wave operator in the presence of trapped particles is defined and its existence proven. 7 references. (JFP)

Instabilities of finite-amplitude oscillations of a plasma in a magnetic field in the presence of trapped particles

A study is made of the instabilities of a plane monochromatic electrostatic wave of finite amplitude in a constant magnetic field due to particles trapped by the electric field of this wave under conditions of Cherenkov resonance or cyclotron resonances. If the particles are trapped near a centre on the phase plane with the transverse or longitudinal velocity and the helical phase as coordinates in the case n not=0 or on the phase plane with the longitudinal velocity and the helical phase as coordinates in the case n=0, and if the particles have a small spread with respect to the velocities and helical phase, there arise coherent instabilities excited by all the trapped particles. If the wave vector k of the unstable oscillations is not near k0, there arises an instability of the two-stream type. If k approximately=k0, there arises a parametric satellite instability. In both cases, the maximal growth rate is gamma max approximately=(n1/n0)1/3 Omega 0, where n1 is the number of trapped particles in unit volume and n0 is the density of the plasma.

Interaction of particles trapped in a magnetic field with coherent electrostatic waves

The study of effects of electrostatic waves on the motion of particles trapped in magnetic wells is of interest in relation to the problem of stationary plasma confinement in toroidal geometries, owing to the numerous consequences of the presence of trapped particles both for the equilibrium and the stability properties of the plasma. The physical effect that is investigated quantitatively is the possibility that a given particle, initially trapped in a static non homogeneous magnetic field, becomes captured by the field of an electrostatic wave and, if this is a wave travelling in the magnetic field direction, escapes from the magnetic mirror.

Transport and particle scattering due to toroidal plasma modes

Electrostatic modes are shown to be excited, in two-dimensional plasma equilibrium configurations, with frequency and parity characteristics such as to significantly affect the orbits of trapped ions by proper mode-particle resonances in regimes where the single-particle collision mean free paths are considerably longer than the periodic particle excursions. In particular, new modes driven by the ion temperature gradient are shown to become unstable in the presence of trapped particles under less restrictive conditions and with considerably larger transverse wavelengths than the ion temperature gradient driven modes arising in a one-dimensional equilibrium configuration. The resulting enhancement in the transport of ion thermal energy across the magnetic field is discussed.

Energy-balance equation and enhanced collisional plasma diffusion

An exact energy-balance equation is derived from the first-order moments of the ion and electron transport equations. In this case the particle loss of a stationary plasma through an isobaric surface is expressed as the sum of several terms each of which contains a specific force (E-field, collisional friction, anisotropic pressure including inertial terms). The equation provides a general frame of interpretation for any stationary diffusion process or loss mechanism of a plasma. In particular, it is shown that Galeev-Sagdeev diffusion in an axisymmetric torus cannot be explained by the friction term η j2 or the Joule term E⃗⋅J⃗, but comes about by pressure anisotropy. This result rests on the assumption that the perpendicular resistivity η⊥ is not altered in order of magnitude by the presence of trapped particles, an assumption that is plausible on account of recent work by Hinton and Oberman. For Tokamak-like parameters the effective pressure anisotropies required for explaining Galeev-Sagdeev diffusion are very small, of the order of 10−5 to 10−6.