Diamagnetic Drift Research Articles

Experiments during AC current transition in ISTTOK and the hypothesis of ballistic runaway electrons

The operation of the Instituto Superior Técnico Tokamak (ISTTOK) in the alternating current (AC) regime is characterized by the presence of a residual plasma density during the current transition, i.e. when the plasma current crosses the zero value. Contrary to what has been reported in other AC experiments, the pressure-like profiles obtained by the heavy ion beam diagnostic (the product of plasma density with a known function of plasma temperature) do not show evidence of the co-existence of two anti-parallel currents. This is also confirmed by the fact that in these experiments the plasma current and the V loop both cross the zero value at the same instant. The Murakami–Hugill plots reveal that during the transition the plasma current decays faster than the Hugill limit and that the subsequent ramp-up phase is sustained by runaway electron (RE) currents. It is hypothesized that during plasma current decay a population of REs can enter the following semi-cycle of the discharge. A trajectory simulation model including the forces at play shows that the retardation effect of the friction and molecular forces can cause the dampening of REs (within 20 μs) with energy below 1 keV inside the chamber (without external vertical correction magnetic fields), thus ionizing the background gas. Faster REs (above 1 keV) can also contribute to the ionization of the background gas (e.g. within 50 μs for 3 keV energy electrons) but require a vertical magnetic field to balance the gradB and curvature drifts. This could explain why tuning the vertical field and dosing the background pressure to obtain successful AC discharges is usually challenging.

Theory of gradient drift instabilities in low-temperature, partially magnetised plasmas

A fluid dispersion theory in partially magnetised plasmas is analysed to examine the conditions under which large-wavelength modes develop in Penning-type configurations, that is, where an electric field is imposed perpendicular to a homogeneous magnetic field. The fluid dispersion relation assuming a slab geometry shows that two types of low-frequency, gradient drift instabilities occur in the direction of the $\boldsymbol {E} \times \boldsymbol {B}$ and diamagnetic drifts. One type of instability, observed when the equilibrium electric field and plasma density gradient are in the same direction, is similar to the classic modified Simon–Hoh instability. A second instability is found for conditions in which (i) the diamagnetic drift is in the direction opposite to the $\boldsymbol {E} \times \boldsymbol {B}$ drift and (ii) the magnitude of the diamagnetic drift is sufficiently larger than the electron thermal speed. The present fluid dispersion theory suggests that the rotating spokes driven by such fluid instabilities propagate in the same direction as the diamagnetic drift, which can be in the same direction as or opposite to the $\boldsymbol {E} \times \boldsymbol {B}$ drift, depending on the plasma conditions. This finding may account for the observation, in some plasma devices, of the rotation of large-scale structures in both the $\boldsymbol {E} \times \boldsymbol {B}$ and $-\boldsymbol {E} \times \boldsymbol {B}$ directions.

Ion-acoustic instability of the cylindrical inhomogeneous helicon discharge plasma with rotating electrons

The kinetic theory for the microinstabilities of a cylindrical plasma, produced by the cylindrical azimuthally symmetric (azimuthal mode number m0=0) helicon wave, based on the Vlasov–Poisson system of equations, is developed. The derived linear integral equation for the Fourier–Bessel transform of the electrostatic potential is the basic equation for the investigations of the parametric and the current-driven instabilities of the radially inhomogeneous cylindrical plasma in the radially inhomogeneous helicon wave. The short-wavelength solution of this equation for the electrostatic potential is derived in the form of the functional equation, which includes an infinite number of its satellites at a frequency separation equal to the helicon wave frequency. The analytical solution is derived for the high-frequency kinetic ion-acoustic instability of the cylindrical helicon plasma, driven by the coupled effect of the electron diamagnetic drift and of the steady azimuthal rotation of electrons relative to the ions with a radially inhomogeneous angular velocity.

Hydrodynamically induced helical particle drift due to patterned surfaces

Advances in microfabrication enable the tailoring of surfaces to achieve optimal sorting, mixing, and focusing of complex particulate suspensions in microfluidic devices. Corrugated surfaces have proved to be a powerful tool to manipulate particle motion for a variety of applications, yet the fundamental physical mechanism underlying the hydrodynamic coupling of the suspended particles and surface topography has remained elusive. Here, we study the hydrodynamic interactions between sedimenting spherical particles and nearby corrugated surfaces, whose corrugations are tilted with respect to gravity. Our experiments show three-dimensional, helical particle trajectories with an overall drift along the corrugations, which agree quantitatively with our analytical perturbation theory. The theoretical predictions reveal that the interaction of the disturbance flows, induced by the particle motion, with the corrugations generates locally a transverse anisotropy of the pressure field, which explains the helical dynamics and particle drift. We demonstrate that this dynamical behavior is generic for various surface shapes, including rectangular, sinusoidal, and triangular corrugations, and we identify surface characteristics that produce an optimal particle drift. Our findings reveal a universal feature inherent to particle transport near patterned surfaces and provide fundamental insights for future microfluidic applications that aim to enhance the focusing or sorting of particulate suspensions.

Modeling study of divertor particle and heat flux asymmetries for EAST H-mode discharges

The BOUT++ transport code is run to study the effects of plasma drifts on the divertor out-in asymmetries (DOIAs) of particle and heat fluxes and their decay widths for EAST lower single null H-mode discharges. The diamagnetic drift seems to have no effects on the DOIAs of total particle and heat fluxes due to its divergence-free nature. However, it could significantly increase the DOIAs of peak particle and heat fluxes and the flux decay widths. The E × B drift is found to induce a large plasma flow to the divertor region, enhancing the DOIAs of both total and peak particle and heat fluxes and the flux decay widths. Both the radial and poloidal components of the E × B drift are necessary in increasing the DOIAs, however, the radial E × B drift seems to play a more important role. The effects on the DOIAs caused by both diamagnetic and E × B drifts are reversed with the reverse of toroidal magnetic field. The heat flux decay width λq and spreading width Sq are important physical and engineering parameters for the divertors and could be obtained by fitting the heat flux profiles at divertor targets. The λq at the outer target from the simulation case with all drifts could well match with the multi-machine scaling proposed by Eich and the DOIA of λq is in reasonable agreement with the scaling proposed by Goldston.

Detecting temporal correlations in hopping dynamics in Lennard–Jones liquids

Lennard–Jones mixtures represent one of the popular systems for the study of glass-forming liquids. Spatio/temporal heterogeneity and rare (activated) events are at the heart of the slow dynamics typical of these systems. Such slow dynamics is characterised by the development of a plateau in the mean-squared displacement (MSD) at intermediate times, accompanied by a non-Gaussianity in the displacement distribution identified by exponential tails. As pointed out by some recent works, the non-Gaussianity persists at times beyond the MSD plateau, leading to a Brownian yet non-Gaussian regime and thus highlighting once again the relevance of rare events in such systems. Single-particle motion of glass-forming liquids is usually interpreted as an alternation of rattling within the local cage and cage-escape motion and therefore can be described as a sequence of waiting times and jumps. In this work, by using a simple yet robust algorithm, we extract jumps and waiting times from single-particle trajectories obtained via molecular dynamics simulations. We investigate the presence of correlations between waiting times and find negative correlations, which becomes more and more pronounced when lowering the temperature.

Directed transport of a particle on a horizontal surface under asymmetric vibrations

Asymmetric unbiased fluctuations coupled with the effect of nonlinear dry (Coulombic) friction are observed to generate a directed transport even in the absence of any net external force or gradient. This phenomenon, called the ratchet effect, occurs when the driving force does not overcome the static friction threshold symmetrically. An elementary model is used to generate directed and unidirectional motion of a particle placed on top of a horizontal rigid surface which is given a zero mean spatially asymmetric motion through a multi-frequency in-plane excitation. We obtain steady-state periodic solutions of the nondimensional transport equations analytically to assess the distinct single/multiple sticking and sliding phases of particle motion and various parametric conditions for phase transitions. These analytical results are then validated by solutions obtained semi-analytically using Galerkin projection and numerically using the fourth-order Runge–Kutta method (RK4) and Newton–Raphson method with a fixed arc-length based continuation scheme. Also, a multibody dynamics model of this particle-on-plate system is built in MSC Adams to help us visualize the actual particle behavior under real dimensional system parameters and compare it with the foregoing analyses. A highly counter-intuitive phenomenon is encountered as the particle drift velocity increases non-monotonically with the increase in friction whereas a very high or a very low amount of friction impedes motion. Maximum transport velocity is found to be independent of the amplitude ratio of the vibratory input for low values of kinetic friction and is observed to manifest at a critical static friction threshold beyond which the particle motion becomes unidirectional. The effect of viscous drag of the ambient fluid on transport is assessed to be quite insignificant. Optimal vibratory input parameters to achieve the best transport current under varying friction conditions are determined. At low kinetic friction, a fixed set of system parameters is observed to generate distinct solution branches that coexisted just beyond the static friction threshold corresponding to a motion transition from “slip” to “stick–slip”.

Distribution of dissolved 137Cs, 131I and 238Pu at Eastern Mediterranean Sea in case of hypothetical accident at the Akkuyu Nuclear Power Plant

A coupled hydrodynamic/lagrangian particle drift model was applied to obtain the distribution of key radionuclides that are dispersed at the surface water (upper 100 m) of Eastern Mediterranean Sea, in case of an accident of Nuclear Power Plant at Akkuyu. The model was applied to simulate the dispersion of key artificial radionuclide concentration, in case of a hypothetical accident to assess issues related to sea health and potential hazards. The released radionuclide was found to have a favorable westward direction, following the prevailing currents. The variability of the predicted pattern is interpreted according to the near surface circulation patterns in the study area. The dispersion of key radionuclides (137Cs, 238Pu and 131I) was studied in a spatial and temporal manner at the neighboring coasts of the location of the NPP. Furthermore, the fate of the plume was predicted for each month after the accident date close to the areas of Akkuyu, namely Cyprus, Rhodes Island, North Cretan coast, Cyclades and Syrian Coast. The annual study revealed that the radioactive plume is present with high concentration (maximum values of 1200 Bq m−3 for the long lived isotopes and 600 Bq m−3 for 131I) in the location of the plant for short period of time (1–2 months), while the other areas are affected during the whole year with low level of activity concentration (20–50 Bqm−3). The ERICA Assessment Tool was also used to evaluate dose rates to common marine biota at the studied areas.

Polarization Attack on Continuous-Variable Quantum Key Distribution with a Local Local Oscillator.

The estimation of phase noise of continuous-variable quantum key distribution protocol with a local local oscillator (LLO CVQKD), as a major process in quantifying the secret key rate, is closely relevant to the intensity of the phase reference. However, the transmission of the phase reference through the insecure quantum channel is prone to be exploited by the eavesdropper (Eve) to mount attacks. Here, we introduce a polarization attack scheme against the phase reference. Presently, in a practical LLO CVQKD system, only part of the phase reference pulses are measured to compensate for the polarization drift of the quantum signal pulses in a compensation cycle due to the limited polarization measurement rate, while the other part of the phase reference pulses are not measured. We show that Eve can control the phase noise by manipulating the polarization direction of the unmeasured phase reference to hide her attack on the quantum signal. Simulations show that Eve can obtain partial or total key rates information shared between Alice and Bob as the transmission distance increases. Improving the polarization measurement rate to 100% or monitoring the phase reference intensity in real-time is of great importance to protect the LLO CVQKD from polarization attack.

On the interplay between interchange turbulence and sheared flows

The presence and the position of an X-point, namely, if the ion diamagnetic drift points toward it or not, strongly impact the edge plasma rotation in tokamaks. In the absence of kinetic effects such as magnetic ripple or ion orbit loss, the shape of the velocity profile results from the balance between neoclassical predictions and turbulent flow generation. In this contribution, we derive a reduced model of turbulence plunged in a shear flow. This model is based on (1) a description of the impact of a sheared flow on the interchange turbulence and (2) a prediction of the poloidal momentum generated by the turbulence. It includes the effects of both the magnetic topology and the finite shear layer width. The model is verified against 2D non-linear flux-driven simulations. Finally, the model predictions of the edge rotation resulting from the equilibrium between the neoclassical prediction and the poloidal momentum generation by the turbulence are invoked to describe the observations from experiment managed in the WEST tokamak. It points out the important role of the magnetic shear in the turbulence tilting and in the flow generation.

Radio Emission of Pulsars. I. Slow Tearing of a Quantizing Magnetic Field

The pulsed radio emission of rotating neutron stars is connected to slow tearing instabilities feeding off an inhomogeneous twist profile within the open circuit. This paper considers the stability of a weakly sheared, quantizing magnetic field in which the current is supported by a relativistic particle flow. The electromagnetic field is almost perfectly force free, and particles are confined to the lowest Landau state, experiencing no appreciable curvature drift. In a charge-neutral plasma, we find multiple branches of slowly growing tearing modes, relativistic analogs of the double-tearing mode, with peak growth rate . Here, B z is the strong (nearly potential) guide magnetic field, J z the field-aligned current density, and is the mode wavenumber normalized by the current gradient scale. These modes are overstable when the plasma carries a net charge, with the real frequency proportional to the imbalance in the densities of positive and negative charges. An isolated current sheet thinner than the skin depth supports localized tearing modes with growth rate scaling as (sheet thickness/skin depth)−1/2. In a pulsar, the peak growth rate is comparable to the angular frequency of rotation, , slow compared with the longitudinal oscillations of particles and fields in a polar gap. The tearing modes experience azimuthal drift reminiscent of subpulse drift and are a promising driver of pulse-to-pulse flux variations. A companion paper demonstrates a Cerenkov-like instability of current-carrying Alfvén waves in thin current sheets with relativistic particle flow and proposes coherent curvature emission by these waves as a source of pulsar radio emission.

The Electric Field outward of Saturn's Main Rings

Cassini data are consistent with a global electric field in Saturn's magnetosphere that points approximately antisunward. The inner radial extent of this field was initially established using Saturn orbit insertion data but measurements of ultrarelativistic electrons from that pass cast some doubt on whether the electric field reaches all the way to the A ring. It was not until the so-called ring-grazing and proximal orbits near the end of the mission in 2017 that relevant data were again obtained on magnetic field lines that connect to the region just outward of the main rings. Here we report on the energetic charged particle data during those orbits, showing that electron observations at a wide range of energies are consistent with an electric field that influences charged particle drift paths near the outer edge of the A ring. We include a very detailed analysis of Cassini's ultrarelativistic electron measurements (channel E7 in the text) and argue they provide no information about the electric field. This result further strengthens the case of several studies that have used the presence of the electric field to explain signatures of acceleration in the data.

Congregation of particles on a plane boundary due to the flow induced by an oscillating sphere

The complex flow-field associated with a spherical structure oscillating adjacent to a solid boundary is significant in ocean engineering. The dynamic interaction between the flow induced by an oscillating sphere and the boundary is evident not only in terms of flow-field deformation around the structures but also on the boundary in the form of shear stress modifications. The present study aims to understand the influence of a plane boundary on the shear stress distributions and particle motion near the boundary induced by the flow due to a submerged sphere oscillating in a viscous fluid. Experiments are conducted using a non-intrusive flow visualization technique, and three-dimensional direct numerical simulations are used to link the time-mean results of the experiments to the simulated Lagrangian particle drift on the plane boundary. The dye tracer in the experiments and the Lagrangian particles in the simulation congregate at the same location. This stagnation location coincides with a ring of zero mean shear stress centered on the point on the plane boundary through which the axis of oscillation of the sphere passes. This ring of zero mean stress is termed the congregation zone. The radius of the particle congregation zone is investigated as a function of the sphere oscillating amplitude, Reynolds number, and the distance between the center of the sphere and the plane boundary. Furthermore, a systematic power scaling law is established for the radius of the congregation zone from the control parameters.

The strong suppression of galactic cosmic rays reaching AU Mic b, c, and Prox Cen b

ABSTRACT The propagation of Galactic cosmic rays is well understood in the context of the Solar system but is poorly studied for M dwarf systems. Quantifying the flux of cosmic rays reaching exoplanets is important since cosmic rays are relevant in the context of life. Here, we calculate the Galactic cosmic ray fluxes in AU Mic and Prox Cen planetary systems. We propagate the Galactic cosmic rays using a 1D cosmic ray transport model. We find for Prox Cen b, AU Mic b, and AU Mic c that the Galactic cosmic ray fluxes are strongly suppressed and are lower than the fluxes reaching Earth. We include in our models, for the first time for a star other than the Sun, the effect of radial particle drift due to gradients and curvatures in the stellar magnetic field. For Prox Cen, we find that the inclusion of particle drift leads to less suppression of Galactic cosmic rays fluxes than when it is excluded from the model. In the case of AU Mic we explore two different wind environments, with a low and high stellar wind mass-loss rate. For AU Mic, the particle drift also leads to less suppression of the Galactic cosmic ray fluxes but it is only significant for the high mass-loss rate scenario. However, both wind scenarios for AU Mic suppress the Galactic cosmic rays strongly. Overall, careful modelling of stellar winds is needed to calculate the Galactic cosmic ray fluxes reaching exoplanets. The results found here can be used to interpret future exoplanet atmosphere observations and in atmospheric models.

Topological phonons in arrays of ultracold dipolar particles

The notion of topology in physical systems is associated with the existence of a nonlocal ordering that is insensitive to a large class of perturbations. This brings robustness to the behaviour of the system and can serve as a ground for developing new fault-tolerant applications. We discuss how to design and study a large variety of topology-related phenomena for phonon-like collective modes in arrays of ultracold polarized dipolar particles. These modes are coherently propagating vibrational excitations, corresponding to oscillations of particles around their equilibrium positions, which exist in the regime where long-range interactions dominate over single-particle motion. We demonstrate that such systems offer a distinct and versatile tool to investigate a wide range of topological effects in asingle experimental setupwith a chosen underlying crystal structure by simply controlling the anisotropy of the interactions via the orientation of the external polarizing field. Our results show that arrays of dipolar particles provide a promising unifying platform to investigate topological phenomena with phononic modes.

Active thermophoresis and diffusiophoresis

Thermophoresis and diffusiophoresis respectively refer to the directed drift of suspended particles in solutions with external thermal and chemical gradients, which have been widely used in the manipulation of mesoscopic particles. We here study a phoretic-like motion of a passive colloidal particle immersed in inhomogeneous active baths, where the thermal and chemical gradients are replaced separately by activity and concentration gradients of the active particles. By performing simulations, we show that the passive colloidal particle experiences phoretic-like forces that originate from its interactions with the inhomogeneous active fluid, and thus drifts along the gradient field, leading to an accumulation. The results are similar to the traditional phoretic effects occurring in passive colloidal suspensions, implying that the concepts of thermophoresis and diffusiophoresis could be generalized into active baths.

Drifts effect on the ordinary plasma wave and the existence of new spin dependent electron drift in the spin quantum plasma

Using a separated spin evolution quantum hydrodynamic model, we have derived generalized formula for drift effects of electrons in the inhomogeneous and nonuniform magnetized spin quantum plasma. We report, for the first time, the appearance of new spin dependent drift along with grad-B drift and diamagnetic drift. These results can be used to study the drift effects on various waves and instabilities. To check the applicability of our results, we have derived the dispersion relation of ordinary-mode (O-mode) incorporating electron spin, gradient in density, and magnetic field. We have studied various new features due to electron spin, perpendicular spin drifts, and grad B on the propagation of O-mode. Numerical analysis is presented for the choice of laboratory plasma parameters.

Temperature anisotropy governed electrical conductivity tensor in a steady state dipole plasma: Spatially resolved experiments and modeling

A generalization of electrical conductivity in a plasma confined in a dipole magnetic field, in the presence of temperature anisotropy is presented. The anisotropy governed by the magnetic field distribution is found to be significant in the strong field region, and has a considerable effect on Pedersen and longitudinal conductivity of electrons over Hall conductivity, whereas the effect of temperature anisotropy on Hall conductivity can be observed in the case of ions. The work reveals new features in the conductivity tensor arising due to the temperature anisotropy and bidirectional nature of the dipole field, by incorporating all possible particle drifts, which would be helpful to enhance the understanding of electrical conduction in both laboratory and space dipole plasmas.

Efficient dust radial drift around young intermediate-mass stars

The radial velocities and direct imaging observations of exoplanets have suggested that the frequency of giant planets may decrease for intermediate-mass stars (2.5 − 8 M⊙). The key mechanism that could hinder their formation remains unclear. From a theoretical point of view, planet formation around intermediate-mass stars may take place on longer timescales, which – coupled with fast migration and efficient photoevaporation – may prevent planetary formation in these environments. In this letter, we investigate the temporal evolution of the radial drift for dust particles in disks when stellar evolution is taken into account. We demonstrate that the particle drift velocity around intermediate-mass stars sharply increases after 1–2 Myr, potentially creating a difficult barrier to overcome in the first steps of planet formation. This high radial drift could explain the lack of disk detections around intermediate-mass stars older than 3–4 Myr, as opposed to low-mass stars (< 2.5 M⊙), where the drift may not be the most impactful factor for the disk evolution. Future high-resolution images of these disks can help us to explain why planets around intermediate-mass stars may be rare. In addition, we can explore whether the role of efficient dust radial drift does in fact hinder planet formation around intermediate-mass stars – or otherwise.

A generalized Hasegawa–Mima equation in curved magnetic fields

We derive a model equation describing electrostatic plasma turbulence in general (inhomogeneous and curved) magnetic fields by analysing the effect of curved geometry on the ion fluid polarization drift velocity. The derived nonlinear equation generalizes the Hasegawa–Mima equation governing drift wave turbulence in a straight homogeneous magnetic field, and may serve as a toy model for the description of turbulent plasmas. The equation is most appropriate for configurations with a small$\boldsymbol {E}\times \boldsymbol {B}$drift velocity divergence, or a mild spatial change in$\boldsymbol {E}\times \boldsymbol {B}$drift velocity. We identify the conserved energy of the system, and obtain conditions on magnetic field topology for conservation of generalized enstrophy. Through numerical examples, we further show how the curvature of the magnetic field reshapes self-organized steady turbulent states.