Magnetic Sheath Research Articles

Investigation of collision effects on ion dynamics in the presheath and sheath of weakly collisional and magnetized hydrogen plasmas

The effect of collisions on the motion of magnetized ions in sheath and presheath plasma regions was investigated through the measurement of ion incident angle of a hydrogen ion at a graphite surface. The experiment was conducted in hydrogen and deuterium plasmas where the ion mean free path is 5–10 times larger than the ion gyro radius and with varying magnetic field angle ψ from 0° to 90° normal to the target surface. The hydrogen ions actively reacted with carbon, leading to the formation of conical tips with axes directed along the incident ion flow direction. The ion incident angle was measured from the etched graphite images taken by scanning electron microscopy. The measured angles were compared to those calculated using Ahedo's fluid magnetic sheath model. In addition, we adopted the nominal Bohm criterion at the electrostatic sheath edge due to the larger ion gyro radius than the sheath. The results show that the ion incident angle was inclined to the normal direction with respect to the magnetic field angle because of the effect of ion collisions on ion motion in the presheath. The collisional effect on the ion motion is drastic for an oblique magnetic field angle ψ > 85°. This study demonstrates that the collisional property of the ions is crucial to guide the ion motion in magnetic (pre)sheath and to determine the ion incidence angle at the surface, even in collisionless and weakly magnetized plasmas.

Effect of the superthermal electrons on the heat flux through a magnetized sheath

A one-dimensional model, in which the ionization and collision are omitted in the Debye sheath region, is applied to investigate the effect of the superthermal electrons on the heat flux through a magnetized sheath. For different temperatures and concentrations of the superthermal electron, and magnitudes and directions of the magnetic field, the profiles of plasma parameters including ion density and flow velocity perpendicular to the wall, the background and superthermal electron densities, and sheath potential in the presheath region are calculated. The variation of the plasma density and sheath potential drop at the Debye sheath entrance with the superthermal electrons and magnetic field modifies the particle and heat fluxes across the Debye sheath to the material surface. The sheath heat transmission coefficient can increase significantly even for a very small superthermal electron population. The dependence of the sheath heat transmission coefficient on the magnetic field angle decreases with the contribution of the superthermal electron in a strong magnetized sheath. When investigation of the heat flux including the superthermal electrons to a water-cooled W/Cu monoblack for the tokamak divertor, compared to the case of without superthermal electrons, it is found that the increase in both heat flux to the material surface and surface temperature of the material is mainly due to the enhancement of the sheath potential drop caused by the superthermal electrons, but the increase in the latter is not as pronounced as the former.

Field-induced interfacial instabilities in a two-phase ferrofluid flow

Forming a stable ferrofluid and non-magnetic interface in a microchannel is essential for biomedical applications with two miscible aqueous solutions. Here, we report the interface profile and its instability of aqueous ferrofluid and water co-flow in a microchannel under the influence of a magnetic field. The scaling law related to diffusion width and dimensionless Pe number is introduced by solving the momentum and diffusion equation and combining the experimental results. It is found that the dimensionless number of diffusion width δ/h ∝ X/Peα, when core flow concentration cf, residual magnetic flux density Br, sheath flow and core flow ratio Qw/Qf, and core flow velocity Qf are changed, α is 0.633–1.071, α ∝ Qβ, β = −0.059,0.147, 0.019 and −0.101, respectively. Altering the core flow velocity and magnetic field intensity can quickly impact the interface's stability by improving the transport efficiency by 32.9 % and counteracting the impacts of material diffusion by 51.8 %, respectively. Elevating the proportion of sheath flow to core flow and magnetic field will impede the diffusion of core flow. This will result in a drop of 11.9 % in the magnetic convection effect and a 20.8 % counteraction of material diffusion effects.

Multi-ion magnetized sheath properties with non-extensive electron distribution

Magnetized plasma sheath plays an important role in semiconductor processing, material surface modification, film deposition, etc. In plasma experiments and discharge applications, multi-ion plasma consisting of more than two kinds of ions often exists. For a long range interacting plasma system, non-Maxwellian electrons can be described by the non-extensive distribution of Tsallis. In this work, a fluid model with one-dimensional spatial coordinates and three-dimensional velocity coordinates is established for the multi-ion plasma sheath. It is assumed that the electron velocity in the sheath follows a non-extensive distribution, and the background helium ions and different kinds of impurity ions are magnetized in a magnetic field with a certain tilt angle. The effects of non-extensive parameters, impurity ions and oblique magnetic field on the number density, velocity, wall potential and kinetic energy of ions in the multi-ion magnetic sheath are investigated by numerical simulation. The results show that in the helium-hydrogen or helium-argon mixed plasma sheath, the ionic velocity along the vertical wall direction decreases with the increase of the non-extensive parameters, the number density of ions and electrons in the sheath, the sheath thickness , and the kinetic energy of ions at the wall decrease. When the concentration of impurity ions increases, the kinetic energy of ions on the wall is independent of the type of ions. With the increase of magnetic field intensity, the number density of helium ions and the velocity along the vertical wall fluctuate along the sheath edge, and the fluctuation amplitude increases with the decrease of non-extensive parameters, while the heavy ions have no obvious fluctuation. In addition, the effects of the types and concentrations of impurity ions on the related properties of the sheath are also analyzed. With the increase of the magnetic field intensity, the number density and the velocity along the vertical wall direction fluctuate at the sheath edge, and the fluctuation amplitude increases with the decrease of the non-extensive parameter, whereas there are no significant fluctuations for heavy ions. In addition, when impurity ions are heavy ions, the absolute value of wall potential increases with the increase of impurity ion concentration and the decrease of non-extensibility parameters, and the kinetic energy of background ions increases at the wall surface. When the impurity ion is a light ion, the absolute value of the wall potential decreases with the increase of the impurity ion concentration and the decrease of the non-extensibility parameter, and the kinetic energy of the background ion at the wall decreases.

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

Fundamental mechanisms governing the erosion and prompt re-deposition of tungsten impurities in tokamak divertors are identified and analyzed to inform the lifetime of tungsten plasma-facing components in ITER and other future devices. Various experiments conducted at DIII-D to benchmark predictive models are presented, leveraging the DiMES removable sample exposure probe capability and the Metal Rings Campaign, in which toroidally symmetric rows of tungsten-coated tiles were installed in the DIII-D divertor. In tokamak divertors, the width of the electric sheath is of the order of the main ion Larmor radius, and a vast majority of sputtered tungsten impurities are typically ionized within the sheath. Therefore, W prompt redeposition is mainly governed by the ratio of the characteristic ionization mean-free path of neutral tungsten to the width of the sheath. In-situ monitoring of the prompt redeposition of tungsten impurities in divertors is demonstrated via the use of WII/WI line ratios and the ionizations/photon (S/XB) method in L-mode discharges. Even with this relatively limited set of emission measurements, net erosion measurements were found to be a consistent upper bound to an analytic scaling based on the ratio of the W ionization length, and the width of the magnetic sheath rather than the ratio of and the W+ gyro-radius. In the far-scrape-off layer (SOL) of the ITER divertor, however, it is calculated that the measurement of photon emissions associated with the ionization of tungsten impurities up to may be required. Finally, W deposition patterns on DiMES collector probes, interpreted via DIVIMP-WallDYN modelling, reveal the key roles of progressive W erosion/re-deposition staps and E × B drifts in regulating long-range high-Z material migration.

Ic measurement of twisted multifilamentary MgB2 wires with non-magnetic sheath over a wide range of temperatures and fields

All-superconducting rotating machines have the potential for meeting the high power density and high efficiency required for electrical aircraft applications. However, very high AC loss encountered in superconducting armature windings could hinder their development. Multifilamentary MgB2 wires are one of the promising candidates for the stator windings, due to their potentially low AC loss properties with small filament size and twist pitches. As the first step, the dependence of critical current and n-value on magnetic fields and temperatures Ic(B, T) and n(B,T), which are basic input parameters for AC loss simulation, needs to be measured. In this work, we present transport Ic measurements in three non-magnetic multifilamentary MgB2 wires (MgB2/Nb/CuNi/CuZn): one large wire with a 0.70 mm diameter and 25 mm twist pitch, and two small wires with a 0.48 mm diameter each and a 10 mm and 30 mm twist pitch respectively. A four-probe direct current method is used to measure Ic of the MgB2 wires with variations in temperature (15 – 35 K) and magnetic field (0 – 5.5 T). Full Ic data for the small wire with 10 mm twist pitch was obtained, and the n-values were mostly less than 20. While the Ic data for the large wire at low fields was more limited due to heating, the n-values were higher and could be up to around 100. The difference is attributed to the different filament sizes. Experiments also found that there is no significant hysteresis in the transport critical current measured by decreasing or increasing magnetic fields due to the non-magnetic sheaths. This non-hysteretic characteristic is critical for lowering AC loss because the additional losses from magnetic sheaths can be eliminated. From the magnetic-field dependence of critical current density, an empirical expression has been developed that provides suitable extrapolations to lower fields for the large wire.

Load Carrying Capacity Enhancing Design and Lubrication Investigation of the Magnetic-Water Double Suspension Elastic Support Thrust Bearing

Aiming at the problem that the traditional water-lubricated bearing cannot carry the heavy load and adapt to the constantly changing operating conditions for the high-power Rim Driven Thruster (RDT), the principle structure of the Magnetic Water-double-suspension Elastic-support Thrust Bearing (MWETB) is designed and the optimal structure parameters of the bearing are selected using simulation. To demonstrate the reliability of the MWETB under the RDTs’ actual working conditions, performance tests, which include the magnetic flux density, magnetic force, and lubrication performance, are carried out. The simulation and experimental results indicate that the optimal offset ratios are in two intervals, and the magnetic alignment and sheath materials have a great effect on the load reduction. The load-carrying force has obvious zoning characteristics with the change in bearing clearance. Besides, compared with the water-lubricated thrust bearings, the MWETB has advantages in terms of minimum film thickness and friction coefficient.

Study of a collisionless magnetized plasma sheath with nonextensively distributed species

A weakly magnetized sheath for a collisionless, electronegative plasma comprising positive ions, electrons, and negative ions is investigated numerically using the fluid approach. The electrons are considered to be non-Maxwellian in nature and are described by Tsalli’s distribution. Such electrons have a substantial effect on the sheath properties. The study also reveals that non-Maxwellian distribution is the most realistic description for negative ions in the presence of an oblique magnetic field. In addition to the negative ion temperature, the sheath potential is also affected by the nonextensive parameters. The present research finds application in the plasma processing and semiconductor industry as well as in space plasmas.

Time-dependent collisional radiative modeling of tungsten in the magnetic sheath for erosion diagnosis

Abstract Tungsten is the material of choice for the divertors in ITER, SPARC and future fusion reactors. Accurate diagnosis of tungsten erosion and migration is important for first wall life time, slag production and core performance. The addition of a magnetic presheath requires time-dependent collisional radiative effects to be included for accurate neutral tungsten collisional radiative modeling. Gross erosion measurements could be modified by a factor of 10 due to the inclusion of time-dependent effects for ITER relevant divertor conditions. A simple sputtering model and sheath density model are developed to investigate time-dependent collisional radiative effects. Neutral tungsten spectral lines populated from different metastable levels depend on model parameters leading to potential spectroscopic diagnostics of plasma parameters. Electron temperatures inferred from spectroscopic line ratios are in agreement with Langmuir probe measurements in the Compact Toroidal Hybrid.

Two-dimensional collisional particle model of the divertor sheath with electron emissive walls

A novel two-dimensional particle-in-cell (PIC) code, named Divertor Edge Simulator of Plasma-wall Interaction with Consistent COllisions (DESPICCO) and developed at CNR-ISTP, is capable of simulating the thin plasma layer of several millimeters, adjacent to the divertor tiles of a Tokamak fusion reactor. Here, kinetic effects and non-neutral plasma physics in the Debye sheath can be self-consistently captured by the PIC approach. The code is firstly benchmarked against literature one-dimensional codes and additional theoretical predictions for a magnetized sheath. Then, it is applied to a realistic divertor scenario featuring an attached plasma with monoblocks (MBs) radial misalignment and gaps, to compute the energy flux amplification factor at the exposed MB edge. A non-ambipolar local current density close to the leading edge and an average sheath heat transmission coefficient larger than the one predicted by classical sheath theory, are found. The effects of electron wall emission and plasma-gas collisions on the ion Mach number and on particle and energy fluxes to the walls are finally estimated to determine future guidelines for simulations. Ion collisions with recycled neutrals and both secondary and thermionic electron emission from the wall are found to have a relevant impact, with the overall effect of reducing by 25% the average ion impact energy, and by 15%–20% the total heavy particles energy flux to the walls, with relevant implications on the divertor wall erosion.

Propagation Characteristics of Terahertz Waves in Magnetized Dusty Plasma

The propagation characteristics, including reflectivity, transmissivity, and absorptivity, of terahertz (THz) waves in magnetized dusty plasmas sheath are investigated theoretically by the analytic method. Both the right-and left-hand circularly polarized (RCP and LCP) waves are considered. The calculation results illustrate that external magnetic field, plasma slab thickness, electron collision frequency, dust particles, and electron relaxation rate can affect the propagation characteristics of THz waves in plasmas. When dust particles are absent, a strong external magnetic field will cause the transmissivity of RCP waves to produce a transmissivity valley at the lower frequency band. Due to the action of external magnetic field and dust particles, a transmissivity peak appears at the lower frequency band for RCP waves. Increasing the density or radius of dust particles can cause an increase in transmissivity peak. For LCP waves, the transmissivity increases monotonously as the frequency of THz wave increases, whether the dust particles exist or not. These simulation results may provide a significant theoretical basis for solving the problem of “blackout.”

Simulation of effect of thermionic emission on magnetized sheath near target plate of tungsten divertor

The high confinement mode (H-mode) is a preferred operation mode of tokamak devices in the future, but the burst of edge localized mode (ELM) will sharply increase the heat load deposited on the divertor target, raising the target temperature rapidly and strengthening surface thermionic emission. In this paper, a one-dimensional fluid model is used to simulate the influence of thermionic emission on the characteristics of the magnetized sheath. The results show that the amplitude of float potential and the electric field strength both decrease under the action of thermionic emission. Plenty of thermionic emission electrons leave the target, resulting in a region with negative charge density near the target plate, and the magnetized sheath is divided into two parts: ion sheath and electron sheath. In the electron sheath, with the rise of the target surface temperature, electrons accumulated in front of the target also increase, the potential distribution is non-monotonic, and a “virtual cathode” structure appears. The reverse electric field formed near the target will confine the thermionic emission electrons leaving the target and slow down the ion movement, leading to a decrease of the ion energy deposited on the target. With the increase of the angle between the magnetic field and the target normal, the potential of the magnetized sheath and the proportion of the thickness of the electron sheath in the magnetized sheath both increase. The virtual cathode potential decreases, the temperature of the target required to form the virtual cathode rises.

Magnetic nanocomposite fabrication using banana leaf sheath Biofluid: Enhanced Fenton catalytic activity towards tetracycline degradation

Impregnation of banana leaf sheath biomass along with its biofluid helps to produce a magnetite nanocomposite of better magnetic strength and tetracycline degradation. • A simple, novel and one-pot fabrication of metal oxide@carbon nanocomposite is demonstrated by utilizing biofluids in biomasses as capping agents. • Capping of ferric ions by biofluid held in banana leaf sheath enhanced their uptake and distribution in the host carbonaceous precursor and assisted fabrication of iron oxide nanoparticles loaded carbon of high magnetic saturation. • Impregnation of iron ions with dry biomass is not as effective as with the fresh one, and subsequent carbonization produced carbon of lesser magnetic strength. • Iron oxide nanoparticles supported on activated carbons are used as heterogeneous Fenton catalysts for the degradation of tetracycline antibiotic. Utilization of biofluids available in plant biomass for uniform distribution of metal cation into carbonaceous precursor, exemplified by that of ferric into banana leaf sheath, followed by carbonization is attempted for the first time in this work. Magnetic iron-oxide nanoparticles incorporated carbon composite prepared by this method (MBLF: Magnetic Banana Leaf sheath carbon-Fresh) is of higher magnetic saturation (26.12 emu. g −1 ), but a similar procedure with dry biomass (MBLD: Magnetic Banana Leaf sheath carbon-Dry) yielded a composite of lesser magnetic strength (12.53 emu. g −1 ). The composites were characterized by FT-IR, N 2 -adsorption, VSM, XRD, PES and TGA techniques and used as heterogeneous catalysts for the activation of hydrogen peroxide (HP) towards degradation of tetracycline (TC) antibiotic. Higher iron content, which is found to be mainly in the form of Fe 3 O 4 , reflected in higher pseudo-first order rate constant for MBLF than MBLD. Oxidations of TC mainly occurred through hydroxy radicals with both catalysts and significant activity is retained after three cycles of operations.

Study on the characteristics of non-Maxwellian magnetized sheath in Hall thruster acceleration region

The secondary electron emission (SEE) and inclined magnetic field are typical features at the channel wall of the Hall thruster acceleration region (AR), and the characteristics of the magnetized sheath have a significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this work, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with SEE in the AR of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized. Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameter and magnetic field on the AR sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly, the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion. Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the direction acting as a resistance decelerating ions becomes larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential, secondary electron number density and sheath thickness.

Rhythmia zero-fluoroscopy workflow with high-power, short-duration ablation: retrospective analysis of procedural data

BackgroundFluoroscopy is commonly used during atrial fibrillation (AF) ablation to guide catheter navigation and placement. Technology improvements have significantly reduced fluoroscopy time, and subsequent radiation dose, necessary to perform successful ablations. However, there is still no amount of radiation exposure known to be completely safe. The aim of this manuscript is to describe a detailed zero-fluoroscopy RHYTHMIA HDx workflow for AF ablation.MethodsThis was an observational, single-center experience to describe the technique, acute procedural success, and safety using a novel zero-fluoroscopy workflow with the RHYTHMIA HDx mapping system and intracardiac echocardiography (ICE). Seventy-two consecutive patients undergoing de novo or redo AF ablation were retrospectively analyzed. Venous access was guided with ultrasound. ICE combined with the mapping system’s magnetic tracking and sheath detection was used for precise catheter placement in the coronary sinus, at the transseptal puncture, and in the left atrium. A high-power, short-duration ablation strategy guided by local impedance was used. Pulmonary vein isolation was performed or touched up for all patients with additional lines added at the operator’s discretion.ResultsUsing this zero-fluoroscopy workflow, all patients achieved acute isolation with no significant procedure-related complications. Average procedure time was 73.7 ± 16.2 min, which included persistent (58%) and paroxysmal (42%) AF cases, and no procedures required conversion to fluoroscopy.ConclusionsIn this experience, a zero-fluoroscopy workflow using the RHYTHMIA HDx mapping system combined with ICE was feasible and safe for ablation in a heterogenous AF population. This approach, in the appropriate patient population, can eliminate radiation exposure to patients and staff.

Plasma-Surface Interactions Within Helicon Plasma Sources

Helicon plasma sources do not require electrodes or grids directly immersed in the plasma, and also present an axial magnetic field confining the plasma discharge. These factors are believed to provide them with long operational lifetimes because of the reduced potential for surface etching. The physics of helicon waves, cylindrical magnetized plasmas, sheaths, and plasma-surface interactions are discussed in the context of this claim. Practical implementation aspects are also reviewed, along with relevant experimental results. It is shown that understanding the distribution of ion density within the source, the presence of induced potentials in its surfaces, and the physics of low-energy sputtering reactions is essential to properly model erosion phenomena within helicons, and consequently predict their performance in practical applications.

Collisionless magnetized sheath resonance heating induced by a transverse magnetic field in low-pressure capacitive rf discharges

Abstract The mechanism of resonance heating between the gyrating electrons and the oscillating sheath induced by a small transverse magnetic field in low-pressure capacitively coupled plasmas is investigated. The gyrating electrons will coherently collide with the expanding sheath if the electron gyro-frequency coincides with half the driving frequency. These electrons will gain substantial energy from collision-less heating, which strongly enhances the electron power absorption and the plasma density at a constant driving voltage. The electron kinetics is revealed at resonant conditions by particle simulations. Our numerical results demonstrate that the relation between the magnetic field and the driving frequency determines this resonance effect. Besides, it is found that the operating pressure, electrode gap, and driving voltage all strongly affect this electron resonance mechanism. The resonance effect is more pronounced at the conditions of low pressure, large gap, and high voltage.

Role of Magnetic Fields in Ram Pressure Stripped Galaxies

Ram-pressure stripping is a crucial evolutionary driver for cluster galaxies and jellyfish galaxies characterized by very extended tails of stripped gas, and they are the most striking examples of it in action. Recently, those extended tails are found to show ongoing star formation, raising the question of how the stripped, cold gas can survive long enough to form new stars outside the stellar disk. In this study, we summarize the most recent results achieved within the GASP collaboration to provide a holistic explanation for this phenomenon. We focus on two textbook examples of jellyfish galaxies, JO206 and JW100, for which, via multi-wavelength observations from radio to X-ray and numerical simulations, we have explored the different gas phases (neutral, molecular, diffuse-ionized, and hot). Based on additional multi-phase gas studies, we now propose a scenario of stripped tail evolution including all phases that are driven by a magnetic draping sheath, where the intracluster turbulent magnetized plasma condenses onto the galaxy disk and tail and produces a magnetized interface that protects the stripped galaxy tail gas from evaporation. In such a scenario, the accreted environmental plasma can cool down and eventually join the tail gas, hence providing additional gas to form stars. The implications of our findings can shed light on the more general scenario of draping, condensation, and cooling of hot gas surrounding cold clouds that is fundamental in many astrophysical phenomena.

Numerical investigation of a radio frequency sheath with supra-thermal electrons in the presence of the magnetic field

The structure of the magnetized radio-frequency (RF) sheath of plasma containing supra-thermal electrons is studied numerically with a one-dimensional hybrid model consisting of a collisionless sheath model and an equivalent circuit model of the sheath. It is found that effect of the external magnetic field on the sheath structure depends on the direction of the disturbance current source. As the plasma magnetization or angle between the magnetic field and the wall is increased, both the sheath thickness and sheath potential drop increase during the positive disturbance current source at the wall, while they change slightly during the negative disturbance current source at the wall. For the different supra-thermal electron populations parameterized by Kappa velocity distribution, both the sheath thickness and sheath potential drop are always larger in the magnetized sheath than in the unmagnetized sheath. Compared with in the unmagnetized sheath, the width of the ion energy distribution (IED) in the magnetized sheath expands towards the high energy regime, and the low-energy peak of the IED decreases while the high-energy peak of the IED changes a little.

Influence of magnetic angle on the E × B drift in a magnetic presheath

In this work, we provide experimental measurements of the E×B flow above a non-magnetic, absorbing boundary in a magnetized plasma. Measurements are taken as a function of the angle between the background magnetic field and the boundary normal. The measurements are compared to computational predictions for oblique magnetic fields based on Chodura's model. Ion flow measurements were obtained with laser induced fluorescence and the presheath potential structure was measured with an emissive probe. The ions were found to accelerate to nearly 30% of the sound speed parallel to the boundary at oblique angles of the magnetic field (∼80°) and the ion speed at the electrostatic sheath edge was found to decrease with angle. The edge of the magnetic presheath has been experimentally determined based on the appearance of the E×B drift and is found to be independent of the angle of the magnetic field. We also demonstrate that laser induced fluorescence is suitable as a non-perturbative diagnostic to measure the electric field in a magnetized sheath.