Rotating Magnetic Field Frequency Research Articles

Design and experimental study of a field-reversed configuration plasma thruster prototype

Abstract The field-reversed configuration (FRC) plasma thruster driven by rotating magnetic field (RMF), abbreviated as the RMF-FRC thruster, is a new type of electric propulsion technology that is expected to accelerate the deep space exploration. An experimental prototype, including diagnostic devices, was designed and constructed based on the principles of the RMF-FRC thruster, with an RMF frequency of 210 kHz and a maximum peak current of 2 kA. Under the rated operating conditions, the initial plasma density was measured to be 5 × 1017 m-3, and increased to 2.2 × 1019 m-3 after the action of RMF. The coupling efficiency of RMF was about 53%, and the plasma current reached 1.9 kA. The axial magnetic field changed in reverse by 155 Gauss, successfully reversing the bias magnetic field of 60 Gauss, which verifies the the formation of FRC plasma. After optimization research, it was found that when the bias magnetic field is 100 Gauss, the axial magnetic field reverse variation caused by FRC is the highest at 164 Gauss. The experimental results are discussed and strategies are proposed to improve the performance of the prototype.

Inductive probe measurements in a rotating magnetic field thruster

The induced magnetic field during acceleration in a pulsed rotating magnetic field (RMF) thruster is experimentally investigated. A two-axis Bdot probe is employed to characterize the time-resolved evolution of the fields in a 5 kW-class test article. This device is operated at an average power of 4 kW with an RMF frequency of 415 kHz, pulse widths of 125 µs, and a repetition rate of 155 Hz. Plasma currents induced in the thruster are shown to reach 2500 A and to have sufficient magnitude to form a field-reversed configuration plasmoid. The Lorentz force resulting from the induced magnetic field contributes ∼25% of measured thrust at this operating condition. Of this Lorentz thrust, ∼58% is due to plasma current interaction with the steady applied bias field, while the remainder is caused by interaction with secondary induced currents in nearby structural elements. This structure force is predicted to scale quadratically with plasma current magnitude. These results are discussed in the context of the historically low performance of these devices and strategies for improving their operation are presented.

Antimicrobial properties of pristine and Pt-modified titania P25 in rotating magnetic field conditions

Effective and cheap water purification is one of the most important tasks facing humanity. Among various methods of water treatment, heterogeneous photocatalysis is probably most recommended. However, the application of artificial sources of irradiation results in high investment and operating costs. Accordingly, either vis-responsive photocatalysts or different ways of photocatalyst activation should be developed. In the present study, rotating magnetic field (RMF) has been tested to inactivate gram-positive (Staphylococcus epidermidis) and gram-negative (Escherichia coli) bacteria in the presence of pristine and Pt-modified titania P25 photocatalyst. Liquid cultures of the bacteria have been exposed to the titania and RMF (RMF frequency of 1–50 Hz, RMF magnetic induction of ca. 19.92 mT, 180 min exposure time, temperature of incubation at 37 °C). It has been found that highly active titania photocatalyst might also work in the absence of photoirradiation but under RMF. Moreover, its modification with platinum besides highly improved photocatalytic activity (tested for two model reactions of oxidative decomposition of acetic acid and anaerobic dehydrogenation of methanol under UV/vis irradiation) results in significant enhancement of antimicrobial effect under RMF. Additionally, photocatalysts with larger content of oxidized forms of platinum show higher antimicrobial activity, and thus it is proposed that platinum oxides are more active than zero-valent platinum under RMF. This study provides evidence of antimicrobial effect of titania without photoirradiation, indicating that RMF might efficiently activate photocatalyst.

Residence time distribution of passive scalars in magnetic nanofluid Poiseuille flow under uniform rotating magnetic fields

The diffusive and convective transport mechanisms that govern the residence time distribution (RTD) of magnetic nanofluid (ferrofluid) capillary Poiseuille flow under an external uniform rotating magnetic field (RMF) were experimentally and numerically investigated. The measured RTDs are reduced and approach plug flow under RMF excitation by shortening the elution time, stiffening the breakthrough rise, and delaying the breakthrough time as the RMF frequency and/or magnetic nanoparticle concentration increase(s). Ferrohydrodynamic (FHD) simulations predict the inception of secondary convective azimuthal flow but fail to explain the impact of the magnetic field on RTD evolution under the assumption of isotropic diffusion transport. The origin of anisotropy in scalar diffusive transport was elucidated by analyzing the rotational properties of magnetic nanoparticles with respect to ferrofluid vorticity and RMF arrangement. Therefore, independent measurement of an anisotropic effective diffusion tensor for RMF-excited quiescent ferrofluids facilitates improved FHD simulations of RTD without compromising the model’s predictive power.

Anomalous anisotropic transport of scalars in dilute ferrofluids under uniform rotating magnetic fields – Mixing time measurements and ferrohydrodynamic simulations

This study reports experimental observations and numerical simulations on the transport of scalars in ferrofluids under rotating magnetic fields (RMF). Mass transport experiments were conducted in a T-mixer capillary with RMF rotational axis parallel to capillary central axis. Significant mass transport enhancement was measured in the transverse direction as RMF frequency and/or magnetic nanoparticles concentration increased. RMF directional control of mass flux enhancement was anisotropic as molecular diffusion was the only transport mechanism parallel to the capillary axis. The significance of ferrofluid advection (spin-up flow) as mass transport enhancement was examined in light of the advection-diffusion transport and experimental results. Hence, the ferrohydrodynamic equations (FHD) of motion were solved for the lower spin viscosity limit [based on Rosensweig’s interpretation of micro-eddies diffusion length scale], and upper limit as reported from earlier spin-up flow studies. FHD simulations predicted lower magnitudes of the linear azimuthal velocities with decreasingly trends as spin viscosity increased. These findings were in disagreement with previously reported ferrofluid flows calculated on the basis of spin diffusion theory. To clarify the inconsistency’s origin and confirm reliability of numerical simulations, the spin field and asymmetric stresses were analyzed to conclude that the small length scale of the capillary could not accredit the role of spin-up flow effects via azimuthal velocities for the improved mixing observations. Mixing time measurements and analysis of the advection-diffusion equation pointed toward lack of current FHD theory to resolve scalar transport due to the nanoparticle-scale secondary circumferential micro-convective flows. A pragmatic approach was therefore proposed to estimate from mixing time measurements the anomalous anisotropic effective diffusion coefficients induced by rotating nanoparticles, and to correlate them to RMF frequency and/or nanoparticles concentration.

Verification of azimuthal current generation employing a rotating magnetic field plasma acceleration method in an open magnetic field configuration

Time-varying, azimuthal electron current is obtained from measured two-dimensional profiles of excited magnetic fields, using the Rotating Magnetic Field (RMF) plasma acceleration method in an open magnetic field configuration. The RMF is applied orthogonally to cylindrical plasma, leading to the azimuthal current drive via the Hall effect. Here, dc azimuthal current, whose magnitude is equivalent to that of ac azimuthal current with twice the RMF frequency, is verified for the first time. In addition, an expected current reversal is found, with the RMF rotation direction changing by 180°.

Modification of bacterial cellulose through exposure to the rotating magnetic field

The aim of the study was to assess the influence of rotating magnetic field (RMF) on production rate and quality parameters of bacterial cellulose synthetized by Glucanacetobacter xylinus. Bacterial cultures were exposed to RMF (frequency f=50Hz, magnetic induction B=34mT) for 72h at 28°C. The study revealed that cellulose obtained under RMF influence displayed higher water absorption, lower density and less interassociated microfibrils comparing to unexposed control. The application of RMF significantly increased the amount of obtained wet cellulose pellicles but decreased the weight and thickness of dry cellulose. Summarizing, the exposure of cellulose-synthesizing G. xylinus to RMF alters cellulose biogenesis and may offer a new biotechnological tool to control this process. As RMF-modified cellulose displays better absorbing properties comparing to non-modified cellulose, our finding, if developed, may find application in the production of dressings for highly exudative wounds.

Axial Dispersion in Nanofluid Poiseuille Flows Stirred by Magnetic Nanoagitators

A Taylor–Aris dispersion in laminar capillary flows of magnetic nanofluids submitted to transverse rotating magnetic fields (RMFs) was analyzed with a simple phenomenological mixing approach. The nanofluid residence time distributions (RTDs) measured under RMFs were used to quantify the deviations, with respect to field-free Poiseuille flows, of the axial dispersion induced by the rotating magnetic nanoparticles (MNPs) as a function of MNPs concentration and diameter, and RMF frequency and strength. The attenuation of axial dispersion due to the magnetic field was ascribed to an enhanced transverse diffusion coefficient that thrust tracer radial transport, owing to nanoconvective streams in the nanoparticle neighborhoods, beyond molecular diffusion capability. To estimate the enhanced transverse diffusion coefficient, a semiempirical model was developed in which the nanofluid domain was viewed as an array of identical cells each containing a magnetic nanoparticle at its center. Owing to the nanoparticle r...

Effects of rotating magnetic field exposure on the functional parameters of different species of bacteria

The aim of the present study was to determine the effect of the rotating magnetic field (RMF) on the growth, cell metabolic activity and biofilm formation by S. aureus, E. coli, A. baumannii, P. aeruginosa, S. marcescens, S. mutans, C. sakazakii, K. oxytoca and S. xylosus. Bacteria were exposed to the RMF (RMF magnetic induction B = 25–34 mT, RMF frequency f = 5–50 Hz, time of exposure t = 60 min, temperature of incubation 37 °C). The persistence of the effect of exposure (B = 34 mT, f = 50 Hz, t = 60 min) on bacteria after further incubation (t = 300 min) was also studied. The work showed that exposure to RMF stimulated the investigated parameters of S. aureus, E. coli, S. marcescens, S. mutans, C. sakazakii, K. oxytoca and S. xylosus, however inhibited cell metabolic activity and biofilm formation by A. baumannii and P. aeruginosa. The results obtained in this study proved, that the RMF, depending on its magnetic induction and frequency can modulate functional parameters of different species of bacteria.

Probe measurements of the three-dimensional magnetic field structure in a rotating magnetic field sustained field-reversed configuration

A translatable three-axis probe was constructed and installed on the translation, confinement, and sustainment upgrade (TCSU) experiment. With ninety windings, the probe can simultaneously measure Br, Bθ, and Bz at 30 radial positions, and can be placed at any desired axial position within the field reversed configuration (FRC) confinement chamber. Positioning the probe at multiple axial positions and taking multiple repeatable shots allows for a full r-z map of the magnetic field. Measurements were made for odd-parity rotating magnetic field (RMF) antennas and even-parity RMF. The steady state data from applying a 10 kHz low pass filter used in conjunction with data at the RMF frequency yields a map of the full 3D rotating field structure. Comparisons will be made to the 3D magnetic structure predicted by NIMROD simulations, with parameters adjusted to match that of the TCSU experiments. The probe provides sufficient data to utilize a Maxwell stress tensor approach to directly measure the torque applied to the FRC's electrons, which combined with a resistive torque model, yields an estimate of the average FRC resistivity.

The Effects of Rotating Magnetic Field on Growth Rate, Cell Metabolic Activity and Biofilm Formation by Staphylococcus Aureus and Escherichia Coli

This work presents results of the study which concerns the influence of the rotating magnetic field (RMF) on the growth rate, cell metabolic activity and ability to form biofilms by E. coli and S. aureus. Liquid cultures of the bacteria were exposed to the RMF (RMF frequency f = 1-50 Hz, RMF magnetic induction B = 22-34 mT, time of exposure t = 60 min, temperature of incubation <TEX>$37^{\circ}C$</TEX>). The present study indicate the exposition to the RMF, as compared to the unexposed controls causing an increase in the growth dynamics, cell metabolic activities and percentage of biofilm-forming bacteria, in both S. aureus and E. coli cultures. It was also found that the stimulating effects of the RMF exposition enhanced with its increasing frequencies and magnetic inductions.

Independent control of electron energy and density using a rotating magnetic field in inductively coupled plasmas

Effects of a rotating magnetic field (RMF) on the electron energy distribution function (EEDF) and on the electron density are investigated with the aim of controlling the radical composition of inductively coupled plasmas. By adjusting the RMF frequency and generation power, the desired electron density and electron energy shift are obtained. Consequently, the amount and fraction of high-energy electrons, which are mostly responsible for direct dissociation processes of raw molecules, will be controlled externally. This controllability, with no electrode exposed to plasma, will enable us to control radical components and their flux during plasma processing.

Control of electron energy distribution function by a rotating magnetic field

The effects of a rotating magnetic field (RMF) on the electron energy distribution function (EEDF) are studied with the aim of controlling the EEDF of radio-frequency (rf) inductively coupled plasmas. In order to obtain the EEDF correctly, the external filter and the reference electrode of the probe system are used to compensate for the effect of the rf (13.56MHz) and the RMF (1.7–2.9MHz) fluctuations of the plasma potential. With increasing RMF frequency, the high energy component of the EEDF increases. Space potential and effective electron temperature also tend to increase with increasing RMF frequency.

Operation of TCSU with Internal Flux-Conserving Rings

The Translation, Confinement, and Sustainment Upgrade (TCSU) device is a facility to form and sustain a field-reversed configuration (FRC) in quasi-steady state using rotating magnetic fields (RMF). Recent campaigns include Ti gettering, the installation of a set of internal flux rings, and RMF frequency scans. The Ti gettering campaign was successful, reduced impurities, and reduced deuterium recycling from the walls allowing density control and hotter FRCs [J.A. Grossnickle et al., Phys. Plasmas 17, 032506 (2010)]. Internal flux rings have been installed to provide a uniform flux surface and minimize plasma-wall contact. Results from the internal flux ring operation and an additional Ti gettering campaign are reported. RMF frequencies of 123 kHz and 170 kHz have been investigated and initial results are reported.

Improved confinement and current drive of high temperature field reversed configurations in the new translation, confinement, and sustainment upgrade device

Previous work in the translation, confinement, and sustainment (TCS) device [Hoffman, Guo, Slough et al., Fusion Sci. Technol. 41, 92 (2002)] demonstrated formation and steady-state sustainment of field reversed configurations (FRC) by rotating magnetic fields (RMF). However, in TCS the plasma temperature was limited to several 10s of eV due to high impurity content. These impurities are greatly reduced in the new TCS upgrade device (TCSU), which was built with a bakable, ultrahigh vacuum chamber, and advanced wall conditioning capabilities. This led to dramatic improvements in TCSU with temperatures well over 200eV, using simple even-parity RMF drive. The higher temperatures, coupled with reduced recycling, allowed plasma to enter into a collisionless, high-ζ (ratio of average electron rotation frequency to RMF frequency) regime. These new FRC states exhibit the following key features: (1) Dramatic improvement in current drive efficiency with ζ approaching 100%, for the first time in TCSU; (2) up to threefold increase in global energy confinement time; and (3) significant reduction in transport rates, accompanied by a striking transition from a Bohm-type transport to a lower hybrid driftlike transport that scales better than gyro-Bohm and is very favorable for the next step FRC development.

Rotating magnetic field current drive of high-temperature field reversed configurations with high ζ scaling

Greatly reduced recycling and impurity ingestion in the Translation, Confinement, and Sustainment—Upgrade (TCSU) device has allowed much higher plasma temperatures to be achieved in the field reversed configurations (FRC) under rotating magnetic field (RMF) formation and sustainment. The hotter plasmas have higher magnetic fields and much higher diamagnetic electron rotation rates so that the important ratio of average electron rotation frequency to RMF frequency, called ζ, approaches unity, for the first time, in TCSU. A large fraction of the RMF power is absorbed by an as yet unexplained (anomalous) mechanism directly proportional to the square of the RMF magnitude. It becomes of relatively lesser significance as the FRC current increases, and simple resistive heating begins to dominate, but the anomalous absorption is useful for initial plasma heating. Measurements of total absorbed power, and comparisons of applied RMF torque to torque on the electrons due to electron-ion friction under high-ζ operation, over a range of temperatures and fields, have allowed the separation of the classical Ohmic and anomalous heating to be inferred, and cross-field plasma resistivities to be calculated.

The suppression of temperature fluctuations by a rotating magnetic field in a high aspect ratio Czochralski configuration

We demonstrate experimentally in a low temperature model that a rotating magnetic field (RMF) has the potential to stabilize the inverse temperature gradient in a medium scale Czochralski facility with a high initial melt level. This stabilization occurs as a result of a flow transition from large scale buoyancy driven to small scale magnetically driven turbulence. The dependency of the required field strength is found vs. the imposed temperature drop for an at least 10 times higher Grashof number than in earlier experiments. The observed scaling agrees well with previous results. It is observed that the flow transition becomes even more pronounced, i.e. occurs in a more narrow range of the magnetic forcing, as the Grashof number is increased. The influence of the RMF frequency is investigated. It is found that an increasing frequency of the RMF gradually eliminates the stabilizing effect. The optimum dimensionless frequency is found minimizing the required strength of the RMF. The characteristic azimuthal velocity of the flow is measured by the temperature correlation technique.

Kinetic modelling of the interaction of rotating magnetic fields with a radially inhomogeneous plasma

The interaction of rotating magnetic fields (RMFs) with a plasma is modelled in the linear approximation. A kinetic Hamiltonian model for the rf plasma conductivity is used. A radially inhomogeneous periodic cylindrical plasma with a rotational transform of the magnetic field is studied with parameters relevant to the dynamic ergodic divertor (DED) of TEXTOR. For the case of a finite electron diamagnetic velocity it is shown that the torque resulting from the RMF tends to bring the electron fluid approximately to the rest frame of this field. This result is in qualitative agreement with long mean-free path drift MHD theory. In contrast to that theory where a resonant behaviour is found at electron and ion diamagnetic frequencies, in the present kinetic analysis, the RMF frequency where the torque passes through zero is smaller than the electron diamagnetic frequency if there is an electron temperature gradient present. The relation of these results with recent experimental measurements of the DED-induced plasma rotation in TEXTOR is discussed.

Electron inertial effects on rotating magnetic field current drive

The effect of finite electron mass on the formation and sustainment of a field reversed configuration (FRC) by rotating magnetic fields (RMF) is studied. The importance of inertial effects is measured by the ratio between the RMF frequency (ω) and the electron-ion collision frequency (ν). In the limit where this ratio is very small (ω∕ν→0), previous results corresponding to massless electrons are recovered. When ω∕ν increases there are significant changes in the value of the minimum external rotating field needed to sustain the FRC and the time required to reach a steady state. Since ν decreases with increasing temperature and decreasing density, these effects are expected to become more important as fusion relevant temperatures are approached.

Resistivity scaling of rotating magnetic field current drive in FRCs

Rotating magnetic fields (RMFs) have been used to both form and sustain low density, prolate FRCs in the translation confinement and sustainment (TCS) facility. The two most important factors governing performance are the plasma resistivity, which sets the maximum density for which toroidal current can be maintained, and the energy loss rate, which sets the plasma temperature. The plasma resistivity has been determined by carefully measuring the amount of RMF power absorbed by the FRC. When the ratio of RMF magnitude, Bω, to external poloidal confinement field, Be, is high, this resistivity is very adversely affected by the RMF drive process. However, when Bω/Be falls below about 0.3, the resistivity returns to values typical of non-driven FRCs. The observed scaling leads to a density dependence of ne ∼ Bω/rsω1/2 where rs is the FRC separatrix radius and ω is the RMF frequency. Since the FRC contains little or no toroidal field, Be is proportional to (neTt)1/2 where Tt = Te + Ti is the sum of the electron and ion temperatures. In the present experiments, except for the initial start-up phase where Tt can exceed 100 eV, the plasma temperature is limited to about 40 eV by high oxygen impurity levels. Thus, low Bω/Be, low resistivity operation was only realized by operating at low values of Bω. The RMF drive sustains particles as well as flux, and resistive input powers can be in the MW range at higher values of Bω, so that high temperature, steady-state operation should be possible once impurity levels are reduced. Changes are being made to the present ‘O-ring’ sealed, quartz chambered TCS to provide bakable metal walls and wall conditioning as in other quasi-steady fusion facilities.