External Field Perturbations Research Articles

Overview of the recent experimental research on the J-TEXT tokamak

The J-TEXT capability is enhanced compared to two years ago with several upgrades of its diagnostics and the increase of electron cyclotron resonance heating (ECRH) power to 1 MW. With the application of electron cyclotron wave (ECW), the ECW assisted plasma startup is achieved; the tearing mode is suppressed; the toroidal injection of 300 kW ECW drives around 24 kA current; fast electrons are generated with toroidal injected ECW and the runaway current conversion efficiency increases with ECRH power. The mode coupling between 2/1 and 3/1 modes are extensively studied. The coupled 2/1 and 3/1 modes usually lead to major disruption. Their coupling can be either suppressed or avoided by external resonant magnetic perturbation fields and hence avoids the major disruption. It is also found that the 2/1 threshold of external field is significantly reduced by a pre-excited 3/1 mode, which can be either a locked island or an external kink mode. The disruption control is studied by developing prediction methods capable of cross tokamak application and by new mitigation methods, such as the biased electrode or electromagnetic pellet injector. The high-density operation and related disruptions are studied from various aspects. Approaching the density limit, the collapse of the edge shear layer is observed and such collapse can be prevented by applying edge biasing, leading to an increased density limit. The density limit is also observed to increase, if the plasma is operated in the poloidal divertor configuration or the plasma purity is increased by increasing the pre-filled gas pressure or ECRH power during the start-up phase.

Electron-phonon coupling dictates electron mean free paths and negative thermal diffusion in metals

The spatiotemporal dynamics of the fundamental energy carriers in metals underpin the efficiency of a wide array of technologies. Although much progress in our understanding of the temporal response of electronic relaxation processes following an external field perturbation has been achieved, the spatial relaxation dynamics of electrons after excitation remains unclear. Here, we employ a scanning ultrafast microscopy with high spatial resolution to directly measure the mean free paths of electrons in different metals. We show that the strength of electron-phonon coupling not only dictates the mean free paths, but also controls a peculiar spatial shrinking of the electronic temperature profile immediately after the electrons have thermalized with the ‘colder’ lattice. More specifically, from our experimental tracking of the spatial relaxation profiles, a clear observation of an effective negative thermal diffusion is apparent for metals with weaker electron-phonon coupling, while for metals with stronger electron-phonon coupling, the negative thermal diffusion effect is not observable. Our spatiotemporal modeling based on the two-temperature approach provides evidence that although negative diffusion occurs universally for materials with two different species that have varying diffusivities and are thermodynamically coupled, effective negative diffusion is masked for cases with stronger coupling between the species.

About the Differential Sweep Method for Measuring of Ferrocolloids Magnetization Curves

Purpose. Justification and description of a laboratory method for measuring static magnetization curves specialized for ferrocolloids.Methods. The measurement method is based on the paramagnetism of magnetic colloids and the quasi-linear response of their magnetisation to small perturbations of the external magnetic field. To obtain the magnetisation curve, the studying ferrocolloid sample is placed in a constant homogeneous field of a laboratory electromagnet with an iron core. By low-frequency modulation of the current in the coils of the electromagnet, a co-directional perturbation is applied to the constant field. Information about the response of the sample to the external field perturbation - the differential magnetic susceptibility of ferrocolloid - is extracted by electrical measurements. These measurements are carried out using a classical compensation device of two counter-connected wire coils, one of which contains the investigated sample. Conducting (sweeping) the measurements in a wide range of applied fields allows to collect a sequence of experimental values of differential susceptibility from which the desired magnetisation curve is reconstructed by numerical integration.Results. The experimental setup for measuring the magnetisation curves of ferrocolloids was assembled. A theoretical description of the compensating electrical measuring device of the setup was proposed. The adjustment of the electrical scheme was carried out within several series of calibration experiments aimed at establishing the material parameters of the setup that were unknown from the theory. On the example of ferrocolloid of the type “magnetite - kerosene – oleic acid” both the process of obtaining primary experimental data and their subsequent processing, including the procedure of numerical integration, were demonstrated. It is established that the use of integration methods of a higher accuracy allows reducing the number of required experimental points and accelerating the measurement process without reducing the quality of the obtained curves.Conclusion: The method applicable for measuring the magnetisation curves of ferrocolloids by differential sweeping is described, substantiated and implemented using laboratory equipment.

Helical mode localization and mode locking of ideal MHD instabilities in magnetically perturbed tokamak plasmas

In H-mode tokamak plasmas, the achievable pressure-gradient is limited by type-I Edge Localized Modes (ELMs), which are projected to cause severe damage to future fusion devices. There are several approaches aiming to mitigate/suppress the occurrence of ELMs, such as the application of an external non-axisymmetric magnetic perturbation field, which breaks the axisymmetry of the tokamak plasma. In this work we use the CASTOR3D code to investigate helical localization and mode locking of edge-localized ideal MHD instabilities in rotating and flow-free magnetically perturbed tokamak plasmas with periodicity. Helically localized instabilities are separated into two classes: quasi-locked and strictly locked. In a non-rotating plasma, the localization of quasi-locked modes is determined by an envelope while their precise location under the envelope is arbitrary, whereas strictly locked modes can only occur at a single helical position. Strictly locked modes only rotate if the toroidal plasma rotation exceeds a critical threshold; above the threshold the forced rotation of the strictly locked modes is non-uniform. For quasi-locked modes, no such critical threshold exists; they rotate uniformly beneath their envelope in the case of finite plasma rotation. The helical localization of both quasi-locked and strictly locked instabilities is determined by the energetic decomposition of the instabilities close to the most unstable flux-surface; for example, strongly current-density driven instabilities are aligned with regions of augmented parallel equilibrium current-density. Finally, we compare the computationally determined localization of MHD instabilities to experimental observations. The determined MHD instability is located at the same position as the experimentally measured modes with respect to the equilibrium corrugation, verifying that ideal MHD can describe the experimentally observed instabilities.

Investigation of MHD instabilities and related dynamic interactions during the major disruption in the HL-2A tokamak

Avoiding the major disruption is of paramount importance in future reactor-level devices, for which understanding the disruption mechanism is essential. In this work, MHD instabilities and related dynamic interactions during the major disruption have been investigated in the ohmically heated plasmas in the HL-2A tokamak. It is reported that the interaction between a kind of edge-oscillating-mode (EOM) perturbation and a rotating m/n = 2/1 tearing mode (TM) inside the plasma plays an important role in inducing the mode locking and the subsequent disruption. The EOM perturbation is oscillating in the laboratory frame, which is proposed to be originally generated by the penetrated error field at the plasma edge and is modulated by the rotating 2/1 TM mode. Before mode locking, the 2D electron cyclotron emission imaging shows that the momentary coupling of the EOM and the 2/1 mode can be decoupled each other and the mode structure does not alter significantly. After the mode locking, the EOM and the 2/1 mode expand and couple each other and induce the heat transfer from the core to the edge. The influence of the TM instability and the EOM perturbation on surrounding plasmas prior to the mode locking has also been presented. The results deepen our understanding of the disruption dynamics related to the external field perturbations, especially in the presence of TMs inside the plasma.

Physical design of a new set of high poloidal mode number coils in the EAST tokamak

An external resonant magnetic perturbation (RMP) field, which is an effective method to mitigate or suppress the edge localized mode (ELM), has been planned to be applied on the ELM control issue in ITER. A new set of magnetic perturbation coils, named as high m coils, has been developed for the EAST tokamak. The magnetic perturbation field of the high m coils is localized in the midplane of the low field side, with the spectral characteristic of high m and wide n, where m and n are the poloidal and toroidal mode numbers, respectively. The high m coils generate a strong localized perturbation field. Edge magnetic topology under the application of high m coils should have either a small or no stochastic region. With the combination of the high m coils and the current RMP coils in the EAST, flexible working scenarios of the magnetic perturbation field are available, which is beneficial for ELM control exploration on EAST. Numerical simulations have been carried out to characterize the high m coil system, including the magnetic spectrum and magnetic topology, which shows a great flexibility of magnetic perturbation variation as a tool to investigate the interaction between ELM and external magnetic perturbation.

Classical Love number for quantum black holes

We present a method for comparing the classical and quantum calculations of the electric quadrupolar Love number $k_2$ and show that our previous derivation of the quantum Love number of a quantum blackhole matches exactly the classical calculation of $k_2$ when quantum expectation values are replaced by the corresponding classical quantities, as dictated by the Bohr correspondence principle. The standard derivation of $k_2$ for classical relativistic stars relies on fixing boundary conditions on the surface of the star for the Einstein equations in the presence of an external perturbing field. An alternative method for calculating $k_2$ uses properties of the spectrum of the non-relativistic fluid modes of the star. We adopt this alternative method and use it to derive an effective description of the interior modes in terms of a collection of driven harmonic oscillators characterized by different frequencies and amplitudes. We compare these two classical methods and find that most of the interior information can be integrated out, reducing the problem of calculating $k_2$ to fixing a single boundary condition for the perturbed Einstein equations on the surface of the deformed star. We then determine this single boundary condition in terms of the spectrum of the object and proceed to identify the relationship between classical quantities and quantum expectation values and to verify the agreement between the results of the effective classical calculation and the quantum calculation.

Measuring radiofrequency field-induced temperature variations in brain MRI exams with motion compensated MR thermometry and field monitoring.

An MR thermometry (MRT) method with motion and field fluctuation compensation is proposed to measure non-invasively sub-degree brain temperature variations occurring through radiofrequency (RF) power deposition during MR exams. MRT at 7T with a multi-slice echo planar imaging (EPI) sequence and concurrent field monitoring was first tested in vitro to assess accuracy in the presence of external field perturbations, an optical probe being used for ground truth. In vivo, this strategy was complemented by a motion compensation scheme based on a dictionary pre-scan, as reported in some previous work, and was adapted to the human brain. Precision reached with this scheme was assessed on eight volunteers with a 5 minute-long low specific absorption rate (SAR) scan. Finally, temperature rise in the brain was measured twice on the same volunteers and with the same strategy, this time by employing a 20-minutes scan at the maximum SAR delivered with a commercial volume head coil. In vitro, the root mean square (RMS) error between optical probe and MRT measurements was 0.02°C with field sensor correction. In vivo, the low SAR scan returned a precision in temperature change measurement with field monitoring and motion compensation of 0.05°C. The 20-minutes maximum SAR scan returned a temperature rise throughout the inner-brain in the range of 0-0.2°C. Brain periphery remained too sensitive with respect to motion to lead to equally conclusive results. Sub-degree temperature rise in the inner human brain was characterized experimentally throughout RF exposure. Potential applications include improvement of human thermal models and revision of safety margins.

REARRANGEMENT OF THE CONFORMATIONAL STRUCTURE OF POLYAMPHOLYTES ON THE SURFACE OF A METAL NANOWIRE IN A TRANSVERSE MICROWAVE ELECTRIC FIELD

Molecular dynamics has been employed to study the rearrangement of the conformational structure of polyampholytes adsorbed on the surface of a gold nanowire with a periodic change in time of its polarity in the transverse direction at an ultrahigh frequency. The radial distributions of the atomic density of the polypeptide and its angular distributions on the nanowire surface have been calculated. At high temperatures, temporary fluctuations in the conformational structure of the adsorbed polyampholyte polypeptide were observed. In this case, for half the period of the nanowire polarity change, the macrochain conformation changed from dense enveloping of the nanowire to an elongated conformational structure along the dipole moment of the nanowire. At low temperature and the nanowire dipole moment, the swelling of the fringe of the adsorbed polyampholyte was observed with a displacement of most of its links to one side with respect to the plane perpendicular to the direction of the nanowire dipole moment and passing through its axis. At low temperature and high values of the nanowire dipole moment, the polyampholyte polypeptide was desorbed from the nanowire surface. An analytical model of conformational rearrangements of a polyampholyte Gaussian chain in the form of an external field perturbation theory is presented.

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and unique advantage of immunity against external field perturbations. To act as the charge-to-spin conversion source in energy-saving spintronic devices, it is of great importance for the AFM material to possess a large spin torque efficiency (ξDL). In this work, using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/NiFe(Py) bilayer system, we systemically study the ξDL of AFM Mn2Au films with different crystal structures. Compared with polycrystalline Mn2Au with effective ξDL < 0.051, we show a much larger ξDL of ~0.333 in single-crystal Mn2Au, which arises from the large spin Hall conductivity instead of electrical resistivity. Moreover, with a further contribution of interfacial effects, the effective ξDL of single-crystalline Mn2Au/Py system increases to 0.731, which is more than two times larger than the value of ∼0.22 reported for the Mn2Au/CoFeB system. By utilizing the large ξDL of Mn2Au in a perpendicularly magnetized MnGa/Mn2Au system, energy-efficient deterministic magnetization switching with a current density at ~106 A cm−2 is achieved. Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

2D HfN2/graphene interface based Schottky device: Unmatched controllability in electrical contacts and carrier concentration via electrostatic gating and out-of-plane strain

Graphene-based van der Waals heterostructure (vdWH) comprising of HfN2monolayer stacked over graphene has been designed and studied based on density functional theory. The vdWH forms an-type Schottky contact with a Schottky barrier height (SBH) of 0.67 eV, while it exhibitsp-type SBH of 0.93 eV. The response of SBH and electrical contact properties to external perturbation, such as, vertical strain and electric field has been investigated thoroughly. Under the application of strain and normal electric field within range of ±0.3 V/Å, the type of electrical contacts, i.e., n/p type Schottky or Ohmic, is found to be interconvertible, while electron/hole doping in graphene is tunable by a doping carrier concentration of up to ~1013 cm−2, which lies between experimentally observed molecular doping (~1012 cm−2) and electrolytic gating (~1014 cm−2). Such an extremely high tunability in electrical contacts, doping carrier concentration along with its excellent optical response in the visible light region shows unrivalled potential of this vdWH in high performance graphene-based futuristic Schottky transistors with high on/off ratio, ultrathin phototransistor with high gain, low-power multivalued optical non volatile memory devices, and nanoelectronics.

Spin Ordering in Low-Dimensional Magnetic System Induced by Model Interaction

The magnetic susceptibility has been studied by using a new trial function for the model interaction between spins arranged in a low-dimensional square pyramidal system. We suggest that the exchange interaction depends on a power exponent which depends on the pyramidal structure in addition to the magnetic energy and thermal energy. We have calculated the magnetization as a function of the temperature and magnetic field keeping the exponent constant. Dependence of the susceptibility on a characteristic length related to the low-dimensional system exhibits novel features. Magnetic field dependence of the susceptibility has a maximum which shifts toward higher field as the temperature is increased. Even at higher fields, the susceptibility can be very small. The variations of the susceptibility with temperature exhibit maximum which depends on the external perturbing field.

Representation of the Third Adiabatic Invariant in Flux Form and Some Consequences of Its Conservation by Examples of Specific Axial Magnetic Systems

The dynamics of invariant magnetic surfaces is considered with examples of axially symmetric magnetic systems. The inner trap (magnetic mirror) and the outer trap (dipole) are distinguished in a system formed by two coils with current. The dynamics of noninteracting particles moving in the plane of the magnetic equator is considered in the drift approximation for the perturbation vector satisfying the third invariant conservation condition. The results are extended to the equator of the geomagnetic trap, in which the external perturbing field is given by the ring current. It is shown that the real effect of the ring current on plasma dynamics in the bounded magnetosphere of the Earth in the approximation of constancy of the third invariant differs from conventional model calculations based on dipole representations of the geomagnetic field. In this case, it is not the magnetic flux that makes practical sense but only its change, which is equal to the difference between the final and initial fluxes calculated by the parameters of the real field. The minimum values of the energies of charged particles in the radiation belts required to satisfy the third condition of adiabatic invariant conservation are estimated for the main phase of a particular magnetic storm.

Beyond the Rosenfeld Equation: Computation of Vibrational Circular Dichroism Spectra for Anisotropic Solutions.

The difference in absorption of left and right circularly polarized light by chiral molecules can be described by the Rosenfeld equation for isotropic samples. It allows the assignment of the absolute stereochemistry by comparing experimental and computationally derived spectra. Despite the simple form of the Rosenfeld equation, its evaluation in the infrared regime remained challenging, as the contribution from the magnetic dipole operator is zero within the Born-Oppenheimer (BO) approximation. In order to resolve this issue, "beyond BO" theories had to be developed, among which Stephen's magnetic field perturbation (MFP) approach offers a computationally easily accessible form. In this work, optical activity is discussed for cylindrically symmetric solutions, which cannot be described anymore by Rosenfeld's equation due to broken spherical symmetry. Mathematical properties of natural and electric-field induced anisotropies are discussed on the basis of the gauge-independent theoretical framework of Buckingham and Dunn. The issue of achiral noise arising from external field perturbations is considered, and potential remedies are introduced. Natural anisotropic vibrational circular dichroism (VCD) equations are solved numerically by applying the MFP approach within the Hartree-Fock (HF) formalism. Properties of anisotropic VCD spectra are discussed for R-(+)-methyloxirane and (1 S,2 S)-cyclopropane-1,2-dicarbonitrile. In particular, by using a group theoretical argument, a gauge-independent lower bound for the quadrupole contribution of C2-symmetric molecules can be identified, which allows the importance of additional quadrupole terms in anisotropic VCD spectra calculation to be assessed.

Spontaneous magnetic field multipolar structure in toroidal plasmas based on 2D equilibrium

The multipolar magnetic field structure is investigated by the momentum conservation equation with self-consistent 3D sheared flows during transition of plasma properties from local paramagnetic to diamagnetic fields. Numerical results show that the traditional poloidal magnetic field () is one part of equilibrium magnetic fields. The non-zero-order quantities are originated from the higher-order terms of 2D equilibrium treatment based on a Fourier expansion of ψ (r, θ). The distributions of magnetic field vectors of the order of 1, 2, and 3 terms are presented respectively in two, four, and six polar fields with the local vortex structures (spontaneous magnetic connection). The excitation mechanisms of the magnetic vortices are the coupling effects of the magnetic fluid structure pattern and the toroidal effects. These results can help us understand the physical mechanism of the interaction between the external perturbation fields and control tearing modes, as well as the radial plasma flow and magnetic vortices.

Effects of two-dimensional magnetic uncertainties and three-dimensional error and perturbation fields on the Small Angle Slot divertor geometry and topology

The Small Angle Slot (SAS) was recently installed on DIII-D as an advanced divertor, promising easier plasma detachment and lower temperatures across the whole target. A twofold study of the SAS magnetic geometry and topology is presented in this paper. On one hand, a two-dimensional uncertainty quantification analysis is carried out through a Monte Carlo approach in order to understand the level of accuracy of two-dimensional equilibrium computations in reconstructing the strike point and angle onto the divertor. Under typical experimental conditions, the uncertainties are found to be roughly 6.8 mm and 0.56 deg, respectively. On the other hand, a three-dimensional ‘vacuum’ analysis is carried out to understand the effects of typical external perturbation fields on the scrape-off layer topology. When the non-axisymmetric I-coils are switched on, poloidally-localized lobes are found to appear, grow, and hit the SAS target, although barely, even for 5 kA; at the same time, the strike point modulation is found to be roughly 1.8 mm and thus negligible for most purposes. Such results complement previous two-dimensional analyses in characterizing typical SAS equilibria and provide useful background information for planning and interpreting SAS experiments.

Effect of large magnetic islands on screening of external magnetic perturbation fields at slow plasma flow

A toroidal resistive magneto-hydrodynamic plasma response model, involving large magnetic islands, is proposed and numerically investigated, based on local flattening of the equilibrium pressure profile near a rational surface. It is assumed that such islands can be generated near the edge of the tokamak plasma, due to the penetration of the resonant magnetic perturbations, used for the purpose of controlling the edge localized mode. Within this model, it is found that the local flattening of the equilibrium pressure helps to mitigate the toroidal curvature induced screening effect [Glasser et al., Phys. Fluids 7, 875 (1975)]—the so called Glasser-Greene-Johnson screening, when the local toroidal flow near the mode rational surface is very slow (for example, as a result of mode locking associated with the field penetration). The saturation level of the plasma response amplitude is computed, as the plasma rotation frequency approaches zero. The local modification of the plasma resistivity inside the magnetic island is found to also affect the saturation level of the plasma response at vanishing flow.

Magnetic Field Data Correction in Space for Modelling the Lithospheric Magnetic Field

The Earth’s magnetic field as it is measured by low-Earth orbit satellites such as Swarm and CHAMP results from the superposition of internal and external source fields overlapping in time and in space. The Earth’s lithospheric field is one of the weakest sources detectable from space and its accurate description requires treatments of rapidly-varying magnetic fields generated by current systems in the ionosphere and magnetosphere. In this paper, we review methods most commonly used in geomagnetism to identify and then to correct for the external perturbation fields at satellite altitudes. We document the pros and cons of Fourier Filtering, polynomial and Spherical Harmonics analyses, Singular Spectral Analysis (SSA) and Line-levelling techniques. The difficulties are illustrated with an application of the methods on a common set of real Swarm magnetic field measurements and with a discussion on the differences between lithospheric field models obtained with each treatment. We finally discuss some perspectives for improvements of external field correction techniques relying on statistical or more explicit assumptions about the geographical distribution as well as the shape and strengths of the external magnetic field structures.

Screening of external magnetic perturbation fields due to sheared plasma flow

Within the single fluid resistive magnetohydrodynamic model, systematic toroidal modelling efforts are devoted to investigate the plasma response induced screening of the applied external 3D magnetic field perturbations in the presence of sheared toroidal flow. One particular issue of interest is addressed, when the local flow speed approaches zero at the perturbation rational surface inside the plasma. Subtle screening physics, associated with the favourable averaged toroidal curvature effect (the GGJ effect (Glasser et al 1975 Phys. Fluids 7 875)), is found to play an essential role during slow flow near the rational surface by enhancing the screening at reduced flow. A strong cancellation effect between different terms of Ohm’s law is discovered, leading to different screening physics in the GGJ regime, as compared to that of conventional screening of the typical resistive-inertial regime occurring at faster flow. These modelling results may be applicable to interpret certain mode locking experiments, as well as type-I edge localized mode suppression experiments, with resonant magnetic field perturbations being applied to tokamak plasmas at low input toroidal torque.

The 2Πg shape resonance of acetylene anion: an investigation with the RAC method

Recently developed method of regularized analytic continuation (RAC) is applied to determination of the 2 Π g resonance of acetylene anion. The method is based on continuation of the electron affinities calculated for the anion in presence of an external perturbation field. Its independence on the correlation treatment of the many-electron system allows application of accurate coupled-clusters methods for electronic structure calculations utilized in determination of the resonance position and width.