Measurements Of Magnetic Fluctuations Research Articles

New high-field side magnetic probe array system for three-dimensional magnetic fluctuation measurements on EAST

To better study the magnetic fluctuations in the high-field side of EAST tokamak, a new high-field side magnetic probe array (HFS-MPA) has been developed on EAST recently. The HFS-MPA consists of 12 identical three-dimensional (3D) magnetic probes, which are mounted on the HFS wall with carefully designed arrangement. The HFS-MPA magnetic probes bring additional toroidal magnetic fluctuation measurements compared with the HFS regular magnetic probes which can only provide poloidal and radial magnetic fluctuation measurements. The upper limits of frequency and toroidal mode number (n) measurements of the magnetic fluctuations have been improved by comparing HFS-MPA with the HFS regular magnetic probes, i.e., 650 kHz vs 100 kHz and n = 23 vs n = 1. In developing the HFS-MPA diagnostic system, many practical challenges have been overcome and many special designs have been developed. These will be mentioned in the main subsystem description of the HFS-MPA diagnostic in this paper, which might be useful for developing new magnetic probe diagnostics in the future on EAST or other magnetically confined fusion devices. The calibration of the effective area and frequency response of the HFS-MPA is also described. The preliminary application in studying the frequency and propagation characteristics of the magnetic fluctuations with HFS-MPA compared with EAST regular magnetic probes shows that the HFS-MPA is well developed for plasma physics studies.

Design of radial interferometer-polarimeter for internal magnetic and density fluctuation measurements at multiple space-time scales in the National Spherical Torus Experiment-Upgrade (NSTX-U).

A Faraday-effect radial interferometer-polarimeter is designed for the National Spherical Torus Experiment-Upgrade (NSTX-U) to measure multiscale magnetic and density fluctuations critical to understanding fusion plasma confinement and stability, including those originating from magnetohydrodynamic instabilities, energetic particle-driven modes, and turbulence. The diagnostic will utilize the three-wave technique with 5MHz bandwidth to simultaneously measure line-integrated magnetic and density fluctuations up to the ion-cyclotron frequency. Probe beams will be launched radially from the low-field side at the NSTX-U midplane, where the measured Faraday fluctuations mainly correspond to radial magnetic fluctuations that directly link to magnetic transport. A correlation technique will be employed to reduce the measurement noise to below 0.01° enabling detection of small amplitude fluctuations. Two toroidally displaced chords with 7° separation will be installed to measure toroidal mode numbers up to n = 25 for mode identification. Solid-state microwave sources operating at 321μm (935GHz) will be used to minimize the impact of the Cotton-Mouton effect.

Quasi-optical design for the cross-polarization scattering diagnostic on the HL-3 tokamak.

As the plasma beta (β) increases in high-performance tokamaks, electromagnetic turbulence becomes more significant, potentially constraining their operational range. To investigate this turbulence, a cross-polarization scattering (CPS) diagnostic system is being developed on the HL-3 tokamak for simultaneous measurements of density and magnetic fluctuations. In this work, a quasi-optical system has been designed and analyzed for the Q-band CPS diagnostic. The system includes a lens group for beam waist size optimization, a rotatable wire-grid polarizer for polarization adjustment, and a reflector group for measurement range regulation and system response enhancement. Laboratory tests demonstrated a beam radius of order 4cm at the target measurement location (near the plasma pedestal), cross-polarization isolation exceeding 30 dB, and poloidal and toroidal angle adjustment ranges of ±40° and ±15°, respectively. These results verify the system's feasibility through laboratory evaluations. The quasi-optical system has been installed on the HL-3 tokamak during the 2023 experimental campaign to support the development of CPS diagnostics.

RF antenna helicity dependent particle heating in a helicon source

Experiments have demonstrated that ion phenomena, such as the lower hybrid resonance, play an important role in helicon source operation. Damping of the slow branch of the bounded whistler wave at the edge of a helicon source (i.e. the Trivelpiece-Gould mode) has been correlated with the creation of energetic electrons, heating of ions at the plasma edge, and anisotropic ion heating. Here we present ion velocity distribution function measurements, electron density and temperature measurements, and magnetic fluctuation measurements on both sides of an m=|1| helical antenna in a helicon source as a function of the driving frequency, magnetic field strength, and magnetic field orientation relative to the antenna helicity. Significant electron and ion heating (up to two times larger) occurs on the side of the antenna consistent with the launch of the m=+1 mode. The electron and ion heating occurs within one electron skin depth of the plasma edge, where slow wave damping is expected. The source parameters for enhanced particle heating are also consistent with lower hybrid resonance effects, which can only occur for Trivelpiece-Gould wave excitation.

A new electromagnetic probe array diagnostic for analyzing electrostatic and magnetic fluctuations in EAST plasmas

A novel electromagnetic probe array (EMPA) diagnostic, which consists of a magnetic probe array and an electrostatic probe array, has recently been developed on EAST. The EMPA is fixed near the first wall at horizontal port P. The magnetic probe array of the EMPA consists of 24 identical magnetic probes, each of them capable of measuring toroidal, poloidal and radial magnetic fluctuations simultaneously, providing additional toroidal magnetic fluctuation measurements compared with the regular magnetic probes on EAST. With a higher sampling rate and self-resonant frequency, the EMPA magnetic probes can provide higher frequency magnetic fluctuation measurements. The magnetic probe array of the EMPA is composed of two parallel layers of magnetic probes with a radial distance of 63 mm, and each layer of magnetic probes is arranged in four poloidal rows and three toroidal columns. The compact arrangement of the EMPA magnetic probe array largely improves the toroidal mode number measurement ability from to and also improves the high poloidal wave number measurement ability of magnetic fluctuations compared with the regular high frequency magnetic probes on EAST. The electrostatic probe array of the EMPA consists of two sets of four-tip probes and a single-tip probe array with three poloidal rows and four toroidal columns. It complements the electrostatic parameter measurements behind the main limiter and near the first wall in EAST. The engineering details of the EMPA diagnostic, including the mechanical system, the electrical system, the acquisition and control system, and the effective area calibration, are presented. The preliminary applications of the EMPA in L-mode and H-mode discharges on EAST have demonstrated that the EMPA works well for providing information on the magnetic and electrostatic fluctuations and can contribute to deeper physical analysis in future EAST experiments.

Electro-optic Bdot probe measurement of magnetic fluctuations in plasma.

We propose a combined use of a Pockels electro-optic sensor with a pickup loop coil (Bdot probe) for the measurement of magnetic fluctuations in plasmas. In this method, induced fluctuating voltage on the coil loop is converted into an optical signal by a compact electro-optic sensor in the vicinity of the measurement point and is transferred across optical fiber that is unaffected by electric noise or capacitive load issues. Compared with conventional Bdot probes, the electro-optic Bdot probe (1) is electrically isolated and free from noise pickup caused by the metallic transmission line and (2) can be operated at a higher-frequency range because of the smaller capacitance of the operation circuit, both of which are suitable for many plasma experiments. Conversely, the sensitivity of the current electro-optic Bdot probe arrangement is still significantly lower than that of conventional Bdot probes. A preliminary measurement result with the electro-optic Bdot probe showed the detection of a magnetic fluctuation signal around the cyclotron frequency range in the RT-1 magnetospheric plasma experiment.

A Highly Sensitive In Situ Magnetic Field Fluctuation Measurement Method Based on Nuclear-Spin Depolarization in an Atomic Comagnetometer

In this article, a highly sensitive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> magnetic field fluctuation measurement method is proposed based on nuclear-spin depolarization in a spin-exchange relaxation-free (SERF) atomic comagnetometer (ACM). The steady-state model of the hybrid atomic spin ensemble is systematically studied. According to the theoretical model, two different states of nuclear-spin polarization and unpolarization lead to different sensitivities of the system to rotation and magnetic fields. The gradient magnetic field (GMF) is employed to change the state of nuclear-spin polarization of the noble gas, transforming the ACM into an atomic magnetometer (AM). The polarization and relaxation rates of the electron spin and nuclear spin under the two measurement modes are measured and compared by the magnetic resonance method and transient response method. At the same time, the performances in the ACM and AM are tested. Detailed experiments verify that the AM can achieve sensitivity for the magnetic field measurement with an average value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$53.6~\mathbf {fT/Hz^{1/2}}$ </tex-math></inline-formula> in the frequency range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10\sim 30$ </tex-math></inline-formula> Hz. The Allan variance method is adopted to evaluate the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -axis magnetic field stabilities of the ACM, and the magnitudes are 61.6 and 55.9 fT at 100 s, contributing about 4.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10\mathbf {^{-3}}\,\,\mathbf {^{\circ }/h}$ </tex-math></inline-formula> and 2.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10\mathbf {^{-4}}\,\,\mathbf {^{\circ }/h}$ </tex-math></inline-formula> to the long-term stability of the ACM. This method can not only realize the high-sensitivity <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> measurement of the close-to-zero magnetic field and its fluctuation but also realize the switching of the two measurement modes of rotation and magnetic field measurements, which is essential for the evaluation of magnetic field error.

Multi‐Spacecraft Measurement of Anisotropic Spatial Correlation Functions at Kinetic Range in the Magnetosheath Turbulence

AbstractWe investigate spatial correlation functions (SCFs) at kinetic range in the terrestrial magnetosheath because of wide angular distributions of the magnetic field fluctuations and the high Signal‐to‐Noise‐Ratio. We employ four‐point measurements of magnetic fluctuations from the Cluster and MMS concerning a spatiotemporally varying background magnetic field to constitute the two‐dimensional (2‐D) SCFs between ion and electron scales. It is found that two populations exist in the 2‐D SCFs, with the minority elongated along the perpendicular coordinate (S⊥) and the majority of population elongated along the coordinate parallel to average magnetic field (S||). This kind of phenomenon indicates that the distribution of magnetosheath turbulence in the wavenumber space is dominantly transverse to the background magnetic field, simultaneously partly along the field (i.e., k⊥ &gt;&gt; k||). In addition, we find that magnetosheath turbulence is the dominance of shear Alfvénic‐like fluctuations, and is quasi‐monofractal intermittent turbulence at kinetic range for both the Cluster case and the MMS case, indicating that strong spatially anisotropic fluctuations may be universal in the magnetosheath. Our observations pose new challenges to the theoretical modeling of turbulence and dissipation at kinetic range in the collisionless magnetized plasma.

The cross-polarization scattering system for the magnetic fluctuation measurement in the Experimental Advanced Superconducting Tokamak.

The cross-polarization scattering (CPS) system for magnetic fluctuation measurements in the Experimental Advanced Superconducting Tokamak (EAST) has been designed and installed. Different from the Doppler reflectometer (DR) system, the CPS system detects the perpendicular polarization of the electromagnetic wave induced by magnetic fluctuations B̃. The CPS system in the EAST has been developed from the existing Doppler reflectometer system, and they are integrated together for simultaneous measurement of magnetic and density fluctuations. Ray-tracing simulations are used to calculate the scattering locations and the wavenumber coverage of the magnetic fluctuation for CPS. In the experiments, the CPS and DR system data were different in Doppler shift, amplitude, and spectrum broadening. In this article, the hardware design, the ray tracing, and the preliminary results of the system in the EAST are presented.

Pedestal magnetic turbulence measurements in ELMy H-mode DIII-D plasmas by Faraday-effect polarimetry

Internal magnetic fluctuation measurements are utilized to identify turbulence associated with micro-tearing modes (MTM) in the DIII-D Edge-Localized-Mode (ELM)-y H-mode pedestal. Using a Faraday-effect polarimeter, magnetic turbulence (150–500 kHz) is directly observed with a typical line-averaged fluctuation amplitude of ∼0.8 G at peak frequency (250 kHz) and ∼15 G integrated over the spectrum from 150 to 500 kHz. Frequency, poloidal wavenumber, and propagation direction of the magnetic turbulence all serve to identify as MTM. Magnetic turbulence amplitude non-monotonically correlates with collision frequency, peaks off mid-plane, and correlates with electron temperature gradient evolution between ELMs, consistent with MTM features identified from theory and gyro-kinetic simulation. The magnetic turbulence growth correlates with confinement degradation in ELMy H-mode plasmas during a slow density ramp. These internal measurements provide unique constraints toward developing physics understanding and validating models of the H-mode pedestal for future devices.

Measurements of Magnetic Field Fluctuations for Plasma Wave Investigation by the Search Coil Magnetometers (SCM) Onboard Bepicolombo Mio (Mercury Magnetospheric Orbiter)

This paper describes the design and performance of the search coil magnetometers (SCM), which are part of the Plasma Wave Investigation (PWI) instrument onboard the BepiColombo/Mio spacecraft (Mercury Magnetospheric Orbiter), which will measure the electric field, plasma waves and radio waves for the first time in Mercury’s plasma environment. The SCM consists of two low-frequency orthogonal search coil sensors (LF-SC) measuring two components of the magnetic field (0.1 Hz – 20 kHz) in the spacecraft spin plane, and a dual-band search coil sensor (DB-SC) picking up the third component along the spin axis at both low-frequencies (LF: 0.1 Hz – 20 kHz) and high-frequencies (HF: 10 kHz – 640 kHz). The DB-SC and the two LF-SC sensors form a tri-axial configuration at the tip of a 4.6-m coilable mast (MAST-SC) extending from the spacecraft body, to minimize artificial magnetic field contamination emitted by the spacecraft electronics. After the successful launch of the spacecraft on 20 October 2018, an initial function check for the SCM was conducted. The nominal function and performance of the sensors and preamplifiers were confirmed, even with the MAST-SC being retracted and stowed in the spacecraft body, resulting in the detection of large interference signals likely from spacecraft electronics. The MAST-SC is scheduled for deployment after the Mercury orbit insertion of Mio in 2025, allowing the SCM to make the first higher frequency measurements of magnetic fluctuations in the Hermean magnetosphere and exosphere, and the local solar wind. These measurements will contribute to the investigation of fundamental problems in the Hermean plasma environment, including turbulence, magnetic reconnection, wave-particle interactions and particle acceleration.

High Resolution Probe for filament transport and current density study at the edge region of W7-X

For the study of electrostatic and magnetic properties of filaments characterizing the edge region of the stellarator experiment W7-X, a specifically designed insertable probe head was constructed within the framework of EUROfusion WP.S1 work package in collaboration between Consorzio RFX, IPP Greifswald and Forschungszentrum Jülich. The probe head, named High Resolution Probe (HRP), was conceived to be installed on the mid-plane multi-purpose fast reciprocating manipulator on W7-X. Electromagnetic filamentary turbulent structures are found to characterize the edge region of different magnetic configurations including Reversed Field Pinch, stellarator and tokamak, where strong currents are associated also to ELM filamentary structures. The study of those phenomena in W7-X stellarator is of particular interest as the electromagnetic features of filaments are expected to become more relevant with the increase of the local plasma beta. In particular the aim is to provide information about the presence and the features of parallel current density associated to filamentary turbulent or ELM-like structures. Furthermore, the possibility to measure the time evolution of the flow radial profiles using the Mach probe array was considered as a further interesting part of the study, given the strong interplay expected between the turbulent fluctuations and the average flows. Further important information provided is the radial propagation of turbulent flux. The contribution will present the design development, the R&D studies and the applied solutions for the sensors embedded in the probe head. In particular, the presence of 140 GHz ECRH plasma environment represents one of the main challenges for reliable magnetic fluctuation measurements. First measurements were performed during the W7-X experimental campaign OP1.2b, where electromagnetic features of turbulence were measured with the probe, at different magnetic configurations.

Correlation polarimetry for broadband magnetic fluctuation measurements.

Correlation techniques have been successfully utilized for plasma diagnostics like electron cyclotron emission to reduce measurement noise. We present the first application of such a technique to Faraday-effect polarimetry measurements on the Madison Symmetric Torus (MST). The MST far infrared (FIR) interferometer-polarimeter diagnostic utilizes 11 vertical chords with a chord separation of 7-8 cm and a heterodyne detection system for fluctuation measurements up to several hundred kHz. The planar-diode mixers viewing each chord represent independent noise sources; modifying the optical setup so that two different mixers view the same chord allows cross correlation between the two independent signals to reduce the noise floor in fluctuation measurements. In this manner, the noise floor in both interferometry and polarimetry measurements in reversed-field pinch discharges has been reduced by a factor of 20-30. The correlation polarimeter provides a sensitive measurement of broadband magnetic fluctuations.

High sensitivity magnetometer using nanocomposite polymers with large magneto-optic response.

Miniaturized magnetic field sensors are increasingly used in various applications, such as geophysical exploration for minerals and oil, volcanology, earthquake studies, and biomedical imaging. Existing magnetometers lack either the spatial or the temporal resolution or are restricted to costly shielded labs and cannot operate in an unshielded environment. Increasing spatio-temporal resolution would allow for real-time measurements of magnetic fluctuations with high resolution. Here we report on a new nanocomposite-based system for miniaturized magnetic field sensing. The sensor is based on Dy3+-doped magnetite and cobalt ferrite nanoparticles dispersed in a polymer matrix. Operation is feasible at room temperature and in an unshielded environment. A compact fiber-optic interferometer is used as the detection mechanism with 20 fT/√Hz sensitivity. We investigated the magnetic field response of the sensor and demonstrated the measurement of the human heartbeat as a potential application.

First step toward a synthetic diagnostic for magnetic fluctuation measurements using cross-polarization scattering on DIII-D

Cross-polarization scattering (CPS) provides localized magnetic fluctuation ( ) measurements in fusion plasmas based on the process where scatters electromagnetic radiation into the orthogonal polarization. The CPS system on DIII-D utilizes the probe beam of a Doppler backscattering (DBS) diagnostic combined with a cross-view CPS receiver system, which allows simultaneous density and measurements with good spatial and wavenumber coverage. The interpretation of the signals is challenging due to the complex plasma propagation of the DBS probe beam and CPS receive beams. A synthetic diagnostic for CPS is therefore essential to interpret data and perform detailed validation tests of non-linear turbulence simulations. This work reports a first step toward a synthetic diagnostic for CPS utilizing GENRAY, a 3-D ray tracing code, to simulate the propagation of the probe and scattered rays. The local wavenumber is calculated from the local O- and X-mode wavenumbers using the wave vector matching scattering condition. The CPS wavenumber values and spatial locations are determined by a complex consideration that includes the local density and level, receive antenna pattern and orientation, scattering volume, wavenumber values detected at the various scattering centers, and alignment of the magnetic wave vector with the plane perpendicular to the magnetic field. The issue of a spurious CPS signal due to polarization mismatches for launch and receive is also discussed. It is suggested that simultaneous O- and X-mode DBS measurements should be utilized for better understanding of the CPS signal contamination when the cutoff locations for both polarizations are close.

Density and magnetic fluctuations in type III-ELM pedestal evolution in JET: experimental and numerical characterization

Density and magnetic fluctuation measurements in low-β type-III ELM discharges are obtained in the Joint European Torus (JET). They are observed during the inter-ELM pedestal evolution, after the LH transition phase, at about 60–70 kHz. Density fluctuations are measured with a correlation reflectometer system installed on the low-field side and they are localized at the pedestal top. Magnetic fluctuations with a spatial scale are measured through a high resolution coil array. The main features and the relations with local plasma parameters are presented. The nature of these fluctuations is discussed along with linear gyrokinetic simulations. Ion temperature gradient (ITG) modes are the dominant instabilities in the frequency range of interest. In terms of radial localization, typical oscillation frequency and qualitative relation with the possible linear drive, ITG modes are consistent with the experimental density fluctuations measurements. Micro-tearing modes (MTMs), found unstable with a lower growth rate, appear a possible explanation for magnetic fluctuations in terms of typical wavenumbers and direction of propagation.

Electromagnetic ELM and inter-ELM filaments detected in the COMPASS Scrape-Off Layer

In fusion devices strong interest is deserved to the edge filament transport both related to turbulent and ELMy structures, because they are believed to provide important interaction with the plasma facing components and divertor plates. Among their features also the electromagnetic (EM) properties are expected to play an important role and in particular for the high beta regimes expected in future devices. The presence of ELM and inter-ELM electromagnetic filaments were detected during H-mode discharges in COMPASS device Scrape-Off Layer, where a new probe head was recently developed and commissioned. The diagnostic allows the simultaneous measurements of electrostatic and magnetic fluctuations with high time resolution, suitable for the identification of EM features of filaments. The method allows in particular the direct measurement of the current density associated to filaments. Detailed electrostatic and electromagnetic features analysis revealed a complex and fragmented structure within a single ELM and strong peaks in parallel current density Jtor are observed to characterize the ELM bunch. Analogous EM structures are observed also in inter-ELM phase, but with more than one order of magnitude lower intensity and time scale.

Simultaneous measurement of magnetic and density fluctuations via cross-polarization scattering and Doppler backscattering on the DIII-D tokamak.

An upgraded cross-polarization scattering (CPS) system for the simultaneous measurement of internal magnetic fluctuations B̃ and density fluctuations ñ is presented. The system has eight radial quadrature channels acquired simultaneously with an eight-channel Doppler backscattering system (measures density fluctuations ñ and flows). 3-D ray tracing calculations based on the GENRAY ray tracing code are used to illustrate the scattering and geometric considerations involved in the CPS implementation on DIII-D. A unique quasi-optical design and IF electronics system allow direct comparison of B̃ and ñ during dynamic or transient plasma events (e.g., Edge Localized Modes or ELMs, L to H-mode transitions, etc.). The system design allows the interesting possibility of both magnetic-density (B̃-ñ) fluctuation and magnetic-temperature (B̃-T̃) fluctuation cross-phase measurements suitable for detailed tests of turbulence simulations.

Measurement of local, internal magnetic fluctuations via cross-polarization scattering in the DIII-D tokamak (invited).

We present new measurements of internal magnetic fluctuations obtained with a novel eight channel cross polarization scattering (CPS) system installed on the DIII-D tokamak. Measurements of internal, localized magnetic fluctuations provide a window on an important physics quantity that we heretofore have had little information on. Importantly, these measurements provide a new ability to challenge and test linear and nonlinear simulations and basic theory. The CPS method, based upon the scattering of an incident microwave beam into the opposite polarization by magnetic fluctuations, has been significantly extended and improved over the method as originally developed on the Tore Supra tokamak. A new scattering geometry, provided by a unique probe beam, is utilized to improve the spatial localization and wavenumber range. Remotely controllable polarizer and mirror angles allow polarization matching and wavenumber selection for a range of plasma conditions. The quasi-optical system design, its advantages and challenges, as well as important physics validation tests are presented and discussed. Effect of plasma beta (ratio of kinetic to magnetic pressure) on both density and magnetic fluctuations is studied and it is observed that internal magnetic fluctuations increase with beta. During certain quiescent high confinement operational regimes, coherent low frequency modes not detected by magnetic probes are detected locally by CPS diagnostics.

Development of a cross-polarization scattering system for the measurement of internal magnetic fluctuations in the DIII-D tokamak.

The design and performance of a new cross-polarization scattering (CPS) system for the localized measurement of internal magnetic fluctuations is presented. CPS is a process whereby magnetic fluctuations scatter incident electromagnetic radiation into a perpendicular polarization which is subsequently detected. A new CPS design that incorporates a unique scattering geometry was laboratory tested, optimized, and installed on the DIII-D tokamak. Plasma tests of signal-to-noise, polarization purity, and frequency response indicate proper functioning of the system. CPS data show interesting features related to internal MHD perturbations known as sawteeth that are not observed on density fluctuations.