Precise Magnetic Field Measurements Research Articles

Precise Measurement of Magnetic Shielding Residual Field and Light Shift Based on 87Rb Atomic Magnetometer

Precise Measurement of Magnetic Shielding Residual Field and Light Shift Based on ⁸⁷Rb Atomic Magnetometer

Dissipative Acousto-optic Interactions in Optical Microcavities.

We propose and demonstrate experimentally the strong dissipative acousto-optic interaction between a suspended vibrating microfiber and a whispering-gallery microcavity. On the one hand, the dissipative response driven by an external stimulus of acoustic waves is found to be stronger than the dispersive response by 2 orders of magnitude. On the other hand, dead points emerge with the zero dissipative response at certain parameters, promising the potentials in physical sensing such as precise measurements of magnetic field and temperature. The strong dissipative acousto-optic interaction is then explored for ultrasensitive detection of broadband acoustic waves. A noise equivalent pressure as low as 0.81Pa at 140kHz in air is demonstrated experimentally, insensitive to cavity Q factors and does not rely on mechanical resonances.

Heading errors in all-optical alkali-metal-vapor magnetometers in geomagnetic fields

The authors present several methods for heading error suppression in all-optical alkali-vapor magnetometers in geomagnetic field ranges. These improvements could result in more precise measurements of Earth's magnetic field and the development of compact and miniaturized sensors.

Magnetic Field Measurement, Shimming and Processing for 230 MeV Superconducting Cyclotron Main Magnet

A 230 MeV compact superconducting cyclotron CYCIAE-230 is being constructed by the China Institute of Atomic Energy, which can be used for proton therapy and space radiation physics research. The technology of magnetic field measurement, shimming and processing is the primary task to realize the isochronous acceleration of cyclotron. The CYCIAE-230 has high magnetic field, high field gradient and dense rotation orbits. Therefore, the isochronous field and tune diagram need to be amended simultaneously. The first harmonic is also shimmed at the same process to ensure a good extraction efficiency by using precessional extraction. The precision of field measurement, shimming and processing is much higher and the algorithm is much more difficult than those in normal temperature cyclotron. In addition, the compact structure of the magnet has very narrow installation space, which makes the design of the mapper more difficult. The process of main magnet construction includes strict control of room temperature and electromagnetic compatibility (EMC), precise measurement of magnetic field, comprehensive research of beam dynamics and multi-dimensional numerical control (NC) processing under the control of complex algorithms. In this paper, the magnetic field measurement, shimming and processing of the main magnet will be illustrated in detail.

Magnetic shielding of long paraboloid structures in the inhomogeneous magnetic field

Shielding efficacy of the high-temperature superconducting (HTS) magnetic shields depends on the superconductor properties and on the orientation of the external magnetic field. For precise magnetic field measurements in areas with changing direction of magnetic noise it is important to reduce both the parallel and perpendicular components of the magnetic field. We have designed and fabricated magnetic shields of 25 cm long paraboloid shape with closed sides from second-generation HTS tapes. We have characterized HTS shields in DC and variable frequency AC magnetic fields at 77 K above a copper electromagnet acting as the source of inhomogeneous magnetic noise. The HTS magnetic shields reduce the magnetic field noise penetration and enhance the sensitivity of magnetic field sensors. The measurements were performed with the magnetic shield placed between the noise source and the sensor. 2D finite element analysis using Comsol model was generated and the results were compared with the experimental data of magnetic field dependences of the shielding factor (SF).

Generation of bright polarization squeezed light at cesium D<sub>2</sub> line based on optical parameter amplifier

Quantum light field is very important source in quantum optics and quantum precision measurement, and the generation of quantum state of light is significant in quantum storage, quantum metrology and studying the interaction between nonclassical light and matter. The polarization squeezed light near the atomic transition has great potential applications in the precise measurement of magnetic field as its Stokes parameter’s noise is less than the standard quantum limit (SQL). Therefore, it is very important to generate the polarization squeezed light at atomic transition. We report in this paper the experiment on generating the bright polarization squeezed light at cesium D<sub>2</sub> line based on an optical parametric amplifier (OPA). The experimental system includes the following three parts: 1) a second harmonic generator (SHG), 2) an OPA, and 3) a detection system. The OPA has a similar structure to the SHG system with four-mirror ring cavity in which only the fundamental wave is resonant. A nonlinear type-I periodically-poled KTiOPO4 (PPKTP) crystal with a size of 1 mm × 2 mm × 20 mm is placed in the center of the cavity waist and its temperature is precisely controlled. The OPA is pumped by the 426 nm blue light which is generated by SHG and this OPA is operating below the threshold. The squeezed light at cesium D<sub>2</sub> line is produced when the crystal temperature is at its optimum phase-matching temperature and the OPA cavity is stabilized based on resonance. The generated squeezed light is combined with the coherent light on a polarizing beam splitter (PBS) to obtain the polarized squeezed light for either <inline-formula><tex-math id="M4">\begin{document}${\hat S_2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M4.png"/></alternatives></inline-formula> or <inline-formula><tex-math id="M5">\begin{document}${\hat S_3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M5.png"/></alternatives></inline-formula> of the Stokes parameter by controlling the type of squeezed light (parametric amplification or de-amplification) and the relative phase (0 or π/2) of two beams. And for <inline-formula><tex-math id="M6">\begin{document}${\hat S_1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M6.png"/></alternatives></inline-formula>, the amplitude-squeezed light (corresponding to parametric de-amplification) is the <inline-formula><tex-math id="M7">\begin{document}${\hat S_1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191009_M7.png"/></alternatives></inline-formula> squeezed light. The maximum squeezing of 4.3 dB (actually 5.2 dB) is observed in a bandwidth range of 2-10 MHz. At present, the squeezing is mainly limited by the escape efficiency of OPA and the detection efficiency, and the OPA escape efficiency is mainly limited by the blue-light-induced loss of PPKTP crystal and the thermal effect of crystal. In the optical atomic magnetometer, increasing the signal-to-noise ratio (SNR) of the system can effectively improve the sensitivity of the magnetic field measurement. This bright polarization squeezed light is expected to be used in the optical cesium atomic magnetometer to improve the sensitivity of the magnetometer.

Development of a CW-NMR Probe for Precise Measurement of Absolute Magnetic Field

Muonium spectroscopy experiment using microwave and J-PARC E34 (J-PARC muon g-2/EDM) experiments aim to measure the hyperfine structure of muonium and anomalous magnetic moment of muon, respectively. Both experiments require homogeneous magnetic fields better than 1 ppm. Therefore, we developed a standard probe to calibrate field measurement probes, with a resolution of 1.92 $\pm$ 0.18 ppb. Moreover, a shift of the nuclear magnetic resonance frequency of the standard probe was also evaluated, and a total uncertainty of the shift is $\pm$ 17.7 ppb.

Probing Magnetic Fields with Square Kilometre Array and its Precursors

Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has hampered its study. The first phase of Square Kilometre Array (SKA-I), will have more than an order of magnitude higher sensitivity than existing radio telescopes. In this contribution, we discuss specific science cases that are of interest to the Indian community concerned with astrophysical turbulence and magnetic fields. The SKA-I will allow observations of a large number of background sources with detectable polarisation and measure their Faraday depths (FDs) through the Milky Way, other galaxies and their circum-galactic medium. This will probe line-of-sight magnetic fields in these objects well and provide field configurations. Detailed comparison of observational data with models which consider various processes giving rise to field amplification and maintenance will then be possible. Such observations will also provide the coherence scale of the fields and measure its random component. Measuring the random component is important to characterise turbulence in the medium. Observations of FDs with redshift will provide important information on magnetic field evolution as a function of redshift. The background sources could also be used to probe magnetic fields and its coherent scale in galaxy clusters and in bridges formed between interacting galaxies. Other than FDs, sensitive observations of synchrotron emission from galaxies will provide complimentary information on their magnetic field strengths in the sky plane. The core shift measurements of AGNs can provide more precise measurements of magnetic field very close (<pc) to the black hole and its evolution. The low band of SKA-I will also be useful to study circularly polarized emission from Sun and comparing various models of field configurations with observations.

Evaluation of using R-SCHA to simultaneously model main field and secular variation multilevel geomagnetic data for the North Atlantic

One efficient approach to modelling the Earth’s core magnetic field involves the inclusion of crossover marine data which cover areas lacking in observatory and repeat station data for epochs when precise three-component satellite magnetic field measurements were not common. In this study, we show how the Revised Spherical Cap Harmonic Analysis (R-SCHA) can appropriately provide a continuous-time field model for the North Atlantic region by using multilevel sets of geomagnetic data such as marine, repeat station, observatory, and satellite data. Taking advantage of the properties of the R-SCHA basis functions we can model the radial and horizontal variations of the main field and its secular variation with the most suitable spatial and temporal wavelengths. To assess the best compromise between the data fit and the model roughness, temporal and spatial regularization matrices were implemented in the modelling approach. Two additional strategies were also used to obtain a satisfactory regional model: the opportunity to fit the anomaly bias at each observatory location, and constraining the regional model to the CHAOS-6 model at the end of its period of validity, i.e. 1999–2000, allowing a smooth transition with the predictions of this recent model. In terms of the root mean square error, the degree of success was limited partly because of the high uncertainties associated with some of the datasets (especially the marine ones), but we have produced a model that performs comparably to the global models for the period 1960–2000, thus showing the benefits of using this regional technique.

Strong ionospheric field‐aligned currents for radial interplanetary magnetic fields

AbstractThe present work has investigated the configuration of field‐aligned currents (FACs) during a long period of radial interplanetary magnetic field (IMF) on 19 May 2002 by using high‐resolution and precise vector magnetic field measurements of CHAMP satellite. During the interest period IMF By and Bz are weakly positive and Bx keeps pointing to the Earth for almost 10 h. The geomagnetic indices Dst is about −40 nT and AE about 100 nT on average. The cross polar cap potential calculated from Assimilative Mapping of Ionospheric Electrodynamics and derived from DMSP observations have average values of 10–20 kV. Obvious hemispheric differences are shown in the configurations of FACs on the dayside and nightside. At the south pole FACs diminish in intensity to magnitudes of about 0.1 μA/m2, the plasma convection maintains two‐cell flow pattern, and the thermospheric density is quite low. However, there are obvious activities in the northern cusp region. One pair of FACs with a downward leg toward the pole and upward leg on the equatorward side emerge in the northern cusp region, exhibiting opposite polarity to FACs typical for duskward IMF orientation. An obvious sunward plasma flow channel persists during the whole period. These ionospheric features might be manifestations of an efficient magnetic reconnection process occurring in the northern magnetospheric flanks at high latitude. The enhanced ionospheric current systems might deposit large amount of Joule heating into the thermosphere. The air densities in the cusp region get enhanced and subsequently propagate equatorward on the dayside. Although geomagnetic indices during the radial IMF indicate low‐level activity, the present study demonstrates that there are prevailing energy inputs from the magnetosphere to both the ionosphere and thermosphere in the northern polar cusp region.

3D Magnetic Field Sensor Concept for Use in Inertial Measurement Units (IMUs)

We report on the design, fabrication, and characterization of a microfabricated 3D magnetic field sensor that is suitable for co-integration with inertial sensors to form single-chip inertial measurement units. In contrast to classical resonant MEMS magnetometers, which are based on Lorentz force measurement, our sensor uses permanent magnetic materials and piezoresistive detection with silicon strain gauges of nanometric section, leading to low power consumption and high sensitivity for small sensor size. Thin multilayers of CoFe and PtMn as ferro- and antiferromagnetic materials are integrated within the MEMS fabrication process. Sensitivities of 1.09 V/T for x- and y- components of the magnetic field and 0.124 V/T for z- component of the magnetic field were measured, respectively. To be sensitive to magnetic fields along all three spatial directions, two permanent magnetization directions on the same die are required. Implementation of the two magnetization directions was validated by a measured correlation of 99.7% between x- and y- sensitivity axes. Power consumption of the 3D sensor is for polarization with a 100 μA dc current. With resolutions of 100 nT/√Hz for x- and y-component of the magnetic field and 350 nT/√Hz for z- component, the sensor is suitable for precise measurement of earth magnetic field.

Single-crystal magneto-optic sensor with electrically adjustable sensitivity

A novel magneto-optic sensor with electrically adjustable sensitivity is proposed that is based on the approximate multiplication correlation between the linear electro-optic phase retardation and the Faraday magneto-optic rotation angle in a single bismuth germanate crystal. The measurement sensitivity and its temperature stability, linear and monotonic measurement ranges of the proposed sensor can be controlled in real time by adjusting the modulating voltage applied to the sensing crystal. In particular, the proposed sensor can be used for the precise measurement of dc magnetic field or dc current. The basic sensing performance is theoretically analyzed and experimentally demonstrated by dc current measurement.

Polarization rotation under two-photon Raman resonance for magnetometry

The polarization rotation and coherent population trapping signal arising due to two photon process using linearly polarized light are found to be significantly enhanced for a Zeeman degenerate system. The zero crossing of the dispersive profile is found to be shifting proportional to the applied magnetic field, albeit the absorptive profile position remains invariant for a slightly imbalanced orthogonal circular polarization component. It provides an alternative method for precise measurement of vector magnetic field without requirement of a bias field. The use of polarization rotation signal for magnetic field measurement offers added advantage due to improved signal to noise ratio.

Precision Signal Conditioning Electronics for Cryogenic Temperature and Magnetic Field Measurements in SST-1

As a part of refurbishment of Steady-state Superconducting Tokamak-1 (SST-1) at the Institute for Plasma Research, India, all 16 toroidal field (TF) magnets of SST-1 have been individually tested in cold with nominal current in a dedicated experimental cryostat. The magnets were cooled down to 4.5 K using either supercritical or two-phase helium, after which they were charged up to 10 kA of transport current. Precise cryogenic temperature and magnetic field measurements in the experimental configuration were mandatory. Temperatures were required to be accurately measured at several locations in the magnet and hydraulic circuits, whereas the field measurements were carried out at few predefined locations on the magnet case. Highly accurate in-house modular signal conditioning electronics had been developed for low temperature and high magnetic field measurements for such large magnets. This paper describes the scheme of the measurement, precautions taken for enhanced reliability, precautions taken for long-term offset stability in cryogenic environment, and the results of SST-1 TF magnet tests.

Revisiting the Rigidly Rotating Magnetosphere model for Ori E - I. Observations and data analysis★

We have obtained 18 new high-resolution spectropolarimetric observations of the B2Vp star sigma Ori E with both the Narval and ESPaDOnS spectropolarimeters. The aim of these observations is to test, with modern data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki (2005), applied to the specific case of sigma Ori E by Townsend et al. (2005). This model includes a substantially offset dipole magnetic field configuration, and approximately reproduces previous observational variations in longitudinal field strength, photometric brightness, and Halpha emission. We analyze new spectroscopy, including H I, He I, C II, Si III and Fe III lines, confirming the diversity of variability in photospheric lines, as well as the double S-wave variation of circumstellar hydrogen. Using the multiline analysis method of Least-Squares Deconvolution (LSD), new, more precise longitudinal magnetic field measurements reveal a substantial variance between the shapes of the observed and RRM model time-varying field. The phase resolved Stokes V profiles of He I 5876 A and 6678 A lines are fit poorly by synthetic profiles computed from the magnetic topology assumed by Townsend et al. (2005). These results challenge the offset dipole field configuration assumed in the application of the RRM model to sigma Ori E, and indicate that future models of its magnetic field should also include complex, higher-order components.

Hinode SOT/SP and SoHO/MDI quiet Sun magnetic field. Implications of their differences on the extrapolated chromospheric field and the height of the magnetic canopy

Aims. We explore the differences in the measurement of the magnetic field of the quiet solar photosphere provided by the Michelson Doppler Imager (MDI) onboard SoHO and the SpectroPolarimeter (SOT/SP) onboard Hinode and the ensuing implications for the extrapolated chromospheric magnetic field and the determination of the location of the magnetic canopy. Methods. We employ potential field extrapolation to reconstruct the chromospheric magnetic field using the magnetic field of the photosphere provided by the two instruments. We also calculate the plasma-β parameter using the VAL C model atmosphere of the quiet Sun to determine the height of the magnetic canopy. Results. MDI underestimates the magnetic field of the quiet Sun sometimes by a factor of five, which leads to an overestimation of the height of the magnetic canopy by up to ∼550km. Although the overall magnetic field configuration does not differ significantly when calculated with either MDI or SOT/SP, the data of the latter lead to lower and more extended canopies. The difference in the resolution of the two instruments does not seem to affect the chromospheric magnetic field higher than 1000km. Conclusions. The height of the magnetic canopy is an important parameter to consider when investigating wave propagation and the oscillatory properties of the quiet Sun regions in the network and internetwork. The canopy height’s derivation depends very much on the sensitivity of the instruments used to measure the photospheric magnetic field. Consequently precise measurements of the photospheric magnetic field are crucial to accurately reconstruct the chromospheric magnetic field and to distinguish between the various wave modes.

Test of an Electromagnetic Shimming Concept for Superconducting Undulators

Accurate magnetic field measurements are a prerequisite for the characterization and optimization of undulators. At the synchrotron light source ANKA there is an ongoing R&D program for superconducting insertion devices. Within this program a test stand for precise magnetic field measurements of superconductive coils was designed, built and installed at the Research Center Karlsruhe. Comprehensive magnetic measurements are performed to confirm the magnet design and to detect or correct field imperfections resulting from the manufacturing process. The magnetic field is measured along a fixed direction by Hall probes with a positioning precision of 3 mum. A NbTi undulator prototype with local correction coils was built to verify the performance of the superconductor and the shimming concept. In this contribution we describe the measuring test station and preliminary tests results including the efficiency of the shimming concept.

Solar zenith angle and merging electric field control of field‐aligned currents: A statistical study of the Southern Hemisphere

High‐resolution and precise vector magnetic field measurements of the CHAMP satellite are used to investigate the field‐aligned currents (FACs) in the southern polar ionosphere. The period considered comprises 2 years, providing a double coverage of the seasons and about a six‐fold coverage of all local times. From more than 11,000 polar passes the average spatial pattern of FACs in the polar ionosphere is derived. The response of features like intensity and positions of large‐scale field‐aligned currents to normal (when merging electric field is less than 2 mV/m) and disturbed (when merging electric field is greater than 2 mV/m) conditions in the Southern Hemisphere are investigated. The influence of the solar illumination‐induced conductivity on the morphology features of field‐aligned currents during normal conditions are also studied. It follows from this analysis that the intensity of field‐aligned currents changes with the merging electric field at all MLT sectors but with the solar radiation‐induced conductivity only at the dayside. Furthermore, a linear relation between the conductivity and the peak FAC density exists, which implies that the dayside FAC densities are directly controlled by the amount of solar radiation. Solar elevation does not affect the nightside FAC density. On the dayside a systematic difference of the footprint latitude between sunlit and dark conditions emerges. Under dark conditions the auroral region retreats about 2° equatorward. On the basis of the above results, we may suggest that the sources of Birkeland currents on the zdayside behave like a voltage source, while on the nightside the response is possibly like a current source.

High quality Hall sensors made of heavily doped n-InSb epitaxial films

In this contribution we confirm that heavily donor doped epitaxial layers of InSb are excellent material for the preparation of Hall sensors working in a broad range of temperatures. In particular, we confirm that InSb epitaxial thin films having the donor concentration of (1–2)×10 18 cm −3 have optimal parameters for manufacturing Hall sensors for precise measurements of magnetic field. Such sensors have the magnetic sensitivity K>0.1 V/T, the temperature dependence of the magnetic sensitivity β≤0.01% per degree and the temperature coefficient of the input/output resistance α<0.1% per degree. These parameters are weakly temperature dependent between 77 and 500 K. The characteristics of the presented Hall sensors are compared with those of commercially available Hall sensors and the Hall sensors based on the 2DEG.

Poloidal magnetic field measurements and analysis with the DIII-D LIBEAM system (invited)

For over 30 yrs, neutral lithium beams have been employed as a localized, noninvasive diagnostic on a variety of plasma experiments worldwide, providing a number of key physics measurements. On DIII-D the LIBEAM diagnostic has been designed to provide precise measurements of the local poloidal magnetic field in the edge region, a parameter of basic importance to understanding the stability of high performance tokamaks. We utilize the Zeeman splitting and known polarization characteristics of the collisionally excited 670.8 nm Li resonance line to interpret local magnetic field components viewed using a closely packed (ΔR∼5 mm) array of 32 view chords. A dual photoelastic modulator/linear polarizer combination serves to amplitude modulate the light in exact correspondence to its input polarization state. Subsequent narrowband spectral filtering using etalons and standard interference filters is used to isolate one of the three Zeeman components, and the polarization state of that component is recovered using a PC-based, multichannel digital lock-in detection system. Edge magnetic pitch angle profiles for a variety of shots have been reconstructed using a small number of chords and detailed analysis of the lock-in and dc signal levels. Present system performance appears to be limited by etalon performance as well as various broadening mechanisms in the beam that tend to decrease the polarization fraction in the observed component. A careful analysis of this effect and some strategies for improving the measured polarization will be presented.