Jan Raagaard Petersen

Scalar Calibration of Vector Magnetometers

The calibration parameters of a vector magnetometer are estimated only by the use of a scalar reference magnetometer. The method presented in this paper differs from those previously reported in its linearized parametrization. This allows the determination of three offsets or signals in the absence of a magnetic field, three scale factors for normalization of the axes and three non-orthogonality angles which build up an orthogonal system intrinsically in the sensor. The advantage of this method compared with others lies in its linear least squares estimator, which finds independently and uniquely the parameters for a given data set. Therefore, a magnetometer may be characterized inexpensively in the Earths magnetic-field environment. This procedure has been used successfully in the pre-flight calibration of the state-of-the-art magnetometers on board the magnetic mapping satellites Orsted, Astrid-2, CHAMP and SAC-C. By using this method, full-Earth-field-range magnetometers (± 65536.0 nT) can be characterized down to an absolute precision of 0.5 nT, non-orthogonality of only 2 arcsec and a resolution of 0.2 nT.

Digital fluxgate magnetometer for the Astrid-2 satellite

The design and performance of the Astrid-2 magnetometer are described. The magnetometer uses mathematical routines, implemented by software for commercially available digital signal processors, to determine the magnetic field from the fluxgate sensor. The sensor is from the latest generation of amorphous metal sensors developed by the Department of Automation at the Technical University of Denmark.

Transverse field effect in fluxgate sensors

Abstract A model of a fluxgate magnetometer based on the field interactions in the fluxgate core has been derived. The non-linearity of the ringcore sensors due to large uncompensated fields transverse to the measuring axis are calculated and compared with measurements. Measurements of the non-linearity are made with a spectrum analyzer measuring the higher harmonics of an applied sinusoidal field. For a sensor with a permalloy ringcore of 1 in. in diameter the deviation from linearity is measured to about 15 nT p-p in the earths field and the measurements are shown to fit well the calculations. Further, the measurements and the calculations are also compared with a calibration model of the fluxgate sensor onboard the ‘MAGSAT’ satellite. The later has a deviation from linearity of about 50 nT p-p but shows basically the same form of non-linearity as the measurements.

Digital detection of the flux-gate sensor output signal

This paper describes an experiment where the flux-gate sensor broad band output signal is digitized with 8-bit resolution at a high rate, and subsequently numerically analysed in order to extract information on the external magnetic field. The results show that it is feasible to obtain a noise level of 1 nT for a data output rate of 100 Hz. The method will allow construction of a magnetometer based entirely on digital techniques, with only a minimum of analogue circuits.

Temperature dependence of the magnetostriction and the induced anisotropy in nanocrystalline FeCuNbSiB alloys, and their fluxgate properties

Making use of the stress induced magnetic anisotropy in some iron-rich FeCuNbSiB nanocrystalline materials we studied the thermal dependence of their magnetostriction which becomes zero below the Curie temperature. The choice of a suitable composition and annealing temperature results in materials with zero magnetostriction at room temperature. Due to the low magnetostriction these materials have very promising fluxgate properties which were studied as well. >

A.c. magnetic-field measurement using the fluxgate

Abstract Fluxgate sensors are mostly used in closed-loop d.c. magnetometer systems; they can also measure alternating fields up to several kilohertz, either in open-loop mode or from an error signal in the slow-feedback loop as in the Thunderstorm rocket magnetometer, which has 0.1 nT resolution up to 3 kHz. The alternative is to use the direct induction effect in the pick-up or feedback coil. While the low L/R constant of the pick-up coil causes a high −3 dB frequency corner, the spherical feedback coil has narrow frequency characteristics and low noise up to 10 kHz when used as a search coil. The noise level achieved is 56 pT r.m.s. from 123 Hz to 10 kHz.

The noise of the Vacquier type sensors referred to changes of the sensor geometrical dimensions

One of the most important properties of the magnetic sensors is the low noise. This contribution focuses on the effects on the Vacquier type sensor noise performance determined by changes of the sensor geometrical dimensions: the length of the magnetic cores; the length of the detector coil; the length of the excitation coil. The results of this study are applied for the optimisation of the Vacquier type fluxgate sensors.

Cryogen-free dissolution dynamic nuclear polarization polarizer operating at 3.35 T, 6.70 T, and 10.1 T

PURPOSE A novel dissolution dynamic nuclear polarization (dDNP) polarizer platform is presented. The polarizer meets a number of key requirements for in vitro, preclinical, and clinical applications. METHOD It uses no liquid cryogens, operates in continuous mode, accommodates a wide range of sample sizes up to and including those required for human studies, and is fully automated. RESULTS It offers a wide operational window both in terms of magnetic field, up to 10.1 T, and temperature, from room temperature down to 1.3 K. The polarizer delivers a 13 C liquid state polarization for [1-13 C]pyruvate of 70%. The build-up time constant in the solid state is approximately 1200 s (20 minutes), allowing a sample throughput of at least one sample per hour including sample loading and dissolution. CONCLUSION We confirm the previously reported strong field dependence in the range 3.35 to 6.7 T, but see no further increase in polarization when increasing the magnetic field strength to 10.1 T for [1-13 C]pyruvate and trityl. Using a custom dry magnet, cold head and recondensing, closed-cycle cooling system, combined with a modular DNP probe, and automation and fluid handling systems, we have designed a unique dDNP system with unrivalled flexibility and performance.

Reply to comment on 'Scalar calibration of vector magnetometers' by V G Semenov

This is a reply to a comment on our paper. The comment suggests an inconsistency in the relation between the magnetic field and the magnetometer measurements. This is resolved, once the existence of two physically different sensor systems is appreciated, as has also been discussed in some detail in our paper.