Precise Magnetic Field Measurements Research Articles

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.

Seasonal and local time dependences of the interhemispheric field‐aligned currents deduced from the Ørsted satellite and the ground geomagnetic observations

The precise magnetic field measurements by the Ørsted satellite revealed a distinct spatial variation of the eastward component residual (ΔBϕ) at mid and low latitudes on the dayside. This magnetic field variation was attributed to the interhemispheric field‐aligned currents (IHFACs) at mid and low latitudes based on its comparison with ground geomagnetic field variations. These IHFACs were found to flow from the summer hemisphere to the winter hemisphere in the dawn sector, and from the winter hemisphere to the summer hemisphere in the noon and the dusk sectors. These IHFACs flip their direction not at equinoxes but in May and November. The current intensity of the IHFACs is largest not at solstices but in February and August. The ground‐based geomagnetic field observations at 17 low latitude stations also support the seasonal dependence and local time distribution of the IHFACs.

Antisunward net Birkeland current system deduced from the Oersted satellite observation

From the precise magnetic field measurements by the Oersted satellite at low altitudes over various local times, a distinct spatial variation at middle and low latitudes was detected in the eastward component residuals (ΔBϕ) after the subtraction of a geomagnetic main field model. The average ΔBϕ decreases on the dayside as latitude increases, and it increases on the nightside. That is, the latitudinal structure of the ΔBϕ has a negative trend in the northward direction on the dayside and a positive trend on the nightside. The slope of the latitudinal profile of ΔBϕ is steep on both dayside and nightside under geomagnetically disturbed condition and has a high correlation with geomagnetic activity index such as the ap. These results strongly suggest the existence of a day‐night net Birkeland current system that flows into the polar ionosphere on the dayside and flows out on the nightside. These net currents seem to distribute in wide local time range, and the center of the net currents is estimated to be around 1000 LT on the dayside and around 2200 LT on the nightside, which is essentially consistent with previous theoretical predictions and ground geomagnetic observations.

The Advanced Technology Solar Telescope

The Advanced Technology Solar Telescope is the largest solar optical facility currently under development. The National Solar Observatory and its partners have just started the design and development phase with first light being planned for late this decade. The 4-m telescope will provide an angular resolution down to 0.025 arcsec, a large photon flux for precise magnetic and velocity field measurements, and access to a broad set of diagnostics from 0.3 to 28 μm. We summarize the currently envisioned scientific capabilities of the telescope and its suite of instruments along with a glimpse at some of the early concepts.

Field‐aligned currents during IMF ∼0

We report on field‐aligned currents (FAC) obtained from precise magnetic field measurements made on‐board the Ørsted and Magsat satellites when the interplanetary magnetic field (IMF) was near zero. We found that Region 1/Region 2 currents persist through southward IMF, IMF ∼ 0, and northward IMF; also the central area of both the northern and southern polar caps is always filled with FACs, even during slightly southward IMF. A statistical distribution of the latter currents resembles the dayside NBZ system with the downward/upward currents at post‐noon/pre‐noon hours, discovered earlier from Magsat observations. We hypothesize that the quasi‐viscous interaction of the solar wind with the magnetospheric lobes may cause a sunward convection of the lobes' field lines through the bunch's core, effectively mapping the FACs of NBZ‐type down to the near‐pole area.

Precise NMR measurement and stabilization system of magnetic field of a superconducting 7 T wave length shifter

The system of measurement and stabilization of the magnetic field in the superconducting 7 T wave length shifter (WLS), designed at Budker Institute of Nuclear Physics are described. The measurementsare performed by nuclear magnetic resonance (NMR) magnetometer at two points of the WLS magnetic field. Stabilization of the field is provided by the current pumping system. The stabilization system is based on precise NMR measurement of magnetic field as a feedback signal for computer code which control currents inside the superconducting coils. The problem of the magnetic field measurements with NMR method consists in wide spread of field in the measured area (up to 50 Gs/mm), wide temperature range of WLS operating, small space for probe and influence of iron hysteresis. Special solid-state probes were designed to satisfy this requirements. The accuracy of magnetic field measurements at probe locations is not worse than 20 ppm. For the WLS field of 7 T the reproducibility of the magnetic field of 30 ppm has been achieved.

Multiple Phase Transitions in CuO Studied by μSR

Precise measurements of the local magnetic field at muon sites in CuO were performed. The temperature dependence of the local field including spatial orientation was obtained in the ordered state. Not only zero field μSR measurements were performed, but also external magnetic fields were applied to obtain an unambiguous determination of field direction. Multiple stages of muon signals were observed at 55 and 80 K. An incommensurate phase between 212 K and 227 K was confirmed from the line shapes of the local fields at the muon sites.

Field fine tuning by pole height adjustment for the undulator of the TTF–FEL

The field of the undulator for the VUV–FEL at the TESLA Test Facility has to meet very tough tolerances in order to guarantee a close overlap between the electron beam and the laser field. Consequently the undulator was designed to have height-adjustable poles in order to allow for fine tuning of the vertical undulator field in such a way that the trajectory is straightened. The signature of local pole height and gap changes on the field distribution was investigated. It was seen that changes are not restricted to the pole itself. Its effect can be seen up to the next eight neighboring poles. In this contribution we describe an algorithm in detail, which allows the prediction of required pole height changes in order to correct for field errors. As input data field errors deduced from precise magnetic field measurements are used together with the signatures of pole movements. A band diagonal system of linear equations has to be solved to obtain the pole height corrections. For demonstration of the method the field of the 0.9 m long prototype structure was optimized to have a straight trajectory. Since only a sparse band diagonal system of equations has to be solved, the method has the potential to be used in very long undulators having 600 –1000 poles.

Contribution of induced and remanent magnetization to long‐wavelength oceanic magnetic anomalies

The Magsat mission has provided very precise measurements of Earth's magnetic field which have been used to derive accurate maps of the long‐wavelength magnetic anomaly field. Analyses of this field over localized areas have shown that induced magnetization is mainly responsible for the anomalies and that remanent magnetization is rarely detectable at Magsat altitude. A careful analysis of the data, taking into account the various effects due to non lithospheric sources, allows this field to be better described and, consequently, suggests two observations: (1) long‐wavelength oceanic anomalies display many similarities with the distribution of seafloor topography, suggesting a relationship between crustal thickening and magnetization enhancement; (2) age‐related pattern (i.e., ridge parallel) of anomalies is observed in two of the larger oceanic basins (Indian and North Atlantic oceans). Several forward models are constructed to investigate these correlations. In the first model, the effect of variations in crustal thickness was modeled assuming uniform susceptibility and that thickness varies proportionally to the seafloor topography. In the second model, the field due to remanent magnetization acquired during the Cretaceous Long Normal period was modeled and shown to contribute significantly to the Magsat field. In order to make the modeled and observed maps fully comparable, several effects must be taken into account: a global magnetization contrast exists between oceans and continents and induces widely distributed anomalies because of the truncated spherical harmonic analysis; the spectral content of the observed map in the north‐south direction is reduced because of the along‐track filtering due to the polar orbit of Magsat, When these effects are taken into account, the model combining the contribution of induced and remanent magnetization display remarkable similarity to observed data. This comparison constrains both induced and remanent magnetization of oceanic lithospheric rocks.

A new two-dimensional coil system compensating X,Y,Z components of leakage field from an aperture of magnetic shields (abstract)

Lightweight magnetic shields of high performance are required for the precise magnetic field measurements such as brain field measurement by SQUID magnetometers. Most of the present magnetic shielding rooms rely on ferromagnetic enclosures to isolate the area to be shielded from outside and, as a result, become heavy and expensive. In this paper, we propose a new method to compensate the leakage of noise field from an aperture of magnetic shields. The proposed method exploits the flux pattern just inside the aperture of the shielding body. Figure 1 depicts this method. Three flat coils mutually orthogonal and to be placed parallel inside the aperture are driven by the feedback signal to cancel each of three components detected by a three-axis fluxgate magnetometer. The important point of this method is that the coil system is flat unlike other active shielding system employing Helmholtz coils. Numerical results obtained for a superconductor vessel showed that the transverse field was attenuated by a factor of 100. Applying this method, the cylindrical shielding case of superconductor or of ferromagnet can be shortened with high shielding factor.

Non-metal hermetic encapsulation of a hybrid circuit

A cheap hermetic non-metal encapsulation was evaluated for use with a high reliability hybrid circuit subject to severe climatic environment for precise magnetic field measurements over 20 years lifetime. For this purpose a literature scan has been carried out establishing roughly the following results: 1. a) Humidity is the largest single risk faktor concerning reliability and expected life time of the device. 2. b) Plastic sealing does not represent a barrier against humidity for a long time. 3. c) There is no off-the-shelf solution available as usual real hermetic encapsulations are ruled out due to electrical, magnetic and price considerations. Therefore a simple ceramic cover soldered by an AuSn preform directly to the ceramic substrate supporting the sensor and the thick film circuit has been developped to meet all requirements of a cheap, hermetic enclosure. This paper presents both the considerations taken into account to evaluate an appropriate encapsulation method as well as the final solution.

Magnetic Field Measurements of the DESY II Magnets

A new method has been developed for the precise magnetic field measurements in ac magnets. The technique has been applied to the DESY II dipoles and quadrupoles. Movable coils of various lengths are used, and voltage induced by the ac excitatition is compensated by reference coil. A fast ADC triggered at different levels of magnet current converts the output of a low drift analogue integrator. Measurement and data handling are microprocessor controlled. The data are sent to the computer centre for analysis and storage. From there the results and a graphic display are sent back to a local interactive terminal for qualitative online checking.

Method of the super-highly precise measurement of strong magnetic field (abstract)

This article describes the method of measuring a strong magnetic field with a nuclear magnetic resonance (NMR) magnetometer using flowing water. In the apparatus, the strong magnetic field measured is taken as the sample of polarizing magnetic field, and frequency synthesizer generator as the source of radio frequency exciting resonance transitions. The apparatus is different from the precise field averaging NMR magnetometer (precision:10−7) for low and high fields using flowing water developed by J. M. Pendlenbury etc., and the wide-ranged NMR magnetometer (precision:10−5) using flowing water developed by V. M. Simonov and colleagues. The minimum step of the frequency alterations of the synthesizer is 0.01 Hz. The stability of frequency is extremely high. The apparatus can quite accurately measure the proton absorption spectrum of the measured field. This provides possibilities for the super-high precision measurement of resonance central frequency. The method of taking the arithmetic mean of frequencies f1 and f2 measured at the half-height of spectrum lines to be the measurement of the central frequency f0 is demonstrated in this paper as well. The precision of measuring results is higher an order of magnitude than that of the direct measurement of f0. The precision of the measuring results of the value 8.455 T of NMR spectrum superconducting magnetic field reaches 0.7×10−9. This technique can be used to precisely measure the strong magnetic field over 0.5 T. So far as the measurement of the distribution of NMR spectrum superconducting field is concerned, it shows its significant advantages.

High altitude measurements of the Earth's magnetic field with a proton precession magnetometer

A nuclear free-precession magnetometer of the Packard-Varian type has been built and used for the precise measurement of the earth's magnetic field (scalar magnitude of total field vector) at high altitudes. The instrumentation, including the magnetometer head, amplifier, and radio telemetering transmitter, was carried to 100,000 feet altitude by a plastic Skyhook balloon. Signals were transmitted to a fixed ground station at one-minute intervals and the nuclear precession frequency was determined by standard frequency measuring techniques. Satisfactory signals were received until 2.5 hours after launching, at which time the balloon was near the radio horizon, approximately 200 miles from the ground station. The data showed a decrease in magnetic field intensity with altitude, as well as large variations in intensity with geographical position. The work described is the first phase in this laboratory's program of adapting the free precession magnetometer to the systematic study of electrical currents in the ionosphere, using rockets as experimental vehicles.