Active Shielding System Research Articles

Analysis of active and passive magnetic field reduction systems (MFRS) of the ITER NBI

In ITER two heating (HNBI) and one diagnostic neutral beam injectors (DNBI) are foreseen. Inside these components there are very stringent limits on the magnetic field (the flux density must be below some G along the ion path and below 20 G in the neutralizing regions). To achieve these performances in an environment with high stray field due to the plasma and the poloidal field coils (PFC), both passive and active shielding systems have been foreseen. The present design of the magnetic field reduction systems (MFRS) is made of seven active coils and of a box surrounding the NBI region, consisting of ferromagnetic plates. The electromagnetic analyses of the effectiveness of these shields have been performed by a 3D FEM model using ANSYS code for the HNBI. The ANSYS models of the ferromagnetic box and of the active coils are fully parametric, thus any size change of the ferromagnetic box and coils (linear dimension or thickness) preserving the overall box shape could be easily reproduced by simply changing some parameter in the model.

Electron Beam Recorder with Nanometer-Scale Accuracy for 100 Gbit/in2 Density Mastering

An electron beam recorder (EBR) was developed for mastering optical disks with a recording density of 100 Gbit/in2 using a multilevel recording format. In this recording format, a nanometer-scale accuracy of relative pit edge position is required in both radial and tangential directions. To achieve the recording position accuracy, a rotation stage with a noncontact vacuum seal and a correction system for rotation errors were developed. In addition, an active magnetic shield system and a learning compensation for beam displacement were adopted to improve the stability of the beam position. As a result, the ability to record with high accuracy and high resolution was confirmed from experimental results. A fine pit pattern with a minimum pit length of 70 nm was formed precisely. The recording accuracy of the EBR was evaluated to be approximately 2 nm (standard deviation) in both radial and tangential directions. Furthermore, a carrier-to-noise ratio of 64 dB was also obtained by reproducing an etched silicon master with a 240 nm monotone pit pattern.

Analysis and optimization of a micro SMES system using multiple parallel solenoids

The micro superconducting magnetic energy storage system is becoming a powerful tool for application in power systems. Nowadays, devices include active shielding systems instead of passive ferromagnetic shields. This calls for sophisticated analysis and design techniques able to provide satisfactory performance in terms of stored energy efficiency, reduced stray field, and/or minimal superconductor's volume. In this paper, the features of an active shielding system using multiple parallel solenoid configurations for superconducting magnets are analyzed and the results of their corresponding stray fields are compared. Two optimization strategies are briefly presented. The results using the proposed optimization strategies for magnet systems of energy capacity of 1 MJ, 5 MJ, and 10 MJ are given, respectively.

Active magnetic shielding for biomagnetic measurement using spatial gradient fields

Biomagnetic measurement performed outside a magnetically shielded room is subject to distortion by strong magnetic fields. Reducing such disturbances can enhance and stabilize biomagnetic measurement conditions in the absence of passive shielding. We have developed an active magnetic shielding system that produces both homogeneous and spatial gradient magnetic fields. The system is composed of anisotropic magnetoresistive sensors, a digital signal processor controller and two different coil systems. In order to improve the measurement environment for a first-order gradient coil SQUID system, the disturbing vertical magnetic fields and vertical field gradients are reduced, thus achieving a shielding factor of approximately 6 at 100 Hz. Our system provides a more flexible and less costly alternative to magnetically shielded rooms.

簡易型磁気シールドルームに適したアクティブシールドシステム設計のための磁界分布と位相遅れの検討

簡易型磁気シールドルームに適したアクティブシールドシステム設計のための磁界分布と位相遅れの検討

Low-Tc magnetometer used for active shielding in the frequency range dc-500 Hz in biomagnetic measurements

Ultra low-noise dc SQUIDS have been used to detect disturbances in biomagnetic measurements and to feed an active shielding system that compensates these disturbances. This work describes some properties of the SQUIDs used and the principle of active shielding that has been improved for frequencies from dc to 500 Hz. So far an additional attenuation of up to 50 dB for near-dc and almost 20 dB for 50 Hz magnetic disturbances has been achieved.

Active shielding to reduce low frequency disturbances in direct current near biomagnetic measurements

Measurements of dc near biomagnetic fields are disturbed by low frequency noise that is not reduced sufficiently by most of the magnetically shielded rooms or gradiometers. For this reason an active shielding system has been developed at the Biomagnetic Center of the University of Jena. This work describes the principle of the active shielding system and demonstrates its properties concerning the attenuation of disturbing fields, frequency range, and some applications in biomedical measurements. We achieved a reduction of external low frequency magnetic fields by more than 50 dB and an attenuation of the field gradient by about 25 dB. This active shielding enables measurements of near dc biomagnetic fields in investigations of periinfarct depolarizations after ischemic stroke and spreading depression in migraine patients.

Magnetometry based on sharpened high T/sub c/ GBJ Fraunhofer patterns

At EUCAS'95, we reported on the large enhancement of the sharpness of the diffraction pattern exhibited by YBCO grain boundary junctions on bicrystal substrates. This effect results from large focusing effects of the superconducting electrodes; the geometry used is a small square washer (630/spl times/630 /spl mu/m/sup 2/) with the junction in the center. For such devices, the Fraunhofer pattern is contracted around the value B/sub n//spl ap/0 where B/sub n/ is the perpendicular magnetic field applied to the washer. A FWHM of the Fraunhofer pattern of 155 mG has been observed leading to a field response of 176 A/T or 50 V/T corresponding to voltage and current biasing modes respectively. A white noise level of 3 pT//spl radic/Hz mas measured. Although a field modulation was applied to remove some of the 1/f noise, an unusual telegraphic noise appearing as jumps of about 20 nT still remained. This is probably associated to vortex hopping between pinning sites in the vicinity of the junction. We propose a possible way for removing such large perturbations. These devices are to be used in active shielding systems and will be very useful in SQUID systems to be operated in an open environment.

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.

Active Shielding System for Removal of Stray Tokamak Magnetic Fields in JT-60 Neutral Beam Injectors

This paper reports that in the JT-60 neutral beam injectors (NBIs), an active method using a set of coils is adopted to shield unneutralized beams from stray magnetic fields, while the usual passive method using high-mu materials is used to shield the ion sources and neutralizers. This active shielding method is a unique solution for the unneutralized beams in the JT-60 NBI under the constraints of the limited space available and minimizing the error file induced by the shielding. A passive shielding method is permissible for the ion sources and the neutralizers because the space to be shielded is limited. The active shielding system is designed by making a one-fourth model of the magnetic system and calculating ion orbits using magnetic fields measured in the model. The shielding characteristics are checked by arrays of thermocouples buried in the beam dump where the unneutralized beams are thermalized. The thermocouple outputs are consistent with those predicted from the ion orbit calculations.

A 5 Telsa imaging magnet for imaging laboratory animals

A report is presented on the construction of the first of a series of magnetic resonance imaging (MRI) imaging magnets for laboratory animals. The magnet has a 33-cm warm bore with a design central induction of 5.5 T without active shielding and 5.0 T with active shielding. The magnet will be used for both imaging and spectroscopy of living animals. The active shield system is designed so that the 5-G line is less than 3 m from the magnet center when the magnet operates at design field. This permits the magnet to be used within an experimental space commonly available within a university building. >

Three-dimensional magnetic preshielding with a single folded coil

For moderate requirements of field uniformity, about +or-2% referred to the centre of a cube of (0.4 m)3, an active shielding system consisting of a single, rectangular, folded loop is proposed. The loop can simultaneously reduce the horizontal and vertical components of the external field down to a level determined by the remaining non-uniformity. The length of the horizontal and vertical legs of the loop must be designed to fit the local field of the experimental region. Some modifications are possible so that a smaller second horizontal component can also be accommodated. The field uniformity of the folded loop compares favourably with that of other similar single or two-loop coil systems of cubic shape.