Akira Haga

Measurement of very fast voltage rise curve due to gap discharge using coupled transmission lines in distributed constant system

A measurement method of very-fast-voltage rise curves due to gap discharge using the coupled transmission lines was examined to discuss a electromagnetic noise source from a viewpoint of electromagnetic compatibility (EMC). The measurement system consists of a distributed constant line system, because the voltage transients were very rapid. A characteristic of the gap electrode, which has a matched impedance for the distributed constant line system, was investigated in the frequency range below 5 GHz. The voltages of power source in experiment were 510 V and 800 V because the Paschens law holds stability in air condition. As a consequence of the experiment with this measurement system, the measurement method made it possible to observe the very fast rise curves of about 100 ps.

12GHz measurement of transition duration and breakdown field due to low voltage ESD

Transition duration of voltage and current rise time due to small gap discharge as the low voltage ESD was investigated in time domain. The measurement system was improved on the band width from 6GHz to 12GHz using the coaxial electrode system. Also, the sensing system was changed from the coupled transmission lines to an E-field sensor and a Hfield sensor. The insertion loss of the experimental system was within about -3dB in frequency range below 12GHz. As a consequence of the experiment using the system, voltage and current rise time of transition duration were shown 32 ps or less. The rise times were changed in configuration of electrodes, source polarity and discharging voltage. Besides, breakdown field was examined to corroborate the very fast transition durations of about 32ps. The breakdown field was very high of about 8×107 V/m in low voltage discharging of below 350V.

A measurement of very fast transition durations due to gap discharge in air using distributed constant line system

Very fast transition durations (rising time in positive polarity and falling time in negative polarity) due to starting of gap discharge were investigated in time domain. The gap space was set very small for voltages below 1500 V as a simulation of the charged device model (CDM) electrostatic discharge (ESD) and the gap discharge of switch devices. The measurement system consists of a distributed constant line system with a tapered coaxial electrode, which has a matched impedance for the characteristic impedance of the distributed constant line system. The insertion loss of the tapered coaxial electrode was within -3 dB in the frequency range below 4.5 GHz. The atmosphere around the electrode is ordinary air. This experimental system enables one to measure the high-speed transients of about 100 ps due to gap discharge in time domain. As a consequence of the experiment, the relationship between the discharge voltage and transition duration was confirmed. The voltage rise time was slowed down gradually in positive polarity, while the voltage fall time was slowed down remarkably in negative polarity for the 0.1-mm needle.

Open-Type Magnetically Shielded Room Combined With Square Cylinders Made of Magnetic and Conductive Materials for MRIs

To improve the comfort of hospital patients, we have developed a new open-type magnetically shielded room (MSR) for magnetic resonance imaging (MRI), constructed by stacking square cylinders composed of magnetic material on the inside and a conductive material on the outside. The magnetic cylinder shields against leakage of the magnetic field from the MRI, while the conductive outer cylinder prevents intrusion of the electromagnetic field from outside by waveguide attenuation. First, the optimal design of the magnetic cylinder was determined, using 3-D magnetic field analysis with a simple cubic MSR. Second, the validity of the analysis and the practical realization of the designed MSR were investigated using a small experimental model. Finally, from a design perspective and the standpoint of saving space, the optimal combination of electromagnetically shielded glass and conductive cylinders made of aluminum was studied by measuring the electromagnetic shielding effect. As the optimal design of an open-type MSR for an MRI, we have adopted a combination of single-layered electromagnetically shielded glass together with stacked cylinders. These cylinders are made of a square cylinder, 144 x 144 mm and 300 mm long, made of silicon steel (thickness t =1 mm) and divided into three parts along the axial direction, placed inside a second square cylinder made of aluminum, 150 x 150 mm and 300 mm long (t = 3 mm). This MSR construction technique creates an open, spacious feeling inside the facility and can be easily fabricated at low cost.

Shield Duct to Prevent Magnetic Field Leakage Through Openings in Double-Layered Magnetically Shielded Rooms

This paper describes an optimal length and combination of short shield ducts to prevent the leakage of magnetic flux through the openings of a double-layered magnetically shielded room (MSR) used for electron-beam lithography systems and biomagnetic measurements. A number of openings as large as 500 times 500 mm must be provided in the walls, ceiling, and floor of such MSRs for air conditioning and cables. First, the magnetic field leakage from the openings, using a small model of an MSR, was measured with short shield ducts of various lengths in order to confirm the validity of the analysis and practical realization. Next, the distributions of the magnetic fields inside the small model were evaluated using 3-D magnetic field analysis and the finite element method. The method of analysis was verified by comparing computed results with those measured. We show the optimal length of shield duct attached around the openings as follows: The optimal combination of short shield ducts is an outer layer duct which is the same length as the width of the opening with an inner layer duct which is half the length of the opening. These lengths of duct accord with the optimal lengths when the outer and inner layer ducts are employed as a single duct. The leakage flux of the optimal single duct through the opening can be decreased to a half by the optimal combination of shield ducts. This also corresponds to the fact that the length of the inner layer duct should not protrude from the outer layer in order to avoid collecting surplus fluxes outside the magnetically shielded room.

A newly designed and constructed 20 kHz magnetic field exposure facility for in vivo study.

Exposure to man-made electromagnetic fields has increased over the past century. As a result of exposure to these fields, concerns have been raised regarding the relationship between electromagnetic fields and human health. Interest in the biological and health effects of intermediate frequency (IF) magnetic fields has grown recently because of the increase in public concern. In order to investigate whether IF magnetic fields have biological effects, we have developed a 20 kHz (IF) magnetic field exposure system for in vivo studies. The exposure facility was designed to study the biological effects of IF magnetic field on laboratory animals. The facility consists of a 9 m x 9 m x 5 m high room containing seven separate rooms including a 5.3 m x 4.5 m x 3 m high specific-pathogen free exposure room. The dimensions of the exposure system are 1.6 m x 1.6 m x 1.616 m high located inside this exposure room. The system is designed to provide magnetic fields up to 200 microT at 20 kHz with the uniformity within +/-5% over the space occupied by animals. After constructing the facility, performance tests were carried out. As a result, it was confirmed that our facility met requirements for evaluation of the biological effects of IF magnetic field on small animal experiments. In this paper, the design, construction, and results of evaluation of an animal exposure facility for the in vivo biological effects of an IF magnetic field are described.

Open Type of Magnetically Shielded Room Combined With Cancelling Coils for Magnetic Resonance Imaging

In this paper, an open type of MSR combined with cancelling coils is investigated. The optimal position of the cancelling coil from the standpoints of the leakage flux density outside the room and the required current is determined using 3-D magnetic field analysis with the finite element method. Moreover, the structure for maximizing the opening of a MSR is proposed.

Characteristics and prediction of magnetic noise due to DC electric railcars for biomagnetic measurements

A magnetically shielded room for biomagnetism should be designed taking account of the S/N ratio of the biomagnetic signal to the environmental magnetic noise generated mainly by dc electric railcars at an installed location. This paper describes the characteristics of the fluctuation of magnetic noise and a method of prediction of its amplitude. The distance dependence of the magnetic noise is measured at three locations near railroad by using fluxgate-magnetometers. A decay factor of the magnetic noise vs. the distance from a railroad can be obtained from such a measurement. The decay factors measured are in fairly good agreement with those calculated. The amplitude of the fluctuation of magnetic noise at any distance from the railroad can be easily predicted if the decay factor is measured.

Design and Construction Method of an Open-Type Magnetically Shielded Room for MRI Composed of Magnetic Square Cylinders

To shield the leakage flux densities of 3 T magnetic resonance imaging (MRI) higher than 1.5 T MRI and to reduce the degradation of magnetic properties during construction, the design of an open-type magnetically shielded room (MSR) combined with square cylinders made of magnetic and conductive materials should be improved. This paper reports on an optimal design for magnetic cylinders to improve the shielding effect, while also considering ease of construction with a view to practical use. Firstly, the structure of the magnetic cylinders, such as their opening width and the slit width were optimized using a 3D linear magnetic-field analysis. Using optimal magnetic cylinders, when the relative permeability (mur) of the magnetic material is larger than 10 000, the reluctance of gaps between the cylinders is the main contributor to the leakage flux. So, the required mur of the magnetic material is 10 000 and oriented silicon steel is adopted instead of the non-oriented type. Secondly, as an optimal construction method, it was determined by experiment that oriented silicon steel sheets, annealed after bending, should be laminated so that the effect of gaps on butt joints between steel sheets on the magnetic property of the cylinder becomes smaller. This enables the relative permeability of 10 000 when the maximum flux density of the material in the cylinder is less than 1.7 T. Finally, the minimum thickness of oriented silicon steel sheet of cylinders depending on the distance from a 3 T MRI is obtained using linear magnetic analysis under the condition that the maximum flux density of the material of the cylinder is less than 1.7 T. In the case of the thickness of 3.8 mm, the leakage flux density of 3 T MRI is decreased to less than a guideline (0.5 mT) when the distance between the cylinders and the center of MRI is larger than 2 750 mm. This indicates that the optimal design and construction method of an open-type MSR for 3 T MRI composed of magnetic square cylinders are realized.

Design Method for Realization of Open-Type Magnetically Shielded Room Composed of Magnetic Square Cylinders for MRI

In order to increase amenity for hospital patients, we introduce an open-type magnetically shielded room (MSR) in tight space composed of magnetic and conductive cylinders for a magnetic resonance imaging (MRI). This paper presents a design for the MSR opening part for an actual 3T MRI that combines magnetic cylinders with magnetic mullions to decrease the leakage flux density to less than 0.5 mT, taking magnetic saturation into account. The design utilizes the homogenization technique to model the opening part which reduces the number of elements. Moreover, a newly optimized design approach is proposed for the design of openings. First, the opening part is designed while treating the magnetic property in linear analysis to obtain an ideal flux distribution without magnetic saturation. Then, the thickness of the magnetic cylinders is determined for magnetic cylinders to stay unsaturated. Finally, the open-type MSR presented in this paper is built in a hospital, and the design method is verified.