A comparative, computational study of the modeling of transcranial magnetic stimulation (TMS) and electroconvulsive therapy (ECT) is presented using a human head model. The magnetic fields from a typical TMS coil of figure-eight type is modeled using the Biot-Savart law. The TMS coil is placed in a position used clinically for treatment of depression. Induced current densities and electric field distributions are calculated in the model using the impedance method. The calculations are made using driving currents and wave forms typical in the clinical setting. The obtained results are compared and contrasted with the corresponding ECT results. In the ECT case, a uniform current density is injected on one side of the head and extracted from the equal area on the opposite side of the head. The area of the injected currents corresponds to the electrode placement used in the clinic. The currents and electric fields, thus, produced within the model are computed using the same three-dimensional impedance method as used for the TMS case. The ECT calculations are made using currents and wave forms typical in the clinic. The electrical tissue properties are obtained from a 4-Cole-Cole model. The numerical results obtained are shown on a two-dimenaional cross section of the model. In this study, we find that the current densities and electric fields in the ECT case are stronger and deeper penetrating than the corresponding TMS quantities but both methods show biologically interesting current levels deep inside the brain.

A study of the effect of local magnetic shear on the drift wave stability is presented. The eigenvalue problem for the drift wave equation is solved numerically in fully three-dimensional stellarator plasma using the ballooning mode formalism. It is found that negative local magnetic shear has a stabilizing effect on the drift wave instability and positive local shear is destabilizing. This is in agreement with the effect of negative global magnetic shear in tokamaks and also agrees with the simple estimates. As a consequence the highly unstable modes found on a specific magnetic surface are localized in the region of positive local magnetic shear.

The linear stability of the ion-temperature-gradient modes (ITG) in the electrostatic limit is examined in the short wavelength region by using a two fluid reactive model in fully three-dimensional Wendelstein 7-X (W7-X) stellarator [G. Grieger et al., Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525] geometry. The spectrum of stable and unstable modes and their real frequencies and eigenfunctions are calculated. The effects of density gradients, temperature gradients, temperature ratios, wavevector, ballooning angle, curvature and local magnetic shear on the ITG mode are also investigated. The frequency and growth rate of the most unstable ITG mode is calculated and visualized for a specific magnetic flux surface. For the equilibrium under investigation both localized and extended eigenmodes are found. The effect of small and large temperature ratios, small and large density gradients as well as large local magnetic shear are all found to be stabilizing. The highest growth rates are found at the outer part of the surface where the local magnetic shear is small and normal curvature is unfavorable.

The ion-temperature-gradient (ITG)-driven drift mode is studied in threedimensional stellarator geometry using a two-fluid reactive model in the electrostatic limit. The model includes first-order FLR effect in the presence of parallel ion dynamics and using the Boltzmann distribution for the electrons. The resulting eigenvalue is solved numerically using the ballooning mode theory. The results are contrasted with the corresponding tokamak results with positive shear. In stellarators, the level of the maximum growth rate of the ITG mode is found to be smaller and the threshold (η i 2.2) is somewhat higher. The effects of small and large temperature ratios and density gradients are found to be stabilizing on electrostatic ITG modes in stellarators.

The influence of plasma geometry on the linear stability of electrostatic ion-temperature-gradient driven drift modes ͑ITG or i ϭL n /L Ti modes͒ is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved numerically. A comparison is made between an H-1NF ͓Fusion Technol. 17, 123 ͑1990͔͒ like stellarator equilibrium, a numerical tokamak equilibrium and the analytical ŝ Ϫ␣ equilibrium. The numerical and the analytical tokamak are found to be in good agreement in the low inverse aspect ratio limit. The growth rates of the tokamak and stellarator are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. The threshold in i for the stellarator is found to be somewhat larger. In addition, a stronger stabilization of the ITG mode growth is found for large ⑀ n (ϭL n /R) in the stellarator case.

Abstract. Drift waves are investigated in a real three-dimensional stellarator geometry. A linear system, based on the cold ion fluid model and a ballooning mode formalism, is solved numerically in the geometry of the stellarator H1-NF. The spectra of stable and unstable modes, as well as localization, are discussed. The dependence of the spectrum of the unstable modes on the wavevector, plasma density variation, and the location in the plasma is presented.

A point measurement of the magnetic field (MF) near a typical spot welding machine showed that the magnetic flux density was above the ICNIRP reference level. To investigate if the basic restrictions were exceeded, the induced body currents in a full 3D human model were calculated. It was found that at an operator position of 34 cm away from the machine the maximum induced current density was below the ICNIRP basic restriction. But if the operator was closer to the machine, the basic restriction was exceeded. An important finding of the present article is that the basic restriction can be exceeded although the MF spatially averaged over the whole body is well below the reference level.

Conventional experimental methods for testing the performance of super-capacitors include the measurement of capacitance through charge and discharge, measurement of equivalent series resistance (ESR) and measurement of self-discharge and the equivalent circuit model (ECM) by electrochemical impedance spectroscopy (EIS). However, the methods are not suitable for the mass production line of supercapacitors since they require a long time for the test and several kinds of different instrument. EIS is an attractive method to evaluate the performance of supercapacitors except that it takes a long time for a single test. In this paper a fast EIS instrument suitable for quality assurance for the mass production of supercapacitors is proposed. In order to reduce the time for the test, a multi-sine sweeping method is used for the EIS test and the results are analyzed by extracting the parameters of the ECM to evaluate the performance of the supercapacitors. The proposed instrument is developed to have multi-channel to further decrease the time for the test with a supercapacitor. It is also presented as to how the extracted parameter values of the ECM can be used to evaluate the performance of the supercapacitor.