AC Transverse Magnetic Field Research Articles

Experiment and modeling of an atmospheric pressure arc in an applied oscillating magnetic field

A set of experiments are carried out to measure and understand the response of a free-burning atmospheric pressure carbon arc to applied transverse dc and ac magnetic fields. The arc is found to deflect parabolically for the dc field and assumes a growing sinusoidal structure for the ac field. A simple analytic two-parameter fluid model of the arc dynamics is derived, in which the arc response is governed by the arc jet originating at the cathode, with the applied J×B force balanced by inertia. Time variation of the applied field allows evaluation of the parameters individually. A fit of the model to the experimental data gives a value for the average jet speed an order of magnitude below Maecker’s estimate of the maximum jet speed [H. Maecker, Z. Phys. 141, 198 (1955)]. An example industrial application of the model is considered.

ポインチングベクトル法による高温超伝導テープ線材の交流損失測定

A new system was developed for measuring losses electrically in short, straight HTS tapes under various external conditions of transport currents and transverse magnetic fields. In this method, Poynting vectors around the straight sample were measured to get loss values by using two movable sets consisting of both a potential lead pair and a miniature pick-up coil on each flat face of the tape. In order to confirm validity of this system, loss measurements on short samples of Ag-sheathed Bi2223 multifilamentary tapes were carried out in liquid nitrogen for three cases as simultaneous sweep of ac transport currents and transverse ac magnetic fields and two single sweep cases of one of them. The results obtained show that this is extremely useful for measuring these ac losses as long as longitudinal change in the magnetic field can be neglected.

Influences of geometrical configuration on AC loss measurement with pickup-coil method

We usually measure AC losses of superconducting wires and cables exposed to a transverse AC magnetic field by a pickup-coil method. In this case, a main pickup coil is coaxially located around the sample wire wound as a solenoidal coil. An additional pickup coil (a cancelling coil) is also placed inside or out side the sample coil. This method is advantageous to obtain detailed electromagnetic properties of the sample. In the present study, we discussed the effect of the geometrical configurations of the sample and pickup coils on AC losses measured by the pickup coil method for three types of arrangements of the cancelling coils. We analytically calculated interlinkage magnetic flux into the pickup coils due to magnetic moments induced in the sample wire, and formulated the geometrical errors in the pickup-coil method. We also prepared some sets of sample and pickup coils, and compared observed AC losses with the theoretical predictions.

Increase of inter-strand coupling losses in superconducting cable-in-conduit conductor under actual condition of sweep rate

Basic studies are carried out on causes of the increase of inter-strand coupling losses in cable-in-conduit conductors used for windings of large scale magnets, such as a single inner vertical (IV) coil in the Large Helical Device. In this paper, the effect of AC transverse magnetic fields periodic changing along a two-strand cable axis on loss properties of the cable is considered as a candidate of the cause. The evidence that the periodic changes produce additional inter-strand coupling losses characterized by another long coupling time constant in addition to the intrinsic one is theoretically shown. We confirmed properties by measurements on two kinds of sample wires, using inter-filament coupling in a strand instead of inter-strand coupling in cable.

First-Order Vortex-Lattice Melting Transition inYBa2Cu3O7−δnear the Critical Temperature Detected by Magnetic Torque

High-resolution magnetic-torque studies on an untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal near its critical temperature ${T}_{c}$ reveal that the first-order vortex-lattice melting transition (VLMT) persists at least up to 0.5 K below ${T}_{c}$. The associated sharp discontinuity in magnetization is detectable even at temperatures where the torque signal deviates from mean-field behavior due to fluctuations. The magnetic irreversibility at the VLMT can be suppressed by applying a weak transverse ac magnetic field. This offers the possibility of separating the irreversibility line from the melting line near ${T}_{c}$.

Collapse of the magnetic moment in a hard superconductor under the action of a transverse ac magnetic field

The suppression of the static magnetic moment M of a superconducting plate by an alternating magnetic field applied perpendicularly to a dc magnetic field has been studied both experimentally and theoretically. Complete disappearance of the hysteresis of the magnetization curve in YBCO melt-textured samples was observed. Nonmonotonous behavior of M with the increase of the ac magnetic field amplitude was found. The dynamics of the collapse of the magnetic moment was studied in the pulse regime of the transverse field. The experimental results indicate homogeneity of the static magnetic induction in the sample regions where the ac field penetrates. The data obtained are interpreted within the framework of a two-liquid hydrodynamic model.

Electrodynamics of hard superconductors in crossed magnetic fields

A model systematically accounting for the cutting of Abrikosov flux lines has been developed for the critical state of a hard superconductor in crossed dc and ac magnetic fields. The electrodynamic equations have been derived by minimizing the Gibbs free energy calculated using the proposed two-velocity hydrodynamic model. One velocity describes the motion of the vortex lattice as a whole, and the other describes the relative motion of the two intersecting sublattices. The resulting equations yield as special cases the previously known electrodynamic equations for hard superconductors. The model provides a natural explanation for the suppression of dc magnetization by a transverse ac magnetic field observed in our experiments.

Dependence of self field AC losses in AC multifilamentary composites on phase of external AC magnetic field

The authors have measured the self magnetic field AC losses of a typical AC multifilamentary superconducting wire in an AC transverse external magnetic field at 50 Hz. The experimental results show that the self field AC losses are much higher than the transverse field AC losses and strongly depend on the phase difference between the transport current and the background field. They derive a theory to estimate the dependence of the self field AC losses on the phase of the external field based on the critical state model. In the paper, the measured loss characteristics are compared with the theoretical results. The theoretical model explains the measured loss characteristics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental evaluation of AC losses in superconducting multifilamentary wires for 50/60 Hz use exposed to a magnetic field with arbitrary angle with the wire axis

In multiple twisted superconducting cables exposed to an external AC transverse magnetic field, a component of magnetic field parallel to the strand axis due to the twisting structure of the cables causes an additional AC loss (longitudinal AC loss) in the strands. The authors experimentally evaluate the longitudinal AC loss by an equivalent measurement with short samples of the strands exposed to an oblique magnetic field to the strand axis. The total loss obtained at 60 Hz was equivalent to a direct sum of the hysteresis loss, the coupling-current loss for the transverse component and the additional loss for the longitudinal one. In triple twisted superconducting cables composed of insulated strands, measured AC losses could be also quantitatively explained in the same manner as for the short samples. These results suggest that the longitudinal loss may be dominant in an AC magnetic field with relatively large amplitude along with the coupling-current losses inside and among strands. The authors also discuss an optimum condition for the twist pitch of the strand to reduce the total AC loss of the strands in the cable. >

Electric field and losses at AC self field mode in MF composites

Studies of the current-voltage characteristics of typical superconducting wires under static and dynamic conditions up to 50 Hz frequency have revealed much higher instantaneous local loss power densities in an external DC magnetic field than those which occur by magnetization in a transverse AC magnetic field. In this paper, the measured loss characteristics are compared with theoretical ones derived from the critical state model, and the self field AC losses are also estimated. Possible errors that can result by applying the critical state model are discussed. >

High-Tc superconducting film in a transverse AC magnetic field: losses and effect of lower critical field

AC losses as a function of the amplitude of the AC transverse magnetic field are measured on both poly- and monocrystalline YBa2Cu3O7- delta superconducting films. The data are consistent with the calculation based on the Bean critical state model adapted to a thin film perpendicular to a magnetic field. An influence of the 'lower critical field' Hc1' (which first penetrates the sample) is observed on the monocrystalline films, the value of Hc1' being about 0.1 times the 'true' Hc1 as known for bulk crystals. The technique described provides a simple method of determining critical current density and penetration fields in high-Tc films.

The Radiation Intensity Distribution of Rectangular Positive Column Plasma by the AC Transverse Magnetic Field.

The rectangular discharge tube will be used for back light sources of a liquid crystal display because the uniform luminance over a wide range is expected. In this tube, the spectral intensity distributed in a cosine function. If a DC transverse magnetic field was applied to the positive column perpendicularly to the tube axis, the spectral intensity distribution was deflected to a tube wall. Therefore, AC transverse magnetic field would make the time averaged spectral intensity into a flat distribution. In this paper, the major axis distributions of the radiation intensity in the rectangular discharge tube were measured for helium, neon and argon as functions of gas pressure and AC magnetic field. The spectral characteristics were also measured for these gases. The tube used for measurements has a rectangular profile of 2×4 cm. The discharge current was 20 mA. The gas pressure was varied from 0.2 to 5.0 Torr. Then the experimental results were discussed qualitatively.

Ac free-electron laser.

A possible new source of tunable coherent radiation is analyzed. By the passing of a relativistic electron beam (relativistic factor ${\ensuremath{\gamma}}_{0}$) through a homogeneous transverse ac electric or magnetic field of frequency ${f}_{0}$, electromagnetic radiation of frequency $f\ensuremath{\sim}2{\ensuremath{\gamma}}_{0}^{2}{f}_{0}$ is generated by its coupling the negative-energy electrostatic beam modes with the high-frequency electromagnetic modes. Existing superconductor cavities which can supply ac fields of frequency ${f}_{0}\ensuremath{\gtrsim}10$ GHz with amplitude ${E}_{max}&gt;20$ MV/m may be used to lead to a new class of submillimeter or even visible generating devices.

Coupling-Current Loss in a Multifilamentary Superconducting Wire Subjected to a Transverse AC Magnetic Field with a Relatively High Sweep Rate

The electromagnetic behavior of a multifilamentary superconducting wire subjected to an external transverse AC magnetic field with a trapezoidal wave form is discussed analytically with the aid of the so-called continuum model. The present expression for the coupling-current loss agrees well with the experimental results reported by Zenkevitch et al. for a practically important range of the sweep rate, whereas the present expression is only an approximate one with a quite simple form. The effects of the presence of a transport current on the coupling-current loss are also discussed analytically. The present analytical method can also reproduce the numerical results on the radius of the internal region and the shape of the saturated region presented by Rem et al. and Ciazynski et al. fairly well up to a relatively high sweep rate of the external magnetic field.

ブロンズ法Ｎｂ３Ｓｎ多しん線の交流損失に及ぼすすず拡散防止層の影響 ＩＩ 大振幅変動磁界中の履歴損失

A theoretical expression for the hysteresis loss in a transverse AC magnetic field with a large amplitude of trapezoidal wave form is derived for a bronze-processed Nb3Sn multifilamentary superconducting wire with a Nb barrier protecting Cu stabilizer from Sn diffusion.The present expression shows a quantitative agreement with our observed results over the whole range of the amplitude of the applied AC magnetic field up to near Hc2. It is to be emphasized that the values of pinning parameters adopted to estimate the values of hysteresis losses are the same as used in our previous paper to explain quantitatively the hysteresis losses when an AC magnetic field of a small amplitude is superposed to a DC bias magnetic field. Hence the hysteresis losses of a bronze-processed Nb3Sn multifilamentary superconducting wire with a Nb barrier can be described systematically over the whole range of the magnetic field by means of the modified Irie-Yamafuji model for the critical current density, if we take into account that the value of the critical current density contributing to the magnetization is usually larger than that estimated from resistive measurements.It is also pointed out that the theoretical expressions for the hysteresis losses used frequently so far give rise to smaller values than the present expressions by a factor of more than 2, and are not adequate for a quantitative description of the hysteresis losses.The present expressions for the hysteresis losses can also be applied to a bronze-processed Nb3Sn multifilamentary superconducting wire with a Ta barrier in which any superconducting layer is not produced.

Nonresonance absorption of a transverse AC magnetic field in nonuniformly broadened paramagnetic systems

Nonresonance absorption of a transverse AC magnetic field in nonuniformly broadened paramagnetic systems

Permeability of ferromagnetic amorphous ribbons carrying AC and DC electric currents

The complex permeability of 2826 Metglas amorphous ribbons carrying sinewave electric current has been analyzed as a function of the instantaneous value of the current. This “dynamic permeability” is generally higher than the corresponding permeability obtained when a dc current is flowing in the ribbon, the “static permeability”. Measurements have been performed by an adapted ac bridge method with an audiofrequency test field. The behavior of both static and dynamic permeability in as-quenched and annealed ribbons is discussed on the basis of the different domain structures of the samples. The high values of the dynamic permeability have been ascribed to the fact that the test field is able to act on the magnetization processes which are being produced by the ac transverse magnetic field associated with the current through the ribbon.

Ac loss of a multifilamentary superconducting composite in a transverse ac magnetic field with large amplitude

A general study is made of ac losses of a multifilamentary superconducting composite carrying no transport current in a transverse ac magnetic field with large amplitude. In the case of applying a magnetic field below the current-saturation condition in an outermost layer, an eddy current loss as well as a hysteresis loss is calculated by taking account of the hysteresis effect of a superconducting filament on these losses with an apparent complex permeability of the filament. It is shown that the frequency characteristics of the eddy current loss are considerably different from those calculated from previous theories. In the case above the saturation condition, a loss calculation is made taking account of the phase difference between the applied magnetic field and the internal one. It is shown in this case that the hysteresis loss increases rapidly with frequency around the critical frequency. These calculated results are confirmed by experiments with Nb-Ti multifilamentary composites. General features of ac losses are also given for the composite design.

AC Loss in the Coil of a Nonideal Type 2 Superconductor. I. A Row of Wires in a Transverse Magnetic Field

A theoretical expression for the AC loss of the row of wires of nonideal type 2 superconductors in a transverse AC magnetic field is derived with the aid of the critical-state model. The applicability of the present calculation is limited to the so-called semi-quasistatic AC loss of the wires with a pinning force of the Bean-London type in a high DC bias field. Compared with the usual simple slab approximation for a coil, a striking difference is found in some cases with regard to the flux penetration and the value of the AC loss.

Magnetic field dependence of ac losses in multi-filamentary superconducting wires

The magnetic field dependence of ac losses due to nonlinear flux penetration into superconducting filaments, ie, nonlinearity between an applied magnetic field and a penetrated flux, has been studied experimentally for multi-filamentary superconducting wires with Nb-Tl filaments and Cu matrix. In order to observe this effect, ac loss measurements were extensively carried out for the cases of applied transverse ac magnetic fields ranging from 0.06 to 50 Hz in frequency and from 10 −3 to 0.2 T in amplitude. Shifting of the frequency corresponding to the peak of the eddy current loss with the amplitude of applied magnetic fields was revealed experimentally. The results obtained were explained by taking into account the magnetic field dependence of the effective permeability of the wire originated from nonlinear flux penetration into superconducting filaments. The associated frequency dependence of the hysteresis loss is also discussed.