Masakatsu Takeo

Abnormal quench process with very fast elongation of normal zone in multi-strand superconducting cables

Abstract An abnormal quench process with apparent high propagation velocity of the normal zone, ·fast quench’, was studied in relation to electromagnetic and thermal couplings among strands for multi-strand superconducting cables. The quench process was numerically evaluated by simultaneous equations of heat balance and equivalent electrical circuit of the multi-strand cables. Experimental results are well explained by the numerical simulation concerning threshold current of the fast quench for a two-strand cable. The quench process of a six-strand cable was also calculated numerically and the relation between the fast quench and current commutation among strands is discussed.

AC Inductive Measurement of Intergrain and Intragrain Currents in High-Tc Oxide Superconductors

Bulk intergrain current and closed intragrain current in sintered Y-Ba-Cu-O superconductors were measured at 77 K by using an ac inductive method. These currents can be separated because of the large difference in penetrating rates of the magnetic flux into specimens with respect to the ac field amplitude. The obtained intergrain current density agreed approximately with critical current density measured resistively. The closed intragrain current density amounted to 4.3×108 A/m2 at B=0.5 T and decreased gradually with increasing magnetic field. The present results show that the ac inductive measurement is one of the available nondestructive methods to characterize sintered oxide specimens.

Estimate of Attainable Critical Current Density in Superconducting YBa2Cu3O7-δ

Attainable critical current density in oxide superconductor YBa2Cu3O7-δ is estimated by an ordinary analyzing method in flux pinning phenomena with superconducting parameters obtained from a magnetization measurement. The pinning mechanisms considered here are an anisotropy in the upper critical field and an electron scattering effect for grain boundaries and twinning planes. While the elementary pinning force of these planar defects is small due to a small thermodynamic critical field at 77 K, a critical current density comparable to that of Nb-Ti at 4.2 K is expected to be attained by increasing the pin concentration.

Flux Jumps in Nonideal Type-2 Superconductors. I. Theory of Flux Jumps in Slabs

A theory of flux jumps in a slab of a nonideal type-2 superconductor is developed by the use of a phenomenological theory of flux motion. To include explicitly in the theory the various quantities which are controllable experimentally, the temperature inside the slab is assumed to be approximately uniform. Then a flux jump is characterized by a catastrophically sharp rise of the temperature of the sample. The criteria for the occurrence of flux jumps are derived under such an assumption, together with the relation between the value of external magnetic field, at which a flux jump starts, and the sweeping rate of the external magnetic field. Various behaviors of flux jumps observed experimentally are reasonably explained by the present theory in a quantitative way.

Critical Transport Current Density in Sintered Oxide Superconductors with High Critical Temperature

Percolative critical transport current density in sintered Y–Ba–Cu–O superconductors is estimated by taking into account the effect of anisotropies in the upper critical field and the pinning structure. This value is much smaller than the estimate for the optimized situation, where misorientation between the magnetic field and anisotropic pinning structures does not occurs. As the volume fraction of the superconducting region becomes small, the critical transport current density decreases drastically due to a decrease in the percolation probability of the current. To improve the transport property, it is necessary to align the orientation of grains or to introduce isotropic pinning centers, in addition to increasing the superconducting volume fraction.

Current non-uniformity in multistrand superconducting cables. Experimental studies and its influence on stability of superconducting magnets

It is widely acknowledged that current non-uniformity is a major source of reduction of quench currents at nonsteady state conditions in multistrand superconducting cables. It is important to study current non-uniformity in experiments. In this paper we present a review of the experimental methods used to study current non-uniformity in CICC and AC multistrand superconducting cables and results received. We discuss the influence of current non-uniformity on stability of superconducting magnets made from multistrand superconducting cables and possible ways to overcome their instability.

Effect of flux creep on current-voltage characteristics of superconducting Y-Ba-Cu-O thin films

Temperature dependence of current-voltage characteristics of Y-Ba-Cu-O thin films is studied experimentally. The observed results are analyzed with the flux creep model, and good agreement is obtained between the theory and the experiment. Using the flux creep model, the pinning potential, U, is obtained as U(T=0)\gtrsim77 meV for thin films with jc(T=0)\gtrsim2×107 A/cm2. The temperature dependence of U is also obtained, and the result is discussed based on the pinning theory.

Relation between frictional loss and combination of thermal coefficient of bobbins and winding tension in AC superconducting coils

Frictional losses in windings are one of the loss types in AC superconducting coils. When an AC current is supplied to the coil, a superconducting wire in the winding start to vibrate. And frictional heat generates at contact segments between the wire and a coil bobbin. And hence, the losses are not electromagnetic losses such as coupling losses but mechanical losses in superconducting coils. We prepared four types of bobbin materials. Two bobbins expanded during a cooling process from room temperature to liquid helium temperature. The other two bobbins contracted during the cooling down. Winding tensions for the four kinds of coils were 0.5, 3.5, and 5.0 N. And then, the AC losses of the twelve coils were measured. When the coils whose bobbins have thermal expansion were used, the AC losses increased with the gain of the winding tensions, in spite that the experimental conditions such as coil currents and background magnetic field were same. On the contrary, in case of the contraction bobbins coils were used, the losses decreased with the tensions increased. To analyze the experimental cases of the winding tensions of the sample coils at the liquid helium temperature were calculated, and the relation between the tensions at the cryogenic temperature and the frictional losses was discussed.

EFFECT OF CUMULATIVE ABLATION ON THE EJECTION OF PARTICULATES AND MOLECULAR SPECIES FROM YBA2CU3O7-X TARGETS

Effects of cumulative ablation on the ejection of particulates and molecular species in pulsed‐laser deposition are studied by Mie scattering and laser‐induced fluorescence spectroscopy, respectively. When a fresh target is ablated, a large amount of particulates are ejected during several initial shots and rapidly decreased within the first ten shots of ablation. This is due to the ejection of powder residues which are struck on the target surface during the polishing process. After this period, ejection of particulates increased gradually and almost saturated after 200 shots. The saturation characteristic is empirically formulated as a function of the number of cumulative ablations. On the other hand, ejection of molecular species rapidly decreases during the initial 500 ablations and afterwards decreases more slowly with further ablation. The effects of cumulative ablation on the particle ejection are discussed in conjunction with the structural modification of the ablated surface observed by the scann...

Influence of sample geometry on amplitude of eddy current oscillation in Rutherford-type cables

Interstrand eddy current induced in the samples of flat two-layer superconducting cables of finite length by ramping up uniform magnetic field are found to be periodic. Corresponding energy losses are compared with those calculated for eddy currents being uniform along the sample length. Dependence of amplitude of eddy current oscillations on the sample geometry is considered.<<ETX>>