Homogeneous magnetic fields are required in different applications. The resolution of MRI techniques depends on the quality of the magnetic field, as well as the efficiency of electron cooling systems used at particle accelerators. Unclosed magnetic shield made of superconducting tapes is able to nullify the radial component of a solenoidal magnetic field, forming the long region of the homogeneous magnetic field. The shield is a lengthwise winding made from YBCO tapes are wound along a carcass generatrix. Then it is positioned coaxially inside an electromagnet. The measurements were carried out under quasistationary conditions, magnetic fields up to 1 kG at 77K. This paper discusses the design of the superconducting shield and presents experimental and numerical studies into the homogeneity of the magnetic field in solenoids with the superconducting shield.

*e-mail: dorofeevgl@mail.ru Abstract The development of high-temperature superconducting (HTS) materials, especially HTS tapes of the second generation (2G HTS tapes) give us new opportunities to use superconductivity in high-energy physics devices and, in particular, in charged particle accelerators. The influence of the unclosed magnetic shield made of 2G HTS tape on the dynamics of a charged particles beam is studied in this paper. Within the framework of the simplest model of the interaction of the magnetic field of a charged particles beam with a superconducting shield, estimates of this influence are made. The superconducting shield shifts of a charged particles beam to its axis. In particular, the beam of ions 197Au79+ energy of 4 GeV per nucleon and a current of 5 A passing at a distance of 5 10 mm from the axis of the superconducting screen with a radius of 50 mm and a length of 5 meter is shifted to the axis of the screen at a speed of about 130 m/sec. That is, the beam itself can be centered for a lot less than 1 second. The possibilities of using an unclosed superconducting shield to compress a charged particles beam are discussed, as well as the possibilities of controlling the shield.

Ensuring the high homogeneity of a magnetic field in the straight solenoid of an electron cooling system is a very important task. In the electron cooling system of the collider in the NICA project, it is planned to use superconducting solenoids for the generation of a longitudinal magnetic field. Using of the superconducting shield is proposed to achieve the required homogeneity of the magnetic field in the cooling section. This article discusses the design of the superconducting shield and presents experimental and numerical studies into the homogeneity of the magnetic field in solenoids with the superconducting shield.

Pinning and magnetic flux diffusion in composites with artificial pinning centers (APC) are studied using a magnetic step method. It is revealed experimentally, that a magnetic flux pinning is determined by boundaries of superconducting filaments and matrix in composites with thickness of filaments less of London's penetration depth. The elementary pinning force on n-s boundary for Cu-NbTi, Nb-NbTi,.. composites is measured. The value of elementary pinning force is controlled by thickness n-s boundary. And the thickness n-s of boundary can be large in result of solid state diffusion during technological process. The thickness n-s boundary is estimated through the dependence of superconducting filaments effective volume from the magnetic field value. These results for elementary pinning force and thickness n-s of boundary are compared to results of direct measurements of critical currents in these APC superconducting composites.

The experimental results of curling effect investigation (sharp increasing of the normal phase propagation speed under influence of the magnetic flux avalanche) in superconducting Nb-Ti and Nb-Zr wires are presented. The external magnetic field was imposed parallel to an axis of a sample of a superconducting wire by thickness from 0.16 mm up to 0.30 mm without a normal stabilizing covering, the transport current was put, and after initiation of a normal phase on a short part of a sample the normal phase propagation and the magnetic flux avalanche along a sample were registered simultaneously. It was found, that the thickness of the magnetic flux avalanche L-mfa and the thickness of the normal phase front L-npf (length of the part of a sample engaged by S-N transition) achieves 3-4 mm, that is more than 10 diameters of the sample. The heating of a superconducting wire by the magnetic flux avalanche is accompanied by increase of the speed of normal phase propagation V-npp. In a narrow range of change of a transport current the sharp increasing of the speed of normal phase propagation V-npp up to the speed of a magnetic flux avalanche V-mfa occurs. The further increasing of a transport current is accompanied by reduction of a distance (spacing) between the normal phase and the magnetic flux avalanche. In turn V-mfa increase with increasing of critical current dencity J(c) and thickness of a superconducting wire. In particular the speeds V-mfa and V-npp achieve the value 9-13 km/s for the wire Nb-25%Zr by thickness of 0.24 mm with the critical current J(c) = 2-4 MA/cm(2) in magnetic fields 0.5-1 T.

Steady state propagation of a magnetic perturbation along a monofilament superconducting wire in circumstances of thermomagnetic instability was investigated. At temperature T=4.2K and magnetic field B=0.5-0.7T the propagation velocity is v = 2.5 - 3.5 km/sec in NbTi wire and this one exceeds a 9 km/sec in NbZr wire.

Magnetic flux flow in layered NbTi/Nb and textured superconducting ceramics Bi2Sr2CaCu2O8+d with the nearly reversible magnetization curve are measured. The characteristic time of the magnetic moment relaxation of such superconductors is determined by the viscous flux flow. It is observed, that in a parallel to superconducting layers external magnetic field the characteristic time of the magnetic flux inflow into the sample and the time of the flux outflow from the sample differ in several times. This difference in the characteristic times is unequivocally due to the flux flow into the sample through the end plates of the thin superconducting layers (plates), where Bean-Livingston barrier is effectively suppressed, but the flux flow out from the superconducting layers (plates) through the wide sides of the plates freely. The experimental proof of the barrier absence for the outflow of the magnetic flux on the border of the superconductor is received. Thus is shown, that the border (surface) of the type-II superconductor is the "semiconductor" for the magnetic flow: it lets the flux to flow out from the superconductor freely, but it interferes with the flux to flow into the superconductor.

The performances of the installation used to measure a.c. energy losses of short superconducting samples in an external pulsed magnetic field are given. The calorimetric method at 4.2 K is used. The measurable range of losses is from 10 up to 1000 mW. The results of loss measurements of superconducting wires and tubular cables with a various diameter of superconducting filaments and twist length are presented. The experimental values of hysteresis and eddy current losses are compared with the calculated ones. The influence of the Nb barrier and solid-state diffusion on the loss values is investigated.

The tuning of the magnetic fluxes to the pinning structure oriented along superconducting filaments is the reason of the existence of the magnetic field component Bperpendicular to perpendicular to the external magnetic field H. The dependence Bperpendicular to from the value and the orientation of an external magnetic field H is measured for multifilamentary NbTi sample. The flux capture by the pinning structure is observed when an external magnetic field is almost parallel to filaments. The dependence Bperpendicular to from the external magnetic field value allows estimating the pinning structure scale of the superconductor.

The sonic range velocities of normal zone propagation in high current density superconductor were observed. The experimental results were obtained on samples of 0.16–0.11 mm thickness of a monofilamentary NbTi wire with critical current density J c about 10 6 A/cm 2 . The s→n change of the sample sections was recorded after the normal zone stimulation by the magnetic field impulse. The voltage increasing speed reached 1.5×10 7 V/s, and the normal zone propagation velocity exceeded 5±1 km/s. This high velocity can be caused by the detonation-like propagation of normal phase in superconductor with high current density.

The current-carrying capacity of traditional NbTi-wires and NbTi superconductors with artificial pinning centers was analyzed. These materials not only have different values of Jc for the same size of flux pinning structures, but different field dependencies Jc(B) too. The authors show that Jc of the traditional NbTi-wires is determined by the inhomogeneity of the beta -matrix. That is the flux pinning is made on the normal regions of the beta -matrix, but the size depends on the value of the external magnetic field.

The nature of current voltage characteristic of NbTi-wires with inhomogeneous filaments was investigated on the wires with finite length filaments and filaments partially damaged by intermetallic particles created on the filament-matrix boundary. The quantitative relation between the quality factor I(o)/I(c) and degree of damage of superconducting filaments d(d)/d(f) was found: I(o)/I(c) = 0.16 x d(d)/d(f). The E(I) shape at low electrical field was assumed to be determined by current distribution between superconducting filaments, and it is nearly linear exponential: E = E(c)eff exp((I - I(c)eff)/I(o)eff), where E(c)eff = 8rI(o)N3l-2, I(c)eff = N (I(c)+I(o) ln(E(c)/E(c)eff))) and I(o)eff = N (I(c)+3I(o)+I(o) ln(E(c)eff/E(c)))16. At high electric field E(I) is near to one for homogeneous wire: E = E(c) exp((I - I(c))/I(o)).

YBa2Cu3Ox and YBa2Cu3Ox + Ag melts were quenched on heated Zr substrates in an oxygen atmosphere. The critical temperature of the samples was about 70 K. We observed a weak magnetic field dependence of the critical current. The current density was about 105 A/cm2 in 0–3 T fields. A liquid quenching method for the fabrication of high-Tc superconductors with large current-carrying capacity at nitrogen temperatures is discussed.

Processes of electromagnetic diffusion in mono- multifilamentary superconducting wires having an exponential transition characteristic from a superconducting to normal state were studied theoretically and experimentally. Corrections arise due to the account taken of the logarithmical dependence of the current density on the local electrical field. These are expressed by a real transient characteri...

Frequency dependence of hysteresis loss in single-filament superconducting wires is studied. (AIP)

The diffusion of a transverse magnetic field is studied in a multifilament nontwisted conductor with superconducting filaments. (AIP)