N. A. Chernoplekov

Some results from the T-7 tokamak superconducting magnet test program

The first results from the superconducting toroidal coil test program for the T-7 tokamak are described. The coils, which have a cold mass of 12 tons and a field volume of 6m3, utilize a force-cooled circulation system. The system was cooled to 4.5K in 4 days using an HGU-250/4.5 refrigerator with a nominal capacity of 400W. The coils were initially charged to 4500A, and then to 4800A, or 75% of the critical current at 4.2K, which was achieved without quenching. At this current the magnetic field at the torus axis was 2.4T, the maximum field at winding 4T, and the stored energy 12MJ. Total charging time was 5 hours and the discharge time 20 minutes. The T-7 installation is now being prepared for plasma physics experiments.

Status and trends of S.C. magnets for thermonuclear research in the USSR

The report deals with some problems of developing superconducting magnet systems for Tokamak T-15. An account was taken of the experience gained in designing and operating superconducting magnet systems for the installations being used for plasma physics research.

Specific heat and electrical resistivity of an icosahedral-structure Zr70Pd30 alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr70Pd30 alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ∼50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr70Pd30.

Angular dependence of the specific heat of La1.85Sr0.15CuO4 in superconducting mixed state

Abstract The specific heat of single crystals La 1.85 Sr 0.15 CuO 4 has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu–O plane and normal to this plane. Measurements were carried out in the temperature range 2–50 K in magnetic fields up to 8 T and with four directions of the magnetic fields: in the a – b plane (along the [100] and [010] directions) and at angles of 45° and 90° with respect to the a – b plane (along [001] and [103] directions). For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field. The dependence of the specific heat on the magnetic field H shows the feature predicted for d -wave pairing: H 1/2 T term. A fourfold symmetry characteristic of the electronic density of states in the crystalline a – b plane and a twofold symmetry in a – c plane was resolved in the magnetic field. The results show unambiguously that the in-plane and out-of-plane electronic density of states in the magnetic field is highly anisotropic and has a minimum when the field is along the a -axis and a maximum when the field makes an angle of 45° with the a and c axes. Using these results, we present an angular mapping of the electronic density of states and the upper critical field H c2 ( T ) estimated from the heat capacity measurements. These results are consistent with d x 2 − y 2 -symmetry of the bulk order parameter.

Thermodynamic and kinetic properties of an icosahedral quasicrystalline phase in the Al-Pd-Tc system

The properties of a quasicrystalline phase in the Al-Pd-Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al70Pd21Tc9 alloy has a face-centered icosahedral quasi-lattice with parameter a=6.514 Å. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al70Pd21Tc9 sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2–300 K. The electrical resistivity is found to be high (600 µΩ cm at room temperature), which is typical of quasicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ=0.24 mJ/(g-atom K2)) and the Debye characteristic temperature (Θ=410 K) are determined. The origin of the specific features in the vibrational spectrum of the quasicrystals is discussed.

Progress in research and development for high temperature and low temperature superconductors

It is noted that the most significant result in the development of Nb-Ti conductors in the last few years is the attainment of the critical current density J/sub c/(5 T, 4.2 K)=(3-4)*10/sup 9/ A/m/sup 2/ in industrial lots of wires. This value is close to the upper limit for traditional thermomechanical treatments with low-temperature annealing. New results on Nb/sub 3/Sn and other A-15 compounds have been obtained using methods that overcome some weak points of the bronze process. However, the bronze technology will remain the most suitable for large-scale production. Active work on the first stage of HTSC (high-temperature superconductor) development has produced some promising results. Critical current density values at 4.2 K and 77.3 K have been obtained in bulk samples such that practical applications of HTSC in high current technologies can be seriously considered. >

Vibrational and electronic characteristics of Zr70Pd30, Zr80Pt20 icosahedral quasicrystals and their amorphous Counterparts

The heat capacity of Zr70Pd30 and Zr80Pt20 icosahedral quasicrystals and their amorphous counterparts is studied in the temperature range 1.5–500 K in order to establish a correlation between the short-range atomic order and the physical properties of these compounds. A comparison of the data made it possible to reveal changes in the vibrational spectra within the low-and high-energy ranges, as well as in the density of states, superconducting characteristics, electron-phonon interaction, and anharmonicity of the lattice thermal vibrations and to calculate the main average frequencies (moments) characterizing the vibrational spectra. The lower superconducting transition temperature Tc of the quasicrystals as compared to that of the amorphous counterparts can be associated with the decrease in the density of states on the Fermi surface, the hardening of the phonon spectrum, and the weakening of the electron-phonon coupling.

Phonon dispersion and soft phonon modes in La1.9Sr0.1CuO4−y

Abstract Using the inelastic neutron scattering method, the low-frequency dynamics of the La1.9Sr0.1CuO4−y compound lattice was studied on a single crystal of about 0.4 cm3 in bulk. It has been established that at the phase transition from the tetragonal structure to the rhombic one, there takes place frequency softening of a phonon with the wavevector q = ( 1 2 , 1 2 , 0) and eigenvector corresponding to oxygen octahedra rotation around copper atoms, which accompanies the structural transition (Tstr=308 K).

Superconducting conductor for T-15 toroidal magnet

Design parameters and material and performance requirements are presented and evaluated for the superconducting toroidal field magnet for the next-generation T-15 tokamak reactor. The superconductor currently under consideration is a composition of an eleven-strand transposed niobium-tin conductor and two copper tubes, combined by a layer of electrolytic copper. The superconducting properties of the niobium-tin bronze matrix composites were analyzed, including critical current density, stability under plasma current flow disruption, critical temperature and field, dependence of the critical parameters on strain, reaction of the current-carrying element to a longitudinal pulsed field, and the smeared nature of the transient characteristic. Testing scenarios are described. Superconducting values were also calculated.

25 tesla hybrid magnet

A hybrid solenoid generating a stationary field up to about 25T in a 28 mm working bore is described. The inner section of the hybrid system represents a conventional water-cooled coil, while the outer coil is made of a superconductor. Some of the insulation and power supplies for the magnet are described. (MOW)