Tom Johansen | University of Oslo (original) (raw)
Papers by Tom Johansen
![Research paper thumbnail of Burning of high T[sub c] bridges](https://attachments.academia-assets.com/43429959/thumbnails/1.jpg)
](Applied Physics Letters, 1997
Superconductor Science and Technology, 2012
ABSTRACT A technique for joining superconducting MgB2 pellets has been developed. For the first t... more ABSTRACT A technique for joining superconducting MgB2 pellets has been developed. For the first time, magneto-optical imaging has been used to visualize the superconducting behaviour of an MgB2 joint. No significant difference was found between the penetration of the magnetic flux in the joint and in the bulk at low magnetic fields and at temperatures less than 35 K. The critical current density of the joint in high magnetic field was determined by measuring the transversal and longitudinal magnetic moment of a bar cut from the sample. A description was developed to extract the critical current density from magnetization measurements. The magnetization measurements reveal a critical current anisotropy in the pressed MgB2 and a high critical current in the joint at low temperatures. A possibility to shape the superconductor by pressing, and the conditions preventing development and spread of fractures have been investigated. The joint technique, which utilizes high pressure, high temperature and high electrical current could be scaled up providing a basis for the wide use of superconductors at liquid hydrogen temperatures.
Journal of Physics: Conference Series, 2008
Superconductive MgB2/Al composite material with low and high volume fractions of particles were f... more Superconductive MgB2/Al composite material with low and high volume fractions of particles were fabricated by our special pre-packing technique and 3-dimensional penetration casting (3DPC) method. The composite material showed homogeneous distribution of MgB2 particles in the Al-matrix with neither any aggregation of particles nor defects such as cracks or cavities. The critical temperature of superconducting transition (TC) was determined by
Journal of the Magnetics Society of Japan, 2008
Journal of Applied Physics, 2000
The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric ho... more The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole is investigated theoretically. Exact expressions for the radial and hoop stress are derived using the critical-state model. Stress profiles during a magnetization process often used to activate high-Tc superconductors as strong trapped-field magnets are presented and analyzed in detail. It is shown that due to the hole the tensile hoop stress is enhanced by a factor of 2 or more, depending on the hole diameter. The dramatic increase in cracking probability is emphasized.
Superconductor Science and Technology, 1998
ABSTRACT Superconducting joints between monofilamentary, Ag-sheathed tapes were investigated by m... more ABSTRACT Superconducting joints between monofilamentary, Ag-sheathed tapes were investigated by means of magneto-optic imaging. Two types of joint were studied; one joint with direct contact between the tape cores, and the other one with an Ag layer between them. The local flux distributions directly reveal the obstacles hindering the current flow through the joints. The direct contact of the tape cores provides joints which can carry about 80% of the current of the original tape, whereas the joints with the Ag layer are considerably worse. This difference becomes even more drastic in applied magnetic fields.
Superconductor Science and Technology, 1997
The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical... more The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical (MO) visualization of flux distributions. One single piece of rolled tape is used throughout all experiments starting from as-prepared, and subsequently the tape is bent to 4 mm diameter and then down to 1 mm diameter in 1 mm steps. Flux patterns are visualized using an intact tape, i.e. the visualization is done through the silver sheath after removing the bending strain. It is shown that bending of the tape causes a series of cracks running perpendicular to the tape axis (rolling direction). This crack pattern remains practically unchanged down to 1 mm bending diameter. The flux penetration changes from nearly homogeneous (as-prepared) to entirely crack-governed. Vortices are found to penetrate the tape preferentially along the cracks and from there into the remaining bulk. In bent samples, a very homogeneous flux distribution is observed when the external field is decreased after having reached the full-penetration field. A further decrease of the field forces vortices to leave the sample along the cracks, and trapped vortices remain within the Bi-2223 grains (grain clusters). With each step of bending, the values for the first flux penetration and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranular currents are still flowing in the sample. Our observations demonstrate the capability of the MO technique for non-destructive testing of superconducting tapes.
Journal of Applied Physics, 1991
Using quasi-static harmonic excitations, the lateral force is measured between a permanent magnet... more Using quasi-static harmonic excitations, the lateral force is measured between a permanent magnet and a Bi-based ceramic high-Tc superconductor (HTSC) for lateral displacement amplitudes down to 1 micron. Clear evidence is found for a transition from elastic (nondissipative) to inelastic interaction, and the effect is attributed to flux pinning. The crossover amplitude can easily reach several micrometers, with the consequence that the laterial disturbance of a levitated magnet decays to this amplitude, whereas further damping is extremely slow. For applications of HTSCs in magnetic bearing systems this can be a very relevant aspect of the interaction, and it can set the limit for precision positioning of levitated objects.
Japanese Journal of Applied Physics, 1991
When a permanent magnet, placed above a high temperature superconductor (HTSC), is moved by an ex... more When a permanent magnet, placed above a high temperature superconductor (HTSC), is moved by an external force, the flux lines in the HTSC will follow the magnet and perform a collective motion due to the interaction between the flux lines and the magnet. Here, the collective motion of flux lines in an Y1Ba2Cu3O7-δ (Y123) HTSC is studied by a mechanical pendulum method for various transport currents I through the sample. The magnet is set in low frequency (typically 1 Hz) oscillations with a mechanical pendulum, and the current I is flowing perpendicular to the direction of the magnet motion. The experiments show that the shift of the stiffness (corresponding the shift of the frequency squared) exhibits a cross-over phenomenon: from independence to power law-like dependence for increasing currents. The results are compared with the theories of collective flux creep in HTSCs.
Magneto-Optical Imaging, 2004
PC controlled setup for magneto- optical (MO) investigations of current- carrying superconducting... more PC controlled setup for magneto- optical (MO) investigations of current- carrying superconducting films was developed. It included a current pulse circuit synchronized with the microscope and PC software for image processing allowing us to determine field distributions across superconducting strip using spatially-dependent calibration and iterative inversion to find current distributions. We study these distributions during and after a current pulse, with and without applied magnetic field, current re-distribution during the pulse, precursors to strip burn-out at high currents, and dendritic instability triggered by the pulse. We find critical-state behaviour accompanied by slow creep of magnetic flux at moderate currents in YBCO films, and fast avalanches related to heating effects at higher currents in YBCO and MgB2.
Physics of the Solid State, 1999
ABSTRACT The paper provides the first demonstration of the efficiency of applying the magneto-opt... more ABSTRACT The paper provides the first demonstration of the efficiency of applying the magneto-optic method to studies of the spatial and temporal magnetic-field relaxation in an YBa2Cu3O7 film strip after the transport current is switched on. It is shown that the evolution of magnetic flux distribution is adequately described in terms of a modified Bean model with time-dependent critical current. At a time 50 ms after the current is switched on, the critical current of the samples studied decreases by ≍15%. This proves the significance of thermally activated magnetic flux motion (creep) in the regime investigated. The magnetic vortex pinning energy has been estimated as U 0≍20 kT.
Physical Review Letters, 2003
Journal of Applied Physics, 2002
The European Physical Journal D, 2005
The determination of inter- and intra-filament transport characteristics in superconducting compo... more The determination of inter- and intra-filament transport characteristics in superconducting composites such as BSCCO-Ag tapes is of great importance for material evaluation towards applications. Most attempts to separate the two contributions have relied on indirect methods based on magnetic measurements such as SQUID or magneto-optic imaging techniques. Here we show that laser patterning of superconducting BSCCO-Ag tapes constitutes a simple approach to measure local transport properties in a direct way, even able to separate inter- and intra-filament contributions to the overall transport behavior of the sample. Our technique is potentially useful for sub-millimeter devices based on superconducting tapes.
Applied Physics Letters, 1997
Superconductor Science and Technology, 2012
ABSTRACT A technique for joining superconducting MgB2 pellets has been developed. For the first t... more ABSTRACT A technique for joining superconducting MgB2 pellets has been developed. For the first time, magneto-optical imaging has been used to visualize the superconducting behaviour of an MgB2 joint. No significant difference was found between the penetration of the magnetic flux in the joint and in the bulk at low magnetic fields and at temperatures less than 35 K. The critical current density of the joint in high magnetic field was determined by measuring the transversal and longitudinal magnetic moment of a bar cut from the sample. A description was developed to extract the critical current density from magnetization measurements. The magnetization measurements reveal a critical current anisotropy in the pressed MgB2 and a high critical current in the joint at low temperatures. A possibility to shape the superconductor by pressing, and the conditions preventing development and spread of fractures have been investigated. The joint technique, which utilizes high pressure, high temperature and high electrical current could be scaled up providing a basis for the wide use of superconductors at liquid hydrogen temperatures.
Journal of Physics: Conference Series, 2008
Superconductive MgB2/Al composite material with low and high volume fractions of particles were f... more Superconductive MgB2/Al composite material with low and high volume fractions of particles were fabricated by our special pre-packing technique and 3-dimensional penetration casting (3DPC) method. The composite material showed homogeneous distribution of MgB2 particles in the Al-matrix with neither any aggregation of particles nor defects such as cracks or cavities. The critical temperature of superconducting transition (TC) was determined by
Journal of the Magnetics Society of Japan, 2008
Journal of Applied Physics, 2000
The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric ho... more The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole is investigated theoretically. Exact expressions for the radial and hoop stress are derived using the critical-state model. Stress profiles during a magnetization process often used to activate high-Tc superconductors as strong trapped-field magnets are presented and analyzed in detail. It is shown that due to the hole the tensile hoop stress is enhanced by a factor of 2 or more, depending on the hole diameter. The dramatic increase in cracking probability is emphasized.
Superconductor Science and Technology, 1998
ABSTRACT Superconducting joints between monofilamentary, Ag-sheathed tapes were investigated by m... more ABSTRACT Superconducting joints between monofilamentary, Ag-sheathed tapes were investigated by means of magneto-optic imaging. Two types of joint were studied; one joint with direct contact between the tape cores, and the other one with an Ag layer between them. The local flux distributions directly reveal the obstacles hindering the current flow through the joints. The direct contact of the tape cores provides joints which can carry about 80% of the current of the original tape, whereas the joints with the Ag layer are considerably worse. This difference becomes even more drastic in applied magnetic fields.
Superconductor Science and Technology, 1997
The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical... more The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical (MO) visualization of flux distributions. One single piece of rolled tape is used throughout all experiments starting from as-prepared, and subsequently the tape is bent to 4 mm diameter and then down to 1 mm diameter in 1 mm steps. Flux patterns are visualized using an intact tape, i.e. the visualization is done through the silver sheath after removing the bending strain. It is shown that bending of the tape causes a series of cracks running perpendicular to the tape axis (rolling direction). This crack pattern remains practically unchanged down to 1 mm bending diameter. The flux penetration changes from nearly homogeneous (as-prepared) to entirely crack-governed. Vortices are found to penetrate the tape preferentially along the cracks and from there into the remaining bulk. In bent samples, a very homogeneous flux distribution is observed when the external field is decreased after having reached the full-penetration field. A further decrease of the field forces vortices to leave the sample along the cracks, and trapped vortices remain within the Bi-2223 grains (grain clusters). With each step of bending, the values for the first flux penetration and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranular currents are still flowing in the sample. Our observations demonstrate the capability of the MO technique for non-destructive testing of superconducting tapes.
Journal of Applied Physics, 1991
Using quasi-static harmonic excitations, the lateral force is measured between a permanent magnet... more Using quasi-static harmonic excitations, the lateral force is measured between a permanent magnet and a Bi-based ceramic high-Tc superconductor (HTSC) for lateral displacement amplitudes down to 1 micron. Clear evidence is found for a transition from elastic (nondissipative) to inelastic interaction, and the effect is attributed to flux pinning. The crossover amplitude can easily reach several micrometers, with the consequence that the laterial disturbance of a levitated magnet decays to this amplitude, whereas further damping is extremely slow. For applications of HTSCs in magnetic bearing systems this can be a very relevant aspect of the interaction, and it can set the limit for precision positioning of levitated objects.
Japanese Journal of Applied Physics, 1991
When a permanent magnet, placed above a high temperature superconductor (HTSC), is moved by an ex... more When a permanent magnet, placed above a high temperature superconductor (HTSC), is moved by an external force, the flux lines in the HTSC will follow the magnet and perform a collective motion due to the interaction between the flux lines and the magnet. Here, the collective motion of flux lines in an Y1Ba2Cu3O7-δ (Y123) HTSC is studied by a mechanical pendulum method for various transport currents I through the sample. The magnet is set in low frequency (typically 1 Hz) oscillations with a mechanical pendulum, and the current I is flowing perpendicular to the direction of the magnet motion. The experiments show that the shift of the stiffness (corresponding the shift of the frequency squared) exhibits a cross-over phenomenon: from independence to power law-like dependence for increasing currents. The results are compared with the theories of collective flux creep in HTSCs.
Magneto-Optical Imaging, 2004
PC controlled setup for magneto- optical (MO) investigations of current- carrying superconducting... more PC controlled setup for magneto- optical (MO) investigations of current- carrying superconducting films was developed. It included a current pulse circuit synchronized with the microscope and PC software for image processing allowing us to determine field distributions across superconducting strip using spatially-dependent calibration and iterative inversion to find current distributions. We study these distributions during and after a current pulse, with and without applied magnetic field, current re-distribution during the pulse, precursors to strip burn-out at high currents, and dendritic instability triggered by the pulse. We find critical-state behaviour accompanied by slow creep of magnetic flux at moderate currents in YBCO films, and fast avalanches related to heating effects at higher currents in YBCO and MgB2.
Physics of the Solid State, 1999
ABSTRACT The paper provides the first demonstration of the efficiency of applying the magneto-opt... more ABSTRACT The paper provides the first demonstration of the efficiency of applying the magneto-optic method to studies of the spatial and temporal magnetic-field relaxation in an YBa2Cu3O7 film strip after the transport current is switched on. It is shown that the evolution of magnetic flux distribution is adequately described in terms of a modified Bean model with time-dependent critical current. At a time 50 ms after the current is switched on, the critical current of the samples studied decreases by ≍15%. This proves the significance of thermally activated magnetic flux motion (creep) in the regime investigated. The magnetic vortex pinning energy has been estimated as U 0≍20 kT.
Physical Review Letters, 2003
Journal of Applied Physics, 2002
The European Physical Journal D, 2005
The determination of inter- and intra-filament transport characteristics in superconducting compo... more The determination of inter- and intra-filament transport characteristics in superconducting composites such as BSCCO-Ag tapes is of great importance for material evaluation towards applications. Most attempts to separate the two contributions have relied on indirect methods based on magnetic measurements such as SQUID or magneto-optic imaging techniques. Here we show that laser patterning of superconducting BSCCO-Ag tapes constitutes a simple approach to measure local transport properties in a direct way, even able to separate inter- and intra-filament contributions to the overall transport behavior of the sample. Our technique is potentially useful for sub-millimeter devices based on superconducting tapes.