Measurement of the decay of the Screening Induced Current Field (SCIF) in a YBCO tape pancake coil. The coil is charged to its nominal current I; the screening currents induce a magnetic field and the change in this field over time is measured.
Discovered 30 years ago, high-temperature superconductors (HTS) have the specific quality of maintaining their superconducting properties at temperatures above that of liquid nitrogen (77 K). Yet the most valuable characteristic for our activities is their current-carrying capacity under intense magnetic fields at the temperature of liquid helium (4 K).
Over the past decade, this current-carrying capacity has been constantly improved, especially for Rare Earth Barium Copper Oxide (REBCO) tapes. Naturally, SACM is involved in the study of these materials as part of its research into intense magnetic fields above 30 T and accelerators for fields close to 20 T.
INTENSE MAGNETIC FIELDS
REBCO high-temperature superconductor tapes offer new prospects for the design of high-field magnets. Yet their use comes with specific issues such as quench protection and screening currents.
As a result of the tape shape of REBCO superconductors, large parasitic currents are generated during field variations, causing disturbances in the field produced. A thesis is currently underway to better understand this phenomenon, measure its effects on the central field of prototype coils in experiments, and model it using numerical simulation tools. The curves show the slow decay, over time, of the magnetic field generated by the screening currents in a 60-turn pancake coil [the decay time constant is several hours (logarithmic over time)]. Several techniques are being explored to accelerate screening current decay (from logarithmic to exponential) through a “vortex shaking” process, with an experiment funded as part of SACM’s internal R&D.
PROTECTION THROUGH CO-WINDING OF METALS
Protecting HTS magnets is more difficult than for magnets made from LTS material because of the low propagation speeds in the resistive zones and the difficulty to detect the quench (very low voltage).
New protection solutions are now emerging, which SACM has naturally endeavored to explore through studies of the MI (Metal as Insulation) solution. The latter consists in replacing the usual insulation (polyimide) with an electrically resistive tape (stainless steel). This solution, which has never been studied from a protection point of view, should allow the same high level of thermal stability of NI (No-Insulation) coils, while considerably reducing the current charging time (to something near that of a standard insulated coil). The NI technique consists of winding the tape without insulation between the turns. The research in progress aims to understand the phenomena so that they can be predicted more easily. Because validation of the numerical models is essential, several highly instrumented samples (pancake coils with several hundred turns)—see photo—are being studied at 77 K (LN2) and 4.2 K (LHe). An example of a quench at 77 K is shown in the graph opposite. The quench is initiated by a heater inserted within the coil (red pulse). The drop in magnetic field (shown in blue), even though the current of the coil (in red) remains constant, shows the transited part being bypassed: the current can avoid the highly resistive zone by spreading into the adjacent turns through the metal insulation.