One of the main goals in development of magnets for FAIR is the reduction of AC losses in coils under pulsed magnetic field. It is well recognized that the main contributions to AC losses in coils during a magnet cycle come from filament hysteresis and inter-filamentary coupling currents. To reduce filament hysteresis loss and inter-filamentary coupling current loss, the wire has to contain small-sized filaments and inter-filamentary matrix with a resistive Cu alloy, and to provide sufficient OFHC copper matrix outside and inside the filamentary region for stability. A wire, with a filament size of 3.5 down to 2.5 microns, with Jc>=2700A/mm2 (5T; 4.2K), Cu/non Cu ratio of 1.8-1.3, and filament twist pitch as small as possible without a degradation of Jc is therefore of interest.
It is well known that maximum Jc and lowest losses are achieved in NbTi wires with a Cu matrix and produced using single stacking. However, the GSI decision to change wire (strand) diameter from 0.65 to 0.825mm for SIS300 magnets, with the aim of providing enough temperature margin for the adopted Rutherford cable configuration, results in a drastic increase in the number of initial stacking elements which must be inserted into the 250mm billet (up to 40,000 for 2.5 microns filaments) and decreases their size to less than 1mm. One could expect certain difficulties with the single stack approach, with such an element size. Therefore, another approach for wire design should be explored which enables the production of the wire in a more industrially feasible way, keeping all super-conducting properties at an acceptable level.
The main objectives of this proposal are:
1. Calculation of a new wire design, meeting the requirements of strands for SIS100 and SIS300 magnets, using both single stacking and double stacking approaches;
2. Fabrication of experimental prototype wires with the new design;
3. Full characterization of wires properties
4. Fabrication and characterization of a model cable, based on these wires.
This project is expected to yield the following results:
- A new wire design that allows the production of low loss wire in an industrially feasible way will be proposed.
- The database of properties of resistive matrices will be extended.
- The dependences of: Jc on filament diameter and twist pitch; hysteresis losses on filament diameter; total losses on field ramp rate and twist pitch; and Jc on temperature will be investigated:
- The results obtained will allow performing the comparison of super-conducting properties of the wires fabricated in different ways.
- Model cables, based on these wires will be fabricated and tested.
The generated knowledge will be disseminated to scientific groups, concerned with superconductors operating in fast-cycling fields. The new approaches, used during this work, could be used in the development of mass production technology for the wires used in FAIR magnets.
Dubna, Moscow Region