S. I. Tyutyunnikov (VBLHEP)
VBLHEP: V. N. Shalyapin, V. V. Efimov, Yu. S. Kovalev, V. A. Artyukh, M. N. Novikov

Investigation of the radiation effect on the field dependence of the critical current of the 2nd generation HTSC tape is aimed at meeting two tasks - to measure the radiation stability (fluence, at which the critical current decreases significantly) and to clarify the possibilities of developing artificial pinning centres using radiation - that is, to increase critical current in magnetic fields. There are reasons to believe that defects in the structure of a superconductor, caused by high energy particles, can not only worsen but also at the initial stages, improve superconducting properties, moreover, in specified ranges of fields and temperatures.
The structure of the 2nd generation HTSC tape is shown in Figure 1. The tape considered in the report has the following parameters: the thickness of the HTSC layer is 1-2 microns, the substrate is Hastelloy 40 microns, the coatings are silver 3 microns, copper 3-7 microns, PIC-61 solder – 5-10 microns, tape width is 4 mm.

Fig. 1. Structure of the 2nd generation HTSC tape

In 2020, two HTSC tape samples from one batch of Superox tape were irradiated with 14.2 MeV neutrons perpendicular to the surface. Two radiation sessions with subsequent measurements of critical currents in fields perpendicular to the tape surface up to 0.1 T at a temperature of 78 K when immersed in liquid nitrogen were carried out.

In the first session, the first sample, on the graphs (Figures 2 and 3) its currents are indicated by blue diamonds, was irradiated with 2x10¹¹ neutrons per cm². The second sample (current values ​​are brown squares in the graphs) was not irradiated. The results of measurements of critical currents are presented in Figure 2. It can be seen that the critical currents of the irradiated sample increase to about 10 mT and more rapidly decrease from a field above 70 mT.

Fig. 2. Field dependence of the critical current of irradiated and non-irradiated HTSC tape samples at a temperature of 78 K

In the second session, both samples were irradiated with a fluence of 2x10¹¹ neutrons per cm². In the first sample with a fluence of 4x10¹¹, the critical current from its own field to about 20 mT decreased by few percent. The current of the second sample with a fluence of 2x10¹¹, in the same fields up to 20 mT, on the contrary, increased by the same few percent. And in fields from 70 mT, the current of the second sample decreased and was equal to the current of the first sample that practically did not change in these fields with extra 2x10¹¹ fast neutrons per cm².

At first sight, there is a clear radiation effect and it is necessary to study the structure of samples and defects, to carry out radiation sessions with large fluences, as well as smaller “steps”, to develop a physical model of pinning for a given type and concentration of defects. However, effects of this scale - a few percent of the critical current can also be explained by the inhomogeneity of the HTSC tape (comparison of samples after the first session), damage caused by resoldering and thermal cycling in nitrogen and to some extent even by measurement conditions, such as the level of liquid nitrogen on the current leads. Therefore, we consider it necessary, in order to verify and further refine the results, in 2021:

1. To carry out a range of irradiations of several samples (to collect statistics) with the same fluences, measuring critical currents before and after each irradiation.
2. To develop measurement techniques both in liquid nitrogen and under other conditions (besides, it is very interesting to see other temperatures and fields, especially 5-30 K and a field of several T at different angles to the HTSC surface).
3. To investigate the structure and defects of these two samples of HTSC tape and samples with different fluences in the following experiments.
4. To develop a physical and mathematical model and to calculate the expected change in the critical current under such irradiations.
5. To consider the possibility of irradiation with higher fluences and not only using fast neutrons with different energies but also charged particles. The size of the created defects depends on the energy and charge that can help to “enhance the pinning” in certain, given, ranges of fields and temperatures.

            The prospects of this research are the development of an HTSC tape with artificial pinning centres for the production of magnets for particle accelerators and other facilities operating on liquid nitrogen or neon; production of magnets for devices operating under conditions of irradiation with fast neutrons and charged particles - particle accelerators, thermonuclear fusion devices, magnets for space applications, development of their protection from radiation. Interested organizations and possible partners may currently be Superox, MEPhI, Kurchatov Institute.