Speaker
Description
Strong efforts to develop magnets for compact fusion reactors using coated conductors (CC) are presently underway. To determine the operational lifetime of fusion magnets, it is necessary to know the effects of radiation damage on the critical current density J$_c$ of the CC. According to plans, fusion magnets will operate between 10 and 30 K in a background field up to 20 T. As portions of the magnet will be exposed to lower fields, and some to slightly higher ones, and considering that CC are anisotropic, we need to investigate J$_c$(B,$\Theta$,T) up to B>20T, for T≥10K, and for all field orientations ($\Theta$). These extensive studies can only be performed in a limited number of facilities, and a complete J$_c$(B,$\Theta$,T) characterization will be time consuming and expensive.
The pinning landscape in CC is complex, consisting of various types of defects, both spontaneously occurring and introduced as artificial pinning centers (APC), which act in combination, producing cooperative and competitive effects. The resulting J$_c$(B,$\Theta$,T) is unique to each deposition method, to the APC nanoengineering, and to processing parameters. The radiation damage will interact with preexisting landscape and reduce the superfluid density in ways that need to be explored and understood in each case. Thus, the studies will have to be repeated for each reactor design and for each CC. Although that detailed investigation cannot be totally avoided, we have designed an approach to make that process as efficient as possible.
In the B-$\Theta$-T space, CC exhibit several pinning regimes that can be identified by the functional dependences of J$_c$(B,$\Theta$,T) and the pinning force F$_p$(B,$\Theta$,T). It is extremely difficult (if not impossible) to infer the effects that adding defects will have in one regime by measuring their effects in another one, thus investigating the irradiation/annealing evolution in a regime different from those to occur in the actual magnets has limited value. However, if the functional dependencies of J$_c$(B,$\Theta$,T) are known, we can apply scaling rules to infer J$_c$ at different B-$\Theta$-T conditions as long as the sample remains in the same pinning regime. Our approach is to map the dependences and boundaries of those regimes in the B-$\Theta$-T space for unirradiated CCs, identify which are relevant to fusion reactor magnets, and then follow their evolution with fluence for different types of irradiations (neutron, proton) at various conditions (room or cryogenic temperature) and after thermal annealing, using a “ladder of experiments” of increasing complexity, from permanent magnets to electromagnets, SQUID up to 7T and VSM up to 14T, and pulsed fields up to 65T; each step informing the next one. Thus, the irradiation effects can be assessed by performing extensive studies using fast and easily accessible systems, complemented with a limited number of the more involved measurements at selected samples and conditions. In this talk I will describe our methodology and present initial results for unirradiated and irradiated samples.