A high-current and low-loss high-temperature superconducting cable for nuclear fusion applications
The primary challenge in applying high-temperature superconducting (HTS) coated conductors (CCs) to AC energy applications, including motors, power transmission, and fusion magnets, is their relatively high AC loss. In this dissertation, a novel high-current and low-loss HTS cable, Patterned superconductors for AC loss minimization and current maximization (PSALM), is proposed and investigated to tackle the AC loss problem. Compared with the existing HTS cables, PSALMs are more compact, more capable of carrying large currents with minimized losses, and easier to manufacture. A 3D finite element method (FEM) model of the PSALM based on the T-A formulation is built to optimize the structure and verify the performance. Samples of PSALMs with a single superconducting layer are manufactured, tested, modeled, and analyzed. In the experiments, the critical current degradation and the transport AC losses are measured. In the simulation, the transport AC losses are calculated and compared with the experimental data, and the current distribution is analyzed in detail. The results show that compared with normal REBCO CCs, PSALMs with optimum patterns can reduce AC transport losses by 60\% through field-canceling and can reduce magnetization losses by the factor of the filament number through patterning. This model is then used to simulate long PSALMs which could be used in practice and calculate the transport AC losses. The losses are found over 30\% lower than that of the normal HTS REBCO tapes, which verifies the loss-reduction performance of the PSALMs. To explore the underlying physics of the PSALM, we then focus on the field-canceling effect of the PSALM introduced by the specially designed patterns on the buffer layers and superconducting layers. From the results obtained by the 2D and 3D models using the T-A formulation, we find this effect can make the current distributed more uniformly and can significantly reduce AC losses. The reasons and the basic principles behind are revealed and analyzed. To sufficiently exploit it to reduce losses, the structure parameters of the PSALM are optimized and an optimum solution is obtained. Then we extend the study to the PSALM with a multiple-layer structure. Current distribution and AC losses of the PSALMs with up to 10 REBCO layers are calculated. The results show that the tapes with even numbers of layers generate lower losses than those with odd numbers, which indicates the superconducting layers should always appear in pairs. Increasing layers can increase the engineering current density of the tape, but the average loss per layer also rises. Therefore, we suggest increasing the distance between the layer pairs or using double-layer PSALM stacks instead. This work provides both experimental and simulation proof of the advantage of the PSALM as a high-current and low-loss HTS cable. It could potentially become the key to compact fusion magnets, which require high power density and suppressed losses.
ac losses, fusion magnet, superconducting cable
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