The idea of transformation inside a null-space region is introduced for the first time, and used to design a novel DC magnetic compressor that concentrates DC magnetic flux greatly and behaves as a DC magnetic funnel. The proposed device can be used as a passive DC magnetic lens to achieve an enhanced DC magnetic field (e.g. 7.9 times or more depending on the size and other parameters of the compressor) with a large gradient (e.g. 400T/m or more) in free space. After some theoretical approximation, the proposed device can be easily constructed by using a combination of superconductors and ferromagnetic materials. Numerical simulations are given to verify the performance of our device. The proposed method (use a null-space region as the reference space) can be extended to reduce the material requirement when designing other devices with transformation optics.
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