Experimental study of shock-wave magnetic cumulation

Experiments with aluminum, copper, and silicon powders are performed to study the mechanism of shock-wave magnetic cumulation. For all substances examined, the magnetic field as a function of the cavity area is described by a power dependence with a constant exponent $\alpha$. The value of $\alpha$ depends substantially on the substance porosity and particle size. For copper and silicon powders and for small-size aluminum powder, the value of $\alpha$ is consistent with the ratio of the particle velocity $u$ to the wave velocity $D$, as is predicted by a simple model of magnetic cumulation. For the fine and coarse aluminum powders, the value of $\alpha$ is noticeably smaller than the ratio $u/D$. The lower effectiveness of magnetic compression can be attributed to insufficiently high electrical conductivity (fine powder) and the emergence of conductivity with incomplete compression of the substance (coarse powder). In the first case, diffusion losses of the magnetic flux in the compressed substance are fairly noticeable. In the second case, the work against the magnetic forces is performed by a layer in the shock-transition region, which has a lower particle velocity. The mechanism of magnetic cumulation involves substance metallization under shock compression and expelling of some portion of the magnetic flux to the non-conducting region ahead of the shock front. The two-stage mechanism of cumulation known in the literature (metallization in the elastic precursor and subsequent compression of the field in the main wave) is not validated by experiments with measurements of the particle and wave velocities and electrical conductivity and by experiments on magnetic cumulation.