Evidence for radiative collapse and magnetic flux pile-up in a radiatively cooled magnetic reconnection experiment*

The Magnetic Reconnection on Z (MARZ) platform achieves strong cooling in a magnetic reconnection experiment driven by pulsed power. Two inverse aluminum wire arrays, simultaneously fielded on the Z machine (20 MA peak current, 300 ns rise time, Sandia National Labs), generate oppositely-directed supersonic and super-Alfvénic plasma flows with anti-parallel magnetic fields. Interaction of the magnetized flows generates a reconnection layer (S_L ≈ 120, normalized reconnection rate ≈ 0.3), where line and recombination-bremsstrahlung losses provide a cooling rate much larger than the Alfvénic transit rate (τ_cool^-1/ τ_A^-1  ≈ 50). Time-resolved characterization of the rapidly-falling >1keV X ray emission, and simultaneously rising visible emission from the reconnection layer, demonstrates decreasing layer temperature and rising density, indicative of radiative collapse. X ray diode and spectroscopic measurements provide constraints on the layer temperature. The reconnection layer is further visualized using multi-frame self-emission imaging of optical (540-650 nm) emission. These images show planar discontinuous regions of enhanced emission upstream of the reconnection layer, providing evidence for shock-mediated magnetic flux pile-up, consistent with the super-Alfvénic inflows.

*SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525