Magnetic field measurements of laboratory-scale, detached, magnetized bow shocks*

Results are presented from the first experiments to successfully measure the magnetic field of laboratory-scale magnetized bow shocks, performed at the OMEGA laser facility. Magnetized bow shocks occur when the magnetic pressure of an obstacle equals the ram pressure of an incoming supersonic plasma flow. These shocks are common in astrophysical systems, notably in the interaction of the Earth’s magnetic field with the solar wind, where the magnetosphere prevents harmful radiation from reaching the surface. In these experiments the magnetic field is generated by a thin, current-carrying wire, and the incoming plasma is produced by the collision and expansion of two counter-propagating laser-generated plasma plumes.

 

The magnetic field was probed via proton imaging, producing 3 MeV and 14.7 MeV proton images at multiple times in the system’s evolution for two imposed magnetic field strengths. These images show caustic features corresponding to the nominal field from the wire, and additional features caused by compression of the field by the incoming plasma. Path-integrated magnetic fields are reconstructed from the caustic proton images in the first experimental application of a new, multi-energy Differential Evolution reconstruction method. The presence of a bow shock and transient magnetopause are inferred from the reconstructed fields. Having demonstrated the controlled formation of magnetized bow shocks, these results pave the way for future experiments to study more exotic astrophysical magnetospheres at laboratory scales.