A high resolution study of the penetration of a magnetic field into a low-resistivity multi-ion-species plasma

We present high-resolution observations of a magnetic-field front (peak magnitude ~ 8 kG) propagating through low-resistivity, multi-ion species plasma (mainly protons and carbon ions, electron density ~ 2.5×10¹⁴ cm^-3 and temperature ~ 6 eV). Diagnostic methods are developed in order to reveal the details of the interaction, including the evolution of the magnetic-field front and plasma properties. These methods are based on controlled injection of trace-element ions (via an optimized laser blow-off) and new analysis approach that allows for obtaining the magnetic field from the velocity evolution of trace-element ions. A sub-mm resolution is achieved, which is comparable to the electron skin-depth. Moreover, the newly developed method enables the determination of relatively low-intensity fields of ~ 1 kG, otherwise impractical to measure spectroscopically by the common Zeeman method under such highly transient, low-density conditions. Here, we briefly describe the diagnostic method and the main results. The structure of the propagating magnetic field front is reconstructed and its width (~ 10 mm) is used for estimating the plasma conductivity. We find that the magnetic-field front structure and velocity remain nearly constant when the field propagates a length scale of the order of the front width. This allows the analysis of the associated electric potential hill in the moving frame of the magnetic field. Using the properties of the potential hill we derive the details of the ion dynamics according to their charge-to-mass (Z/m) ratios. Ions of relatively low Z/m ratios (C II-III) are penetrated by the magnetic field, whereas ions of high Z/m ratios (protons and C V-IV) are reflected off the field-front at different field magnitudes. The measured electron density evolution agrees with the predicted ion dynamics.