Study of Fields Structure at Strong Shock Front in Low-density System
Shocks are essential components in astrophysical systems and inertial confinement fusion (ICF). In recent decades, with the development of the proton probing technology, the fields associated with shock fronts have attracted great attention. However, the tiny scale of ICF relevant shocks and the finite resolving power of proton probing highlights the difficulty of measuring and differentiating between electric and magnetic fields. In addition, the hydrodynamic treatment, which is frequently used to predict the evolution of plasmas in inertial confinement fusion (ICF), may not be sufficient in simulating the shock convergence phase of the implosion, as the plasma can become kinetic in the high temperature, low-density conditions. Therefore, quantitative experimental measurement of the strong shock structure and the associated fields in low-density plasma system is crucial for further understanding the shock physics and benchmarking the theoretical and numerical models.
This thesis proposes the experiments to study shock propagation in a cylindrical gas-tube along the central axis. This series of experiments were conduced on OMEGA EP Facility. strong shocks were generated using long pulse beams from their laser system. Two primary diagnostics, proton radiography and a soft x-ray spectrometer, were fielded to probe the fields structure and plasma conditions at the shock fronts. Another short pulse laser was used to generate protons for the proton radiography.
Given the difference in responses of charged particles to magnetic and electric fields, protons were designed to project the shock front region from multiple angles in order to distinguish the magnetic field from the electric field. For a Mach 6 shock propagating in pure helium gas, the oblique incident protons revealed the domination of the magnetic field on the order of couple tesla, while the normal incident protons disclosed the electric field on the order of couple hundreds of volts. Simulations indicate the Biermann battery effect and the electron pressure gradient at the shock front respectively accounts for the generation of the magnetic field and electric field.
In addition to the field structure, spatial profiles of density and temperature of strong shocks including the entire precursor regions are also measured by the soft x-ray spectrometer respectively in the helium and neon gases. With comparable peak electron temperatures at the shock fronts, a precursor layer, where electron temperature is far in excess of ion temperature, was found to be highly dependent on the atomic number Z as it extended much longer in the helium gas than in the neon gas.