Measurements of Weibel magnetic fields in optical-field ionized plasmas*

Generation and amplification of magnetic fields in plasmas is a long-standing topic that is of great interest to both fundamental and applied physics. Weibel instability is a well-known mechanism responsible for self-generating magnetic fields in anisotropic plasmas and has been extensively investigated in both theory and simulations, yet its experimental verification has proven challenging. Recently, we have demonstrated a new experimental platform that enables the initialization of the plasma electron velocity distribution in a controllable manner and then measures the evolution of Weibel magnetic fields using an external ultrashort relativistic electron probe. Here we will first present experimental results on time-resolved measurements of Weibel magnetic fields in non-relativistic plasmas produced by optical field ionization using an ultrashort IR laser (0.8 μm). It was found that the Weibel magnetic fields self-organize into a quasistatic structure consistent with a helicoid topology within a few picoseconds and such a structure lasts for tens of picoseconds. The magnetic fields show a well-defined wave vector. The growth rate and saturated magnetic field magnitude were measured and agree well with kinetic theory predictions. We then discuss the feasibility of extending the study to the quasi-relativistic regime by using intense CO₂ (10 μm) lasers to produce much hotter anisotropic plasmas. The platform we have demonstrated is suitable for exploring a broad range of plasma phenomena such as magnetic reconnection, annihilation, and island formation thereby opening a new avenue for studying astrophysical phenomena in the laboratory.