Probing thermal Weibel instability in optical-field-ionized plasmas using relativistic electron bunches

Abstract

Thermal Weibel instability driven by anisotropic velocity distributions is an important mechanism for self-generating magnetic fields in both laboratory and space plasmas. However, there is a lack of experimental data on thermal Weibel instability due to the difficulty of initializing anisotropic distributions in a controllable manner as well as the challenge of probing the magnetic fields with high spatiotemporal resolution. Here we show that the initial electron velocity distribution of optical-field-ionized plasmas can be easily manipulated by changing laser polarization and such plasmas are unstable to the thermal Weibel instability. The topology of the self-generated magnetic fields depends on the laser polarization. We propose to use ultrashort relativistic electron beams such as those produced by a laser wakefield accelerator as a probe to record the spatiotemporal evolution of the magnetic fields. By taking a series of snapshots of the magnetic fields at different times, the wavevector spectrum and growth rate of the instability can be deduced and compared with kinetic theory.