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.
Electron surfing acceleration by the electron two-stream instability in a weak magnetic field