A critical review of theories to explain the consistent observation of filaments in laser ablated plasmas

What have been variously described as filaments, hairs, jets or spokes in electron density and electromagnetic fields have been observed in underdense plasma in planar, cylindrical and spherical laser-solid experiments, for all materials, intensities >10¹³ W cm^-2, pulse durations from 1 ps to 6 ns, and wavelengths from 0.35 μm to 10 μm, regardless of laser uniformity. We review existing theories. The magnetothermal and electrothermal instabilities can be ruled out. The radiation cooling instability can only contribute after or outside the laser pulse and for mid-Z or greater materials. Electron return currents lack a sound theoretical basis. Weibel instability can only be a self-consistent explanation with a physical mechanism to generate an anisotropic velocity distribution. Anisotropy can be generated by collisionless laser absorption, temperature gradients, and expansion cooling. Magnetohydrodynamics can capture the development of filaments, if the plasma is magnetized. We conclude that magnetization by the Weibel instability, due principally to expansion cooling, is most likely the seed for the formation of filaments.