Self‐focusing of short intense pulses in plasmas

The self‐focusing of relativistically intense laser light pulses is analyzed, where the pulse length is short enough that ion inertia prevents any significant motion of ions. Self‐focusing occurs as a result of an increase of the wave refractive index arising from two effects: the mass increase of electrons caused by their relativistic quiver velocity in the light wave, and the reduction of the electron density as a result of ponderomotive force expulsion of the electrons. The latter effect is significant even for rather small values of (P−P_L)/P_L, where P is the laser beam power and P_L is the critical power above which self‐focusing occurs. In fact, for (P−P_L)/P_L≳0.1 the effect is so strong that all electrons are expelled within a core radial region of the self‐focused laser light channel (this new phenomenon is called electron cavitation).