“…Following threshold intensity for relativistic self-focusing was derived [9] for a Nd:glass laser: I rel = 3.66 × 10 18 W/cm 2 and (since Iλ 2 ) is a similarity parameter), I rel = 3.66 × 10 16 W/cm 2 for a CO 2 laser. However, it has been stated [9] that the relativistic self-focusing also occurs at laser intensities much lower than the calculated relativistic threshold (10 −3 I rel is accepted in [11] and even lower intensities in [10,28]). The self-focusing length has been calculated for Nd:glass laser [8,9,11]: it can be very long at low plasma density (n e n cr ), [9], it is expected to be ∼ 500 µm in [8], and it is very short (as low as two times the beam diameter), if the laser intensity is between 3 × 10 16 and 3 × 10 18 W/cm 2 , [9,11].…”