Figure 1

Side images of filaments (a) without the aluminum target and with the aluminum target located at the (b) beginning, (c) middle, and (d) end of the filament, recorded using a digital camera (Nikon D7000: f number (F) = 5.6, light sensitivity (ISO) = 800,shutter speed (S) = 1/13 s). The dashed line outlines the position of the geometric focus. The arrow indicates the propagation direction of the laser pulses.