(a) The spatial distribution of filaments along the dashed line in Fig. 3(a). The black line is the experimental result. With different band FFT (Fast Fourier Transformation) filters, the red line presents all (short and long) filaments spaced by 90 μm–250 μm, while the blue line presents only long filaments spaced by 400 μm–600 μm as indicated by the solid triangles. (b) The FFT spectrum of the spatial distribution of filaments. It is noted that after eliminating the instrumental magnification effects, the real electron current widths or the spaces between all and only long filaments are around 120 μm and 260 μm, respectively.