(a) The electron density distribution and (c) the energy spectrum of electrons at t = 5 ps. (b) The electron density distribution and (d) the energy spectrum of electrons at t=4.2 ps. The electron density in panel (a) decreases linearly to zero from the radius r = 5 μ m to r = 2 μ m , while the electron density in panel (b) decreases linearly to zero from the radius r = 5 μ m to r = 3 μ m . The dashed lines in panel (a) and panel (b) show the boundaries of the hollow plasma channel.