Numerical calculations for 2.5-MeV electrons (Renkai Li, personal communication). (a) The RMS beam diameter (in μm), (b) the percentage energy spread, and (c) the pulse length (in fs) as a function of position z along the optic axis, for a pulse containing one million 2.5-MeV electrons focused by a solenoid at z = 10 cm. The calculations are for four values of the maximum field strength B
0 within the solenoid. (d) The calculated shape of the focused pulse (initial energy spread 0.001%) with the horizontal (z) and vertical (transverse) scales in μm, with colors representing the electron energy, red being the highest.