L_vac 120[cm] "Width of vacuum chamber" R_vac 10[cm] "Height of vacuum chamber" L_cyl 6[cm] "Length of cylinders in lens" T_cyl 0.25[cm] "Thickness of cylinders in lens" R_cyl_fil T_cyl/2 "Radius of fillet for cylinders in lens" R_cyl 2[cm] "Radius of cylinders" d_lens 40[cm] "Downstream distance of start of lens" cyl_sep 2[cm] "Separation of cylinders" initial_beam_radius 1[cm] "Radius of beam at inlet to vacuum chamber"