lcls_gun: e_gun, 
   L = 0.135,
   tracking_method = time_runge_kutta,  mat6_calc_method = tracking, 
   rf_frequency = 2856.0e6, 
   field_calc = fieldmap, 
   grid_field = call::lcls_gun.h5,
   PHI0_AUTOSCALE = 0.5930461945054, FIELD_AUTOSCALE = 1.1475096482006E8, ! Help out initial scaling, phasing
   aperture_at = continuous, aperture_type=wall3d,
   !wall = {section={s=0, v(1) = {0, 0, 0.012}}, section={s=0.135, v(1)={0, 0, 0.012}} }
   wall = call::lcls_gun_wall.bmad

lcls_gun[voltage] = 6e6 - m_electron