Simple Electrostatic Quadrupole Lens (1/8th geometry)
Circular poles with Radius = 1.15 * Radial Aperture
Pole-tip voltages = +/- 20 KV
Uses one eighth of the geometry between 0 and 45 degrees
Neumann boundary along bottom; Dirichlet boundary along 45-degree line

; Copyright 1998, by the University of California. 
; Unauthorized commercial use is prohibited. 

&reg kprob=0,      ! Poisson or Pandira problem
xjfact=0.0,        ! Electrostatic problem
dx=0.05,           ! Mesh interval
icylin=0,          ! Cartesian coordinates
nbslo=1 &          ! Neumann boundary condition at lower edge

&po x=0.,y=0. &
&po r=4.,theta=45. &
&po nt=2,r=4.,theta=0.,x0=0.,y0=0. &
&po x=3.3,y=0. &
&po nt=2,r=1.15,theta=180.,x0=2.15,y0=0. &
&po x=0.,y=0. &

&reg mat=0,ibound=-1,voltage=20000.0 &
&po x=3.3,y=0. &
&po nt=2,r=1.15,theta=180.,x0=2.15,y0=0. &

&reg ibound=0 &    ! Set Dirichlet boundary on the 45-degree line
&po x=0.,y=0. &
&po r=4.,theta=45. &

stop  ! Voltage = 0.0 on the outer arc is the default, so the last
      ! REG section is not needed.

&reg ibound=-1,voltage=0.0 &
&po r=4.,theta=45. &
&po nt=2,r=4.,theta=0.,x0=0.,y0=0. &