D-cone core: what limits the focusing?
R
h
Rc
Curvature at r ~ 1/r
B ~    k ˜ 2p r dr 1/r2
    ~    k log Rp/Rc
Rp
Core has gaussian curvature
          Þ  stretching
Ecore  ~  k  (Rc/h)x
x > 0
Minimize B + Ecore Þ
Rc ~ Rp0  Rp0 h
How does it work in practice?  Simulation.