twt{  Simple TWT (travelling wave tube).  The helix is represented by a  cylindrical surface with the "polarizer" BC.  This is the standard  "sheath helix" model.  A beam is emitted from the left and interacts with the slow-wave  structure through electrostatic fields.  A solenoidal field is used to help confine the beam.}Region{Grid{   n2 = 1.000e+00   x2f = 0.545e-01   x2s = 0.000e+00   K = 18   n1 = 1.000e+00   x1f = 3.720e-01   x1s = 0.000e+00   J = 74   Rule   {     Limit     n1 < 2.500e-01     Fatal -- n1 < 0.25 grid spacing too nonuniform to ensure accuracy   }}Control{   B01 = 6.000e-1   dt = 5.000e-12   Rule   {     Limit     dt <= 0.000e+00     Fatal -- time step must be positive   }}CylindricalAxis{        normal = 1        k2 = 0        j2 = 74        k1 = 0        j1 = 0}Conductor{        normal = 1        k2 = 0        j2 = 0        k1 = 18        j1 = 0}Polarizer{	normal = -1        k2 = 15        j2 = 74        k1 = 15        j1 = 0	transmissivity = 0.5	Phi = 15}Conductor{        normal = -1        k2 = 18        j2 = 74        k1 = 18        j1 = 0}Dielectric{        normal = -1        QuseFlag = 0        k2 = 0        j2 = 74        k1 = 18        j1 = 74}Species{        name = electrons        m = 9.11e-31        q = -1.6e-19}VarWeightBeamEmitter{        np2c = 3.000e+09        I = 1.2e3        speciesName = electrons        v1drift = 2.480e+08        normal = 1        k2 = 12        j2 = 0        k1 = 0        j1 = 0}}