A three-dimensional (3-D) multifrequency large signal model of the beam-wav
e interaction in a helix TWT is described. The beam is divided into a set o
f discrete rays, or "beamlets", instead of the disks or rings used in one-d
imensional (1-D) or two-dimensional (2-D) models, The RF fields supported b
y the helix are represented by a tape helix model that uses a modal expansi
on including the full (Bessel function) radial dependence of the fields; bo
th forward and backward synchronous space harmonics are included in the mod
el. RF space charge fields are obtained from solutions of the Helmholtz equ
ations for the RF electric and RF magnetic fields, using the beam current a
nd charge densities as sources. The de space charge electric field is simil
arly obtained from a solution of Poisson's equation.
This model has been implemented in a code called CHRISTINE 3D, a generaliza
tion of the one dimensional CHRISTINE code. The full three dimensional trea
tment permits the accurate computation of large signal gain and efficiency,
taking into account the self-consistent variation of beam radius along the
interaction space. The code also computes helix interception current and t
ransverse beam distributions at the entrance to the collector-important des
ign data that are unavailable from a 1-D model.
Results from the CHRISTINE 3D code are shown to compare very favorably with
measurements of output power, efficiency, and interception current vs. dri
ve power. Its predictions for spent beam distributions also compare very we
ll with measurements.
Run times for the code are problem dependent, but for a single case of inte
rest are typically 1 to 5 min on a 450 MHz PC, orders of magnitude shorter
than that required for a comparable 3D particle-in cell simulation.