Nonlinear theory of helix traveling wave tubes in the frequency domain

A nonlinear helix traveling wave tube (TWT) analysis in the frequency domai n is presented for a sheath helix surrounded by a conducting wall. Dielectr ic- and vane-loading of the helix are included as is circuit tapering and e xternal focusing by either a solenoid or a field produced by a periodic per manent magnet (PPM) stack. The electromagnetic field is treated as a superp osition of waves where the amplitudes and phases vary slowly in z. The fiel d equations are solved in conjunction with the three-dimensional Lorentz fo rce equations for an ensemble of electrons. Beam space-charge waves are inc luded using a superposition of solutions of the Helmholtz equation. The dc (direct current) self-fields of the beam are also included. The simulation is compared with linear theory as well as a time-domain simulation, each of which is applied to an example of a tube built at Northrop-Grumman Corp. T he frequency- and time-domain simulations are in substantial agreement. The advantage of the frequency-domain formulation is a shorter run time than f or the time-domain simulation. Hence, the frequency-domain simulation can m ore easily treat beam thermal effects and multiple waves with closely space d frequencies. (C) 1999 American Institute of Physics. [S1070-664X(99)03009 -8].