Computer simulation of ion trapping and detrapping in a PPM focused traveling wave tube

The processes involved in the trapping and detrapping of ions in a PPM focused traveling wave tube have been simulated in a time-dependent, finite element particle-in-cell (PIC) code. The entire system is assumed to be axially symmetric, with the simulated macro particles being rings. Their motion is then followed in the axial-radial position plane, subject to electrostatic forces from boundary conditions and space charge, and to magnetic forces from a periodic permanent magnet (PPM) field. This program was developed from a time-dependent electron gun simulation program by adding several capabilities: 1) the ability to inject, at user-specified times and positions, multiple species of ions. Simultaneously, the neutralizing low-energy electrons which would be introduced by ionization of a neutral gas are introduced. 2) An ionization mechanism to introduce ions over the time of the simulation. This mechanism assumes that the ionization is caused by collisions of the main beam electrons with a constant density background neutral gas. As ions are produced, an equal number of neutralizing electrons that would be freed by such ionizations are also injected. 3) An ion-electron viscous force in the angular direction, taken from the collisional term of the Fokker-Planck equation. This was added on the speculation that such a force might provide significant energy to the ions over time for detrapping.