Effect of DC Space Charge Fields in the Interaction Gaps of Coupled Cavity TWTs

Summary form only given. During the passage of an electron beam through an interaction gap in a cylindrical beam tunnel, the beam experiences first a decelerating and then a re-accelerating force due to the axial self-electric field produced by the DC beam charge density in the vicinity of the gap. In the absence of RF fields, the beam returns to the same velocity at the exit of the gap that it had at the entrance. The net effect, however, is a slight reduction in the average axial velocity of the beam, compared to what the velocity of the beam would have been in the absence of the gap. This (current dependent) velocity reduction changes the degree of synchronism between the beam velocity and the phase velocity of the slow wave supported by the coupled cavity circuit. One practical effect is a current dependent shift in the location of the peak in the small signal gain vs. voltage curve to higher voltage. In this paper we will present an analysis of this effect and discuss several examples. The DC potential and electric field due to a cylindrical beam propagating in a perfectly conducting cylindrical pipe with a gap has been computed analytically from Poisson´s equation for the two different idealized gap geometries. A detailed comparison of results from the analytic expressions for the electric field with numerical results from the MICHELLE gun/collector code shows very good agreement between the two. The analytical expressions for the field have been implemented in the CHRISTINE-CC large signal coupled cavity simulation code. The analytical results for the DC field and sample small and large signal numerical results from the CHRISTINE-CC code will be presented.