Multipactor in Minimum Electric Field Regions of Transmission Lines and Superconducting Rf Cavities*

Multipactor in beam-pipe transitions of superconducting RF cavities can be explained using RF potential well theory [1]. In this paper we present simulation results supporting this explanation for both RF cavities and transmission lines. INTRODUCTION Curved-shaped transition regions between a cavity and a beam pipe were thought to be multipactor-free. Thus recent experimental observations of multipactor (MP) in two such transitions in Cornell ERL injector cavity [2] and KEK Ichiro cavity [3] surprised experimenters. The possibility of multipacting in those geometries was later confirmed in computer simulations. Analyzing these cases we have noticed that the electric field along the cavity profile has a minimum at the locations of MP. We proposed an explanation based on the Gaponov-Miller theory [4]. According to the theory an electric field minimum is associated with the local potential well, thus attracting electrons. This creates conditions favorable for multipacting. More details can be found elsewhere [1]. To check this explanation, we performed MP simulations for cavity geometries with different transition shapes, which confirmed that multipactor is suppressed for transitions with no electric field minimum. The potential well theory can be applied to transmission lines too. For example, Miller force was used in [5] to derive an analytic solution of electron motion in a coaxial line and also was invoked in discussion of MP in a waveguide iris [6]. This force can also explain drift of multipacting electrons from the waveguide midline to sidewalls, that was observed in computer simulations [7] and, in case of partial or full standing wave in a transmission line, migration of electrons toward the standing wave minimum. We explore electron migration in the latter case for coaxial line and rectangular waveguide later in this paper. Simulation results presented here were obtained with computer codes MultiPac [8] (for cavity-to-beam-pipe transitions and a coaxial line) and XingRK4 [9, 10] (for a rectangular waveguide.) MP IN CAVITY-TO-BEAM-PIPE TRANSITIONS Figure 1 shows MP trajectories at the electric field minimum in the transition form the Cornell ERL injector cavity end cell to a beam pipe. The contour line of the geometry, shown in Figure 2, consists of elliptic arcs connected with tangential straight segments. Ae, Be, Ai, Bi and so on are half-axes of the ellipses, i refers to the inner half of the cell, e refers to the outer half, R is the radius of the circle smoothening the transition; Req is the equatorial radius, Rbp is the radius of the beam-pipe. Figure 1: MP in the ERL injector cavity [2]. Figure 2: Geometry of the cavity-to-beam-pipe transition. We have examined a transition from the end iris aperture Rae = 37 mm to the beam-pipe radius Rbp = 55 mm with different radii R. Half-axes of the end iris ellipses were ae = at = 12.53 and bt = be = 20.95 mm. Other dimensions of the cavity are chosen to tune its frequency to 1300 MHz and the ratio of the peak electric field to the accelerating field to Epk/Eacc = 2.0. Figures 3 and 4 show dependence of the maxima of the enhanced counter function A on the radius R and corresponding values of the peak electric field E. Three sets of points correspond to three different MP bands. Analyzed values of field levels were in the range from 25 to 35 MV/m as it has the most distinct maximum of the function A. Two points from Figures 3 and 4 corresponding to R = 12 mm are further looked at in Figure 5. Two maxima of the normalized enhanced function 0 20 c e and corresponding impact energies and trajectories are presented. These trajectories can be related to two kinds of MP: three-periodic for 25 MV/m, and two-periodic MP for 33.5 MV/m (with some deviations from exact periodicity). Both are located in the flat minimum of the electric field. ___________________________________________ * Work is supported by the NSF grant PHY 0131508. [email protected] THP035 Proceedings of LINAC08, Victoria, BC, Canada Technology 860 3A Superconducting RF 0.0E+00 5.0E+03 1.0E+04 1.5E+04