MIT, Cambridge, Massachusetts, USA
MIT/PSFC, Cambridge, Massachusetts, USA
MIT/PSFC, Cambridge, Massachusetts, USA
Three novel ideas for wakefield acceleration (WFA) of electrons with metallic periodic subwavelength structures will be presented. The first idea is a deep corrugation structure for collinear WFA. A design for the Argonne Wakefield Accelerator is shown. An analytical model is developed and it agrees with the CST wakefield solver. A scaling study has been performed, and ways to increase the gradient will be discussed. The deep corrugation structure can generate a higher gradient than a dielectric tube with the same beam aperture when excited by the same bunch. The second idea is an elliptical structure for two-beam acceleration (TBA). The unit cell is an elliptical cavity, and the drive beam hole and the witness beam hole are located around the two focal points. The TBA process has been calculated and will be presented. The third idea is a metamaterial ‘wagon wheel' structure for a power extractor design. The fundamental mode is a TM mode with a negative group velocity. A power extractor at 11.7 GHz based on the structure can reach a GW power level when a train of 40 nC bunches with 1.3 GHz rep rate are sent in.
MIT/PSFC, Cambridge, Massachusetts, USA
We present calculations of the dark current in a high gradient accelerator with the intent of understanding its role in breakdown. The initial source of the dark current is the field emission of electrons. For a 17 GHz single-cell standing wave disk-loaded waveguide structure, the 3D particle-in-cell simulation shows that only a small portion of the charge emitted reaches the current monitors at the ends of the structure, while most of the current collides on the structure surfaces, causing secondary electron emission. In the simulation, a two-point multipactor process is observed on the side wall of the cell due to the low electric field on the surface. The multipactor approaches a steady state within nanoseconds when the electric field is suppressed by the electron cloud formed so that the average secondary electron yield is reduced. This multipactor current can cause the ionization of the metal material and surface outgassing, leading to breakdown. We report first results from an experiment designed to extract dark current directly from an accelerator cell from the side through two slits. First results show that the dark current behavior deviates from the field emission theory.