RF Cell Modeling and Experiments for Wakefield Minimization in DARHT-II
  Electron beams of linear induction accelerators experience deflective forces caused by RF fields building up as a result of accelerating cavities of finite size. These forces can significantly effect the beam when a long linac composed of identical cells is assembled. Recent techniques in computational modeling, simulation, and experiments for 20 MeV DARHT-II (Dual Axis Radiographic Hydrodynamic Test) accelerator cells were found to reduce the wakefield impedance of the cells from 800 ohms/meter to 350 ohms/meter and experimental results confirm the results of the modeling efforts. Increased performance of the cell was obtained through a parametric study of the accelerator structure, materials, material tuning, and geometry. As a result of this effort, it was found that thickness-tuned ferrite produced a 50% deduction in the wakefield impedance in the low frequency band and was easily tunable based on the material thickness. It was also found that shaped metal sections allow for high-Q resonances to be de-tuned, thus decreasing the amplitude of the resonance and increasing the cell s performance. For the geometries used for this cell, a roughly 45 degree angle had the best performance in affecting the wakefield modes. 
Scott D. Nelson, Michael Vella