An Ultra-High Gradient Travelling Wave Gun

High brightness beams enable novel applications like x-ray free electron lasers and ultrafast electron microscopes. High brightness beams essentially consist of a large number of electrons in a small phase space volume, i.e. a high peak current. When such beams are generated from the cathode, there is a strong space charge force, which elongates the bunch and reduces its brightness. An optimal solution is to raise the accelerating voltage in the gun; however, the maximum gradient is limited by the effects of RF breakdown. The probability of RF breakdown is reduced as the RF pulse length decreases. We propose to develop an electron photo injector operating with short RF pulses. However, standing-wave cavities are inefficient to operate at short RF pulse lengths. For this reason, the proposed gun will consist of high peak gradient cathode half-cell with low loaded Q-factor, followed by a travelling-wave accelerating section, which is more efficient operating at short RF pulse lengths. In Phase I, we will design a gun prototype in X-band. This effort will include RF design, beam quality optimization, and engineering design. The design will be based on a short RF pulse generated at the Argonne Wakefield Accelerator Facility for two-beam acceleration experiments. We will manufacture an aluminum prototype to measure its microwave properties, most importantly, fill time. The proposed short RF pulse gun is an effective high gradient electron source fed by short RF pulses. These pulses can be formed by radar magnetrons with compact pulse compression systems that have a short delay line. The high brightness beam source can be used as the main beam in wakefield accelerators. It will find commercial applications in ultrafast electron diffraction and microscopy systems.