Improved DC Gun Insulator Assembly

Abstract

Many user facilities such as synchrotron light sources and free electron lasers require accelerating structures that support electric fields of 10-15 MV/m, especially at the start of the accelerator chain where ceramic insulators are used for very high gradient dc guns. These insulators are difficult to manufacture, require long commissioning times, and often exhibit poor reliability. Two technical approaches to solving this problem will be investigated. Firstly, inverted ceramics offer solutions for reduced gradients between the electrodes and ground. An inverted design will be presented for 350 kV, with maximum gradients in the range of 5-10 MV/m. Axial symmetry is important to minimize emittance growth of the beam, but the replacement of the photo-cathode, in a gun with an inverted ceramic base, presents a significant mechanical engineering problem. Also, any field emission at all from the gun will ionize gases and poison the cathode. Secondly, novel ceramic manufacturing processes will be studied, in order to protect triple junction locations from emission, by applying a coating with a bulk resistivity. The processes for creating this coating will be optimized to provide protection as well as be used to coat a ceramic with an appropriate gradient in bulk resistivity from the vacuum side to the air side of an HV standoff ceramic cylinder. Example insulator designs are being studied through computer modeling, and insulator samples are being manufactured and tested.