Electron Beam Focusing in Gas and Plasma-Filled Transport Cells on Rits

Summary form only given. In flash X-ray radiography, one of the key goals is to focus an electron beam onto a high-Z-target to generate X-rays. One of several goals is to create a small radiographic spot. The paraxial diode, which utilizes a gas-filled transport cell to focus the beam into a small spot, has been investigated for several decades. It has been shown in simulations using LSP, a particle-in-cell code, that the primary limitation to achieving a small spot size is due to time-dependent net currents in the transport cell which cause the beam\´s focal plane to shift axially during the timescale of the pulse. This movement, often referred to as \´\´beam sweep", smears out the radiographic spot, leading to a larger size than is desired. Recently, calculations with LSP, demonstrate that replacing the gas with a pre-ionized plasma with density on the order of 10¹⁶ cm^-3 significantly slows down the net current evolution, and yields a more stable focal plane and thus decreased time-integrated spot size. Recent experiments utilizing gas and plasma-filled transport cells have been performed on the RITS accelerator (4.5 MV, 20-30 kA beam current 60 ns FWHM) to study these predictions. The most recent results and analysis from these experiments are presented