Electron Beam Transport in Gas and Plasma-filled Cells on RITS

Over the past 30 years, the paraxial diode has utilized a gas-filled transport cell to focus an electron beam onto a high-Z target in order to generate intense x-rays for flash radiography. One of the key objectives is to create a small radiographic spot, which in turn is related to the focusing dynamics of the beam. For gas-filled transport cells, the primary limitation to achieving a small time-integrated spot size is believed to be due to beam sweeping through the focal plane on the timescale of the pulse. This results in a larger than desired time-integrated radiographic spot. Recent simulations using LSP, a particle-in-cell code, have found that if a pre-ionized plasma transport cell is utilized with density on the order of 10¹⁶ cm^-3, then the beam focal plane is essentially "frozen" in time, thereby limiting spot growth due to beam sweep. Recent experiments at Sandia National Laboratories have been conducted to investigate the spot behavior as a function of time for both gas and plasma-filled transport cells. For the plasma-filled experiments, a z-discharge of static hydrogen is used to create a highly ionized plasma in the transport cell. For the gas-filled experiments, the transport cell is filled with either hydrogen or air with pressures of 0.5-5 Torr. Time-resolved as well as time integrated measurement of the focal spot are presented and compared for both the gas and plasma-filled transport cells.