Flash X-Ray (FXR) Accelerator Optimization Beam-Induced Voltage Simulation and TDR Measurements

Lawrence Livermore National Laboratory (LLNL) is evaluating design alternatives to improve the voltage regulation in our Flash X-Ray (FXR) accelerator cell and pulse-power system. The goal is to create a more mono-energetic electron beam. When an electron beam crosses the energized gap of an accelerator cell, the electron energy is increased. However, the beam with the associated electromagnetic wave also looses a small amount of energy because of the increased impedance seen across the gap. The beam-induced voltage at the gap is time varying. This creates beam energy variations that we need to understand and control. A high-fidelity computer simulation of the beam and cell interaction has been completed to quantify the time varying induced voltage at the gap. The cell and pulse-power system was characterized using a Time-domain Reflectometry (TDR) measurement technique with a coaxial air-line to drive the cell gap. The beam-induced cell voltage is computed by convoluting the cell impedance with measured beam current. The voltage was checked against other measurements to validate the accuracy.cell features. The injector voltage has added complexity because of the reflections in the cathode and anode stalks. The third term is defined as the beam-induced gap voltage that launches an electromagnetic (EM) wave into the cell and pulse-power system. A portion is reflected back from the different cell components and appears in the gap again. This is related to beam loading, but the impedance mismatches in the cell and pulse-power system creates a much more dynamic process than the name "loading" implies. This report focuses on the beam-induced energy variation.