The role of ions during stable impedance operation of the immersed-Bz diode at 4 to 5 MV

The immersed-Bz diode is being developed as a high-brightness, flash x-ray radiography source at Sandia National Laboratories. This diode is a foil-less electron-beam diode with a long, thin, needlelike cathode that is inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces forward-directed bremsstrahlung. In addition, electron beam heating of the anode produces surface plasmas allowing ion emission. Two different operating regimes for this diode have been identified: a nominal operating regime where the total diode current is characterized as classically bipolar and an anomalous operating regime characterized by a dramatic impedance collapse where the total diode current greatly exceeds the bipolar limit. Data from a comprehensive series of experiments fielded at 4 and 5 MV, where the diode operates in the nominal or stable impedance regime, with beam currents ranging from 20-40 kA on target are presented. In this mode, both the measured diode current and experimental radiation production are consistent with physics based models including two-dimensional particle-in-cell simulations. The analysis indicates that intermediate mass ions (e.g., 12-18 amu) control the nominal impedance evolution rather than expected lighter mass ions such as hydrogen