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ABSTRACT: 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.
Physics of Plasmas 11/2007; 14(11):113107-113107-10. · 2.15 Impact Factor
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D. C. Rovang,
M. D. Johnston,
J. E. Maenchen,
B. V. Oliver,
S. Portillo, E. Puetz,
N. Bruner,
D. V. Rose,
D. R. Welch,
G. M. Cooper,
J. McLean
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ABSTRACT: Sandia National Laboratories is investigating and developing high-dose, high-brightness flash radiographic sources. One of the diodes we are developing is the immersed-B z diode. This diode is a foil-less electron-beam diode with a long, thin, needle-like cathode 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 bremsstrahlung. Recently, we reported on an extensive series of experiments<sup>1</sup> where an immersed-B z diode was fielded on the RITS-3<sup>2,3</sup> pulsed power accelerator, a 3-cell inductive voltage generator that produced peak voltages between 4 and 5 MV, ∼140 kA of total current, and power pulse widths of ∼50 ns. The analysis of those results is on-going. A summary of those results and analyses are presented.
Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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ABSTRACT: The magnetically immersed (B<sub>z</sub>) diode [1] is being investigated as a source for pulsed-power driven flash radiography. Experiments fielding this diode have revealed a limit on its achievable current density on target. Either a small spot produces a low dose, or a high dose is achieved with a large spot. It has been proposed that this limit is due to non-protonic ions liberated from the anode surface and subsequently ionizing to higher states [2,3]. The three-dimensional particle-in-cell code LSP [4] is used to investigate this proposal. Data from the recent immersed diode experiments conducted on the RITS-3 accelerator are compared to LSP models of the experimental configuration, including the B<sub>z</sub> field map. We report on how the non-protonic and protonic ion models compare to data, and proposals for future investigation.
Pulsed Power Conference, 2005 IEEE; 07/2005
