| Flash radiography continues to be valuable in investigating compression under explosive force where brightness, small source size, and short time duration must be maintained. The TriMeV pulsed electron accelerator delivers 30 kA at 3 MV over a 20 ns time window. In order to maximize brightness and minimize source size the treatment of the electron beam must be considered. Emitted from a cathode tip, the electrons accelerate across the anode-cathode (A-K) gap while ion plumes and electric and magnetic fields focus the beam onto a converter where Bremsstrahlung is produced.
A description of the system and a model for TriMeV circuitry is presented including a comparison of simulation and experimental measurements. Multiple methods for spot size and dose are described and recommendations made to improve and expand the characterization of this radiography source. The figure of merit (FOM) is calculated and represents a gauge of the potential of the system to produce a quality radiograph.
The presence of a cathode sleeve and anode shroud tune the focus of the beam by per-turbing fields and conditioning ion plumes in the A-K gap. The cathode sleeve appears to collimate the beam and produces more reliable spot sizes shot to shot but reduces the FOM from 15.8 ± 0.8 rad @ 1m/mm2 for shots without a cathode sleeve to 5.4 ± 0.2 rad @ 1m/mm2 with a sleeve. With an anode shroud we noted a 2/3 reduction in output dose but little change in dose for a smaller shroud aperture. Compared to source spots observed without a shroud, the diameter was reduced to 1/3 for similar anode shroud bore/cathode tip diameters and to 1/4 when the anode shroud bore diameter was half that of the cathode tip. The FOM improved from 4.58 ± 0.26 rad @ 1m/mm2 for no shroud shots to 5.42 ± 0.37 rad @ 1m/mm2 for 25.4 mm shroud bore and further to 13.7 ± 1.32 rad @ 1m/mm2 for shots with shroud bore of 12.7 mm. Additionally, with the anode field enhancements we noted an average spot diameter of 0.73 mm ± 0.1 mm and an average FOM of 114 rad @ 1 m/mm2 (std. deviation: 86.7 rad @1 m/mm2).
Key words: flash radiography, TriMeV, pulsed power, electron beam diode, figure of merit |