Efficient low-impedance high power electron beam diode

The new low impedance pulsed power machines, such as SPEED, require efficient electron beam diodes if these machines are to be used to generate bremsstrahlung radiation for x-ray diagnostic systems. A pinched beam diode typically transports a large fraction of the total charge as ions. Therefore, we are investigating the triaxial diode, which consists of inner and outer cylindrical anodes, a cylindrical cathode, and a planar anode connecting the two cylindrical anodes. These experiments use the 4.8 ohm Hydramite accelerator to drive the triaxial diode. In addition to measuring the electrical parameters, we use carbon activation to measure the proton fluence. Computer simulations support the experimental work. In these experiments, the diode operated over the range of 2 to 8 ohms at cathode voltages between 0.9 and 1.6 MV. The diode produced an annular electron beam with the cathode diameter and virtually no ion current. That is, carbon activation proton measurements and computer simulations are consistent with no ion current in the diode. Moreover, the planar cathode emitted in the Langmuir-Child unipolar mode. Therefore, the cathode is emitting in the unipolar mode. We plan experiments to more fully explore the role of the radial gaps and post-hole convolutes. Also, we are investigating diodes that consist of two or more concentric triaxial diodes in parallel.