Nanosecond imaging of relativistic electron transport in a paraxial diode gas cell

The paraxial diode is routinely fielded on a number of AWE pulsed-power accelerators to provide a flash X-ray source for use in radiography experiments. In these diodes, a relativistic 30-40 kA electron beam is created in a vacuum diode gap, passed through a thin anode foil, and then transported, in air at 1.1 Torr, to a high-Z metal target, producing Bremsstrahlung radiation. At present, this diode does not meet AWE´s future X-ray source requirement of >1000 R@lm in a <2 mm spot. In order to achieve this, a program of research has commenced to improve the understanding of the diode´s operation. One of the factors believed to significantly affect the diode´s operation is the focusing behaviour of the electron beam within the gas-filled transport region of the diode. LSP (Large Scale Plasma) PiC simulations have suggested that the electron beam´s focal position moves axially within the gas cell throughout the duration of the pulse. This movement is believed to significantly increase the time-integrated spot size produced by the diode. In an attempt to verify this behaviour, 10 ns images of the light emission generated by the electron beam´s propagation within the gas cell have been taken at different stages of the 80 ns pulse, produced by one of AWE´s 5MV Superswarf accelerators. These presented images show that the beam pinches rapidly as the current rises, after which the beam appears to remain tightly pinched.