20 TORR Gas Cell Focussing in the Paraxial Diode.

Summary form only given. The paraxial diode is routinely used to convert high intensity electron beams to bremsstrahlung X-radiation for use in experiments requiring flash radiography at AWE (Aldermaston). The paraxial diode is one of the diodes being considered for use on an inductive voltage adder (IVA) driver in the proposed core punch facility (CPF). The operation of the diode using a 20 torr air-fill in the gas cell as opposed to the traditionally used value of around 1 torr air will be presented. At present the performance of the diode is believed to be limited by significant focal sweeping which causes the radiographic spot to increase with time. It is anticipated that at the higher air pressures the magnetic diffusion timescale in the gas cell will be lower than the pulse length of the beam. Under these conditions the plasma return current will quickly diffuse away because of the high plasma resistivity. This causes the net current in the gas to quickly become equal to the beam current. It will be shown using particle in cell (PIC) modeling that under these conditions focal sweeping can be reduced which in turn results in smaller radiographic spot sizes. A comparison between the experiments carried out on Mogul D and the PIC models will be presented and the differences highlighted