Emittance and its Role on the Radiographic Spot in a Paraxial Diode

Summary form only given. The paraxial diode is one of several candidate diodes being investigated to meet the X-radiography requirements of the CPF at AWE. The long term requirement is to produce a 2 mm X-ray source with a dose of 1000 rads at 1 metre. The paraxial diode currently produces around 500 R @ 1 m in a 5 mm spot. To help meet the long term requirement stated above the paraxial diode is currently being studied in detail with the aid of the large-scale-plasma PIC code Lsp in order to see if improvements can be made to spot and dose. It will be shown how the concept of emittance can be used as a powerful tool in understanding the behavior of the beam as it propagates in the diode. Emittance results in degradation of an electron´s trajectory, which ultimately leads to an increased spot on the high Z, target. Previous work suggests that focal sweeping, because of time-varying net currents, is the main contributor. However, it will be demonstrated with the aid of Lsp simulations that emittance growth from non-linear focussing and foil scatter will set fundamental limits on the spot size achievable (typically 5 mm when operating at 1.1 torr air in the drift cell). Suggestions to reduce emittance and spot are presented by using a high pressure air fill ~20 torr inside the drift cell