Ion emission at the target of the radiographic devices PIVAIR and AIRIX

On an AIRIX facility, a high-intensity electron beam is used to generate an x-ray pulse for radiographic purposes. The measured electron beam spot size is found to be much larger than its computed value. It is shown that this discrepancy is removed considering ion emission at the target, under electron beam interaction. A model has been developed to simulate the plasma produced during the interaction of the electron beam with the target. The predictions have been compared with ion identification results developed on the PIVAIR prototype accelerator as well as with the x-ray spot size and dose measurements obtained on AIRIX. The simulations show that the ion effect can be reduced by placing a thin foil (100 µm thickness) at an appropriate distance from the target to catch the ions emitted and to stabilize the electron beam spot size. The X-spot size is then reduced, as found experimentally on AIRIX. Moreover, for a smaller thickness barrier foil (5 µm), the ion effect can even be suppressed, producing an X-spot size close to that obtained without ion emission.