Spot size reduction on radiographic devices

Summary form only given. On the radiographic device AIRLX, the high-intensity relativistic electron beam is focused on a tantalum target (2 kA, 20 MeV, 100 ns) to produce, via the bremsstrahlung process, the X-ray emission used for radiographic purposes. The Ta target is covered by an aluminium foil used to measure the spot size by the OTR diagnostic. At the shot instant, the pressure near the target is about 10/sup -6/ Torr, so that about 1.3 monolayers of water remains adsorbed at the Al foil surface for a roughness factor r=3, following the mean experimental outgassing rate data on Al surfaces. The energy deposit by the beam, focused with an angle /spl theta/ on a diameter spot size /spl phi/=2 mm, causes the water vaporisation and the production of H/sup +/ ions by dissociation of the water molecules by electron impact. Afterwards, the Al foil is vaporized and later the Ta target. Further, these layers are ionized by the beam, to form three distinct plasma layers that expand from the target position. The protons production induces a partial beam charge neutralisation, reducing the average radial electrical field. The beam then pinches due to the self-consistent azimuthal magnetic field. The ions thus produce a lens effect that increases with the ratio /spl rho/ of the ion density to the electron beam density. The ions are subsequently accelerated downstream in the self-consistent axial electric field of the beam, inducing a displacement of the fictitious lens. Thus, the waist moves away from the target with a velocity V that increases with the lens strength, i.e. with /spl rho/. It results to a prejudicial increase of the spot size diameter /spl Phi/ at the target (and of the radiographic image) with a velocity v=/spl theta/V. The PIC code M2V is coupled with the model VAPOR, that calculates the production of the ions, to simulate their transport in the electron beam. This ab-initio model found that the H/sup +/ current represents only 10% of the Child-Langmuir- (CL) current, so that v is about 20 times smaller than in the CL case, as found experimentally on the PIVAIR accelerator prototype with a simple Ta target (hydrogen getter). The spot size is in good agreement with the AIRIX experimental results. These results thus explains why the assumption, of CL emission of protons, made to simulate the experimental results on ITS failed. Moreover, the hypothesis of the emission of heavy metallic ions, issued from the target, to interpret the experimental results on ITS does not hold because the H/sub 2/O plasma screens the metallic plasma, preventing the heavy ions to be accelerated.