The effect of trapped backstreaming ions on beam focus and emittance in radiographic accelerators

The use of electron beam accelerators in advanced X-ray radiography requires that the beam be focused to a very small spot size on a metallic bremsstrahlung converter target. The energy deposition of the beam into a small volume of target material rapidly converts the target into a high-density plasma. The space-charge density of the focused beam sets up a strong electric field at the surface of the grounded target, which then accelerates ions of target plasma and lighter contaminants into the beam. These "backstreaming ions" form a long channel which partially neutralize the space charge of the beam, disrupting the beam focus and degrading the radiographic performance. One solution to this problem is to confine the backstreaming ions in a short channel with a self-biased inductive trap. Such a trap has been proposed for testing on the ETA-II accelerator. We present numerical simulations which show the effect of the high trapped-ion charge density on the beam spot size and emittance.