Effect of trapped backstreaming ions on beam focus and in radiographic211 accelerators

摘要

The use of electron beam accelerators in advanced X-ray radiography requires that211u001ethe beam be focused to a very small spot size on a metallic bremsstrahlung 211u001econverter target. The energy deposition of the beam into a small volume of target 211u001ematerial rapidly converts the target into a high-density plasma. The space-charge 211u001edensity of the focused beam sets up a strong electric field at the surface of the 211u001egrounded target, which then accelerates ions of target plasma and lighter 211u001econtaminants into the beam. These backstreaming ions form a long channel which 211u001epartially neutralizes the space charge of the beam, disrupting the beam focus and 211u001edegrading the radiographic performance. One solution to this problem is to 211u001econfine the backstreaming ions in a short channel with a self-biased inductive 211u001etrap. Such a trap has been proposed for testing on the ETA-II accelerator. They 211u001epresent numerical simulations which show the effect of the high trapped-ion 211u001echarge density on the beam spot size and emittance.