Ion clearing in an ERL

The rest-gas in the beam-pipe of a particle accelerator is readily ionized by effects like collisions, synchrotron radiation and field emission. Positive ions are attracted to electron beams and create a nonlinear potential in the vicinity of the beam which can lead to beam halo, particle loss, optical errors or transverse and longitudinal instabilities. In an energy recovery linac (ERL) where beam-loss has to be minimal, and where beam positions and emittances have to be very stable in time, these ion effects have to be avoided. Here we investigate three measures of avoiding ion accumulation: (a) A long gap between linac bunch trains that allows ions to drift out of the beam region, a measure regularly applied in linacs; (b) a short ion clearing gap in the beam that leads to a time varying beam potential and produces large excited oscillations of ions around the electron beam, a measure regularly applied in storage rings; (c) Clearing electrodes that create a sufficient voltage to draw ions out of the beam potential, a measure used for DC electron beams and for antiproton beams.For the parameters of the X-ray ERL planned at Cornell University we show that method (a) cannot be applied, method (b) is technically cumbersome, and (c) should be most easily applicable.