  We have recently performed experiments to test the effectiveness of three
ion-clearing strategies in the Cornell high intensity photoinjector: DC
clearing electrodes, bunch gaps, and beam shaking. The photoinjector reaches a
new regime of linac beam parameters where high CW beam currents lead to ion
trapping. Therefore ion mitigation strategies must be evaluated for this
machine and other similar future high current linacs. We have developed several
techniques to directly measure the residual trapped ions. Our two primary
indicators of successful clearing are the amount of ion current removed by a DC
clearing electrode, and the absence of bremsstrahlung radiation generated by
beam-ion interactions. Measurements were taken for an electron beam with an
energy of 5 MeV and CW beam currents in the range of 1-20 mA. Several
theoretical models have been developed to explain our data. Using them, we are
able to estimate the clearing electrode voltage required for maximum ion
clearing, the creation and clearing rates of the ions while employing bunch
gaps, and the sinusoidal shaking frequency necessary for clearing via beam
shaking. In all cases, we achieve a maximum ion clearing of at least 70 percent
or higher, and in some cases our data is consistent with full ion clearing.
