Detection and clearing of trapped ions in the high current Cornell photoinjector

TL;DR

This paper evaluates ion-clearing strategies in the Cornell high current photoinjector, demonstrating effective methods to mitigate ion trapping at high beam currents through experimental measurements and theoretical modeling.

Contribution

It introduces and tests three ion-clearing techniques in a high current photoinjector, providing quantitative analysis and models for their effectiveness.

Findings

At least 70% ion clearing achieved with each method

Maximum ion clearing approaches 100% in some cases

Theoretical models accurately predict clearing requirements

Abstract

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.