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

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.

Compact −300 kV dc inverted insulator photogun with biased anode and alkali-antimonide photocathode

“…5) similar to those described in Ref. [36] were also tested as a means to preserve photocathode QE, but alone (i.e., without the biased anode), these ion-clearing electrodes did not prevent rapid QE loss and photocathode damage, although the damage threshold beam current increased from 1 to 3 mA. Significantly improved lifetime was largely attributed to the biased anode.…”

Beam Commissioning Results from the CBETA Fractional Arc Test (CBETA Note 030)

“…Construction of the CBETA machine at Cornell began in earnest with the disassembly and removal of the injector [6][7][8][9]19] from its original experimental hall in early 2015. At this time, the Injector Crymodule (ICM) was temporarily removed for maintenance, and a short beamline constructed to study high current operation (up to 45 mA) from the original Cornell DC gun in the CBETA experimental hall.…”

Ultrafast High-Voltage Kicker System for Ion-Clearing Gaps

“…Even with improvements in vacuum technology, ions can fully neutralize the electron beam within seconds for vacuum pressures as low as 10 −9 Torr [8]. Therefore, it is necessary to directly remove the trapped ions in order to avoid or mitigate these unwanted effects.…”