Search

GaAs photocathodes in dc high-voltage photoguns are highly susceptible to ion back-bombardment, which reduces the photocathode quantum efficiency and limits the useful operating lifetime for producing polarized electron beams. This paper demonstrates that applying a small positive bias to the photogun anode can significantly suppress ion back-bombardment and increase charge lifetime. This technique was studied extensively using the Continuous Electron Beam Accelerator Facility photogun, where highly polarized electron beams created using a strained-superlattice GaAs/GaAsP photocathode were used and charge lifetimes improved by almost a factor of 2. A new simulation code ionator was developed to model ion production and tracking in order to better understand and explain the factors that led to the performance improvement. Results of the experiments and simulations are discussed in detail.

The Radio Frequency Quadrupole (RFQ) is a linear accelerator that focuses, bunches, and accelerates a continuous input of charged particles while preserving the beam emittance. This paper focuses on the study of the transverse acceptance of an RFQ and how this concept can be used in the design of frontend structures. A simple and fast system to qualify a source and low energy transfer line has been developed in terms of the number of particles delivered in the RFQ acceptance. Multi-particle simulation results show a dependence of the RFQ transverse acceptance on the particle phase in the radio frequency period. The usually referred-to acceptance value is, in fact, just an average value over the 360° phase range, whereas a modulation has been found between more and less favorable phase values, with different patterns depending on the specific structure. We use as a study case three RFQs designed and operated at CERN to investigate such correlations.

The need to extract the maximum beam intensity with low transversal emittance often comes with the drawback of operating the ion source to limits where beam current instabilities arise, such fluctuations can change the beam properties producing a mismatch in the following sections of the machine. The space charge compensation (SCC) generated by the beam particles colliding with the residual gas reaches a steady state after a build-up time. This paper shows how once in the steady state, the beam ends with a transversal emittance value bigger than the case without compensation. In addition, we study how the beam intensity variation can disturb the SCC dynamics and its impact on the beam properties. The results presented in this work come from 3-D simulations using tracking codes taking into account the secondary ions to estimate the degree of the emittance growth due to space charge and SCC.