Abstract
Degradation of the photocathode materials employed in photoinjectors represents a challenge for sustained operation of nuclear physics accelerators and high power free electron lasers (FEL). Photocathode quantum efficiency degradation is due to residual gases in the electron source vacuum system being ionized and accelerated back to the photocathode. These investigations are a first attempt to characterize the nature of the photocathode degradation, and employ multiple surface and bulk analysis techniques to investigate damage mechanisms including sputtering of the Cs-oxidant surface monolayer, other surface chemistry effects, and ion implantation. Surface and bulk analysis studies were conducted on two GaAs photocathodes, which were removed from the JLab FEL DC photoemission gun after delivering electron beam, and two control samples. The analysis techniques include helium ion microscopy, Rutherford backscattering spectrometry (RBS), atomic force microscopy, and secondary ion mass spectrometry (SIMS). In addition, two high-polarization strained superlattice GaAs photocathode samples, one removed from the continuous electron beam accelerator facility (CEBAF) photoinjector and one unused, were also analyzed using transmission electron microscopy (TEM) and SIMS. It was found that heat cleaning the FEL GaAs wafer introduces surface roughness, which seems to be reduced by prolonged use. The bulk GaAs samples retained a fairly well organized crystalline structure after delivering beam but show evidence of Cs depletion on the surface. Within the precision of the SIMS and RBS measurements, the data showed no indication of hydrogen implantation or lattice damage from ion back bombardment in the bulk GaAs wafers. In contrast, SIMS and TEM measurements of the strained superlattice photocathode show clear crystal damage in the wafer from ion back bombardment.
Highlights
Scientific productivity at user-based electron accelerators depends on robust photocathode operation to generate high electron beam current, as is the case with high power free electron lasers (FELs), or highly polarized continuous wave (CW) electron beams for nuclear physics research
Surface and bulk analysis studies were conducted on two GaAs photocathodes, which were removed from the Jefferson Lab (JLab) FEL DC photoemission gun after delivering electron beam, and two control samples
The superlattice GaAs samples were analyzed using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS), with the cross sectional depth samples prepared by an focused ion beam (FIB) milling and lift-out technique
Summary
Scientific productivity at user-based electron accelerators depends on robust photocathode operation to generate high electron beam current, as is the case with high power free electron lasers (FELs), or highly polarized continuous wave (CW) electron beams for nuclear physics research. ; where i is current (A), is quantum efficiency, Plaser is incident laser power (W), is laser wavelength (m), e is elementary charge (C), and h is Plank’s constant (J s). This shows that the quantum efficiency of a photocathode is dependent on the power of the laser, and on the wavelength The goal of this work is to analyze the surface morphology, composition, and crystalline quality of negative electron affinity GaAs photocathodes used to produce hundreds or thousands of coulombs of charge in DC photoemission guns, in order to better understand the degradation mechanisms. Increasing operational lifetime through understanding photocathode damage mechanisms can help focus future improvements for the electron sources and could improve accelerator availability in both machines at Jefferson Lab (JLab) and elsewhere
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