Abstract
Metal photocathodes are commonly used in high-field rf guns because they are robust, straightforward to implement, and tolerate relatively poor vacuum compared to semiconductor cathodes. However, these cathodes have low quantum efficiency (QE) even at UV wavelengths, and still require some form of cleaning after installation in the gun. A commonly used process for improving the QE is laser cleaning. In this technique the UV-drive laser is focused to a small diameter close to the metal's damage threshold and then moved across the surface to remove contaminants. This method does improve the QE, but can produce nonuniform emission and potentially damage the cathode. Ideally, an alternative process which produces an atomically clean, but unaltered, surface is needed. In this paper we explore using a hydrogen ion (H-ion) beam to clean a copper cathode. We describe QE measurements over the wavelength range of interest as a function of integrated exposure to an H-ion beam. We also describe the data analysis to obtain the work function and derive a formula of the QE for metal cathodes. Our measured work function for the cleaned sample is in good agreement with published values, and the theoretical QE as a function of photon wavelength is in excellent agreement with the cleaned copper experimental results. Finally, we propose an in situ installation of an H-ion gun compatible with existing s-band rf guns.
Highlights
AND DESCRIPTION OF THE EXPERIMENTImproving and maintaining the quantum efficiency (QE)of a metal photocathode in an s-band rf gun requires a process for cleaning the surface
Measurements of the QE vs wavelength, surface contaminants using x-ray photoelectron spectroscopy, and surface roughness were performed on a copper sample, and the results showed a significant increase in QE after cleaning with a 1 keV hydrogen ion beam
Measurements of the QE as a function of optical wavelength were performed as a function of the integrated exposures of a copper sample to the hydrogen beam
Summary
Of a metal photocathode in an s-band rf gun requires a process for cleaning the surface. In this type of gun, the cathode is typically installed and the system is vacuum baked to 200 C. While laser cleaning does increase the cathode QE, it requires fluences close to the damage threshold and rastering the small diameter beam, both of which can produce nonuniform electron emission and potentially damage the cathode. Measurements of the QE vs wavelength, surface contaminants using x-ray photoelectron spectroscopy, and surface roughness were performed on a copper sample, and the results showed a significant increase in QE after cleaning with a 1 keV hydrogen ion beam. These results and a method for installing an H-ion cleaner on existing s-band guns are described
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