In situcleaning of metal cathodes using a hydrogen ion beam

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.

Experimental investigation of thermal emittance components of copper photocathode

“…It is well known that the work function is sensitive to crystal structure and perfection, and ion bombardment damage can produce work function reduction on the order of several hundred meV [40,41]. The low work function in our experiment may be caused by the aggressive ion bombardment during rf breakdowns, and is also supported by the highest QE for copper measured at this wavelength [37].…”

“…The copper cathode QE rose by a factor of $3:5 to 10 À4 after gun conditioning, which is the highest QE reported at this wavelength for a copper cathode [37]. The increase of the cathode QE during gun conditioning is also observed in other groups [5,38], which indicates gun conditioning along with rf breakdowns or high intensity UV laser illumination on the cathode has similar effects as ion cleaning or laser cleaning [37,39]. The QE fitting to Eq.…”

“…(2) shows p ¼ 12:2 AE 1:3 ðMV=mÞ 1=2 , and this means the upper limit of w is 4:20 AE 0:05 eV after the cathode cleaning. The work function upper limit is much lower than the nominal 4.65 eV of polycrystalline copper [40], and a similar phenomenon was reported for both copper and other metals after ion cleaning [37,40,41]. It is well known that the work function is sensitive to crystal structure and perfection, and ion bombardment damage can produce work function reduction on the order of several hundred meV [40,41].…”

Reduction of Intrinsic Electron Emittance from Photocathodes Using Ordered Crystalline Surfaces

“…From the same 3-step based photoemission model, it can be shown that the quantum efficiency (QE) is proportional to the square of the excess energy [12]. In the past decade, experimental studies of metal cathodes with normally incident light have validated this model both in terms of QE [12] and emittance [11,13,14]. A consequence of this model is that the QE depends on the 4th power of the emittance that is required.…”

Photoelectron emission from metal surfaces induced by VUV-emission of filament driven hydrogen arc discharge plasma

“…The spectrum is not corrected for spectral transmittance. (b) Photoelectron yield quantum efficiencies for aluminium, stainless steel, gold [5] and copper [6].…”