Metallic photocathodes

Metal cathodes are commonly used in RF Guns because they work robustly and tolerate poor vacuum compared to semiconductor photocathodes. Metal photocathodes have the advantage that they represent a low contamination risk to the SRF gun cavity.
At ELBE, magnesium (Mg) photocathodes are made of a Ø 10 mm poly-crystalline Mg bulk plug that is mirror-like polished and is used for medium bunch charge applications with bunch charges up to 300 pC.

However, metal cathodes only provide low quantum efficiencies under UV excitation and the main prerequisite for improving the QE is to produce an atomically clean surface. At ELBE, a successfully established process for improving the QE of Mg is laser cleaning [1] in which a high-power laser cleans a Ø 4 mm area in the middle of the Mg plug center.
Although the laser cleaning improves the QE, a non-uniform surface and potential damage to the Mg photocathode surface arise at the same time. Ideally, an alternative process producing an atomically clean, smooth, and damage-free surface is desired.

Therefore, ELBE studies different surface cleanings influencing the Mg photocathode quality. Different cleanings on the Mg surface such as hydrogen ion cleaning, argon ion bombardment, and thermal surface cleaning are compared with the help of in-situ X-ray photoelectron spectroscopy (XPS) without leaving the UHV environment.

XPS photoelectronO and C 1s spectra of Mg cleaned surfaces
Photoelectron O and C 1s spectra of a thermal cleaned Mg surface (left) and an argon-bombarded Mg surface (right).

XPS photoelectron spectra of O 1s revealed surface oxygen (Os) species that could be successfully removed either by thermal cleaning or by argon ion bombardment. Vice versa, bulk oxygen (Ob) species appeared.
The thermal cleaning was not able to remove all carbon contaminations and showed lower QE (0.1 % at 275 nm, 100 µW) than for an argon-bombarded surface (0.35 %), where carbon contaminations were removed completely.

SEM images of the thermally cleaned Mg surface (left) and the argon-sputtered Mg surface (right).
SEM images of the thermally cleaned Mg surface (left) and the argon-sputtered Mg surface (right).

The SEM images show a different morphology for the thermally cleaned Mg surface and the argon-sputtered surface. The SEM of the thermally cleaned sample shows a smooth surface and the measured RMS roughness is about 5 nm, whereas the argon-sputtered surface shows a periodic structure with an RMS roughness of about 8 nm.

In fact, the images might not show the real surface under UHV conditions because the samples were taken out of the UHV vacuum and measured ex-situ with SEM.

The spectra and images shown are the first, preliminary results of the Mg surface studies.