For many disciplines in basic and applied research, medicine and industrial development accelerators are an important driving force. Especially electron accelerators as synchrotron sources are among the brightest sources of radiation from the infrared to the X-ray regime and thus fundamental to a broad range of analytical techniques. Photoinjectors as electron sources for accelerator applications are a key component for the development of light sources such as free electron lasers as well as new accelerator concepts like energy-recovery linacs (ERLs). The photocathode and drive laser define the quantum efficiency and intrinsic emittance of the photoemission process and thus central figures of merit of the photoinjector. This work focuses on the development of alkali antimonide photocathodes for the application in a superconducting radio frequency photoinjector. Alkali antimonides generally exhibit a high quantum efficiency and cesium potassium antimonide (Cs-K-Sb) specifically is expected to release electrons with a low intrinsic emittance as the photoemission threshold is close to the photon energy of common, green, drive laser wavelengths.
A preparation and analysis system has been commissioned for the deposition of Cs-K-Sb thin film photocathodes and their analysis regarding quantum efficiency and chemical composition. A new deposition technique, the alkali metal co-deposition, was established and compared to the sequential deposition in terms of process reliability and quality of the produced samples. For our setup, the co-evaporation procedure was found to yield samples with higher QE in a reproducible manner. The correlation of quantum efficiency with the stoichiometry as determined by X-ray photoemission spectroscopy was investigated and the Cs2KSb composition yields higher QE than CsK2Sb. In view of the operation of photocathodes in the cryogenic environment of the photoinjector, the influence of cooling on the spectral response was measured and no strong influence at the relevant photon energies was found. The intrinsic emittance of a cesium antimonide (Cs-Sb) photocathode was measured in a static accelerating setup and is discussed with reference to recent photoemission models. The expected decrease of the intrinsic emittance with photon energy was observed and a lower limit of the mean transverse energy manifests at the photoemission threshold.
The work concludes with a report of the commissioning of a prototype of the photoinjector and successful cathode transfers in ultra-high vacuum, which represents an important technological advancement towards the operation of an accelerator with the combination of Cs-K-Sb photocathodes and an SRF injector. This combination makes the generation of an electron beam with low emittance and high average current possible which is necessary, e.g. for the operation of an ERL like bERLinPro.
|Advisor:||Jankowiak , Andreas , Raoux , Simone , Aulenbacher , Kurt|
|School:||Humboldt Universitaet zu Berlin (Germany)|
|Source:||DAI-C 81/7(E), Dissertation Abstracts International|
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