Jankowiak, A.

Kamps, T.

Kühn, J.

Schmei er, M.

HZB Repository

CsK2Sb PHOTOCATHODE DEVELOPMENT FOR bERLinPro∗M. A. H. Schmeißer#, A. Jankowiak, T. Kamps, J. Kühn Helmholtz-Zentrum Berlin, GermanyAbstractIn order to generate high brightness and high-current elec-tron beams for bERLinPro, an SRF photoinjector is beingdeveloped at HZB. Normal conducting CsK2Sb cathodeswill be used due to their high quantum efficiency (QE) atvisible wavelengths and fast response time. We report onthe commissioning of a preparation and analysis system thatallows investigation of the surface and near-surface chemicalcomposition of the cathodes using XPS and ion scattering.In addition, the design of an UHV transport system for cath-odes is presented.INTRODUCTIONThe ERL concept imposes strong demands on the injector.For bERLinPro [1], we intend to operate an SRF photoinjec-tor in CW mode with high bunch charge in order to achievean average current of 100 mA [2]. The use of CsK2Sb pho-tocathodes with high QE in the visible allows to drive theinjector with a laser wavelength of 532 nm and average powerof only 10s of Watts, thus relaxing the requirements on thecryogenic and drive laser system. The preparation of thecathodes will be performed in a dedicated growth chamber,where the cathode material is deposited on a plug that willbe inserted in the back wall of the photoinjector cavity. Thecathodes must be transferred between the two systems inultra high vacuum conditions of ≤ 10−10 mbar.Alkali antimonides have been used in phototubes andlight-sensitive devices for decades and a number of empiri-cally optimized deposition recipes are available. However,many aspects of their growth, like the diffusion of alkalis inthe material and the crystallization as well as correlations be-tween composition, crystal structure, and electronic structureand emission properties have yet to be understood. The de-mand from accelerator physics for highest QEs, reproduciblegrowth, smooth surfaces, and stable operation (as desiredby future user facilities) calls for dedicated investigations ofthese topics.Using in-situ surface analysis with the system describedbelow it is intended to study the influence of growth parame-ters on the composition of the material and find correlationswith electronic structure and emission properties.COMMISSIONING OF THEPREPARATION SYSTEMThe preparation and analysis system was commissionedat BNL and is now back in Berlin. It consists of a prepa-ration chamber and a surface analysis chamber which areconnected via a sample manipulator. Evaporation can be∗ Work supported by the German Bundesministerium für Bildung undForschung, Land Berlin and grants of Helmholtz Association.# martin.schmeisser@helmholtz-berlin.deperformed from a thermal evaporation cell for Sb and twoports for Alkali deposition from SAES dispensers. The anal-ysis chamber is equipped with a X-Ray source (Al and Mganodes), an ion source and a SPECS Phoibos 100 hemispher-ical analyzer. Both chambers have a base pressure below3 · 10−10 mbar.Figure 1: Photograph of the photocathode preparation andanalysis (PPA) system.PHOTOCATHODE CHARACTERIZATIONThe photocurrent of a sample is collected by a biasedanode and measured with a picoamperemeter. Currently, thesample is excited by a 532 nm solid state laser with 1 mWoptical power. It is planned to set up a Xe-arc lamp withmonochromator which would allow to resolve the spectralsensitivity of a sample. The spectral intensity of commercialtunable light sources is about 10 µW per nm bandwidth,which would allow us to measure QEs down to 10−5.A first cathode sample was prepared in April 2015 on aMo foil (Sigma Aldrich, 99.9%). The substrate was cleanedby heating it to 450◦C for 1 h and Ar+ sputter cleaning at3 keV for 30 min. After the cleaning procedure, only smallO and C contaminations were still present. An Sb layer wasdeposited at a deposition rate of 0.5 Å/s, which correspondsto 1 · 10−7 mbar vapour pressure. During deposition, thepartial pressure of H2O was 2 · 10−9 mbar, O2 and CO2 werebelow 1 · 10−10 mbar. The film proved to be very clean andno contaminations were found in the XPS spectrum.Following a sequential growth procedure, K and Cs weresubsequently deposited. For both alkalis, the vapour pressurewas very low and could hardly be resolved in the mass spec-trometer (<10−13 mbar). During the K deposition the partialpressure of H2Owas 2 · 10−9 mbar, O2 and CO2 were below1 · 10−10 mbar. The Cs dispenser had to be heated above itsspecified current to obtain a reading on the quartz microbal-ance. A too high temperature of the dispenser led to highbackground pressure of atomic O and N in the 10−9 mbarrange. An XPS spectrum was taken after the deposition ofboth Alkalis where traces of Cr and O are discernible. Allthree spectra can be compared in fig. 2.1000 800 600 400 200 0Cs 4dCs 4pK 2pK 2sSb 3dSb 3p1/2Intensity (arb. u.)Binding energy (eV) Mo Sb Cs-K-SbMg KαO KLLO 1sMo 3sMo 3pMo 3dC 1s Mo 4sMo 4pSb 3dSb 4dSb MNNSb 3p3/2K LMM Cs 3dFigure 2: XPS survey spectra of three steps of a cathodepreparation. The Mo substrate after cleaning (see text), afterSb deposition and after K and Cs deposition is shown.SUBSTRATE PREPARATION ANDCATHODE TRANSFERThe preparation system is located in a laboratory roomapart from the injector, thus the cathodes must be transportedin a movable vacuum vessel and transferred into the injectorcryomodule using a load-lock. The transfer system is basedon a setup that is in use at HZDR.Figure 3: Drawing of the pug holder that will fit into the backwall of the gun cavity. The small CuBe spring that holds theplug is locked tightly and can be opened by a mechanisminside the holder rod.The cathode puck (red cap in fig. 3) can be made ofcopper or molybdenum. Copper plugs will be used for thecommissioning of the injector with low bunch charge andmolybdenum plugs will be used as substrates for the CsK2Sbcathodes. In order to maintain the low surface resistivity ofcopper, but exploit the better substrate behavior of refractorymetals, the use of a Cu plug with Ta or Mo evaporated onthe surface as a barrier substrate layer will be investigated.Here, field emission properties need to be considered [3].Figure 4: Drawing of the transfer system at the PPA system,as-planned.In the preparation system and during transport the plugswill be mounted on omicron style sample holders. The trans-fer system (fig 4) at the injector allows to take the plugsoff their sample holders and mount them on a cathode stalkshown in fig. 3. Both transfer systems can handle a tray offour sample holders which can be inserted to the vacuumsuitcase (fig. 5) through a load-lock.Figure 5: Photograph of the vacuum suitcase with 4 cathodepositions, as recently commissioned. Base pressures in thelow 10−11 mbar range can be maintained.Ongoing work will be focused on studying the QE depen-dence on the composition, optimization of growth proce-dures, photocathode life time studies and the developmentof a reproducible growth procedure of CsK2Sb photocath-odes with a smooth surface and high QE. A prototype ofthe photoinjector and a diagnostics beamline will be avail-able in spring 2016 [4], where in-gun experiments and post-operation analysis will be performed.ACKNOWLEDGMENTThe authors gratefully acknowledge the engineering workof K. Martin, D. Böhlick and A. Frahm and P. Murzcek(HZDR).REFERENCES[1] M. Abo-Bakr et al. “STATUS OF THE HZB ERLPROJECT bERLinPro.” In: Proceedings of IPAC 2014.MOPRO106. Dresden, 2014, pp. 2014–2016.[2] A. Neumann et al. “Towards a 100mA Superconduct-ing RF Photoinjector for bERLinPro.” In: Proceedingsof SRF 2013. MOIOB02. Paris, pp. 42–49.[3] S. Lagotzky et al. “Prevention of electron field emis-sion from molybdenum substrates for photocathodesby the native oxide layer.” In: The European PhysicalJournal Applied Physics 70.2 (2015), p. 21301. doi:10.1051/epjap/2015150167.[4] J. Völker et al. “Introducing GunLab- A Compact TestFacility for SRF Photoinjectors.” In: Proceedings ofIPAC 2014. MOPRI020. Dresden, 2014, pp. 630–632.

CsK2Sb Photocathode Development for BERLinPro

http://www.helmholtz-berlin.de/pubbin/oai_publication?VT=1&ID=89088

CsK2Sb Photocathode Development for BERLinPro

Abstract

Similar works

Full text

Available Versions

HZB Repository