IPAC2012 - List of Authors (Schubert, S.G.)

Paper	Title	Page
MOPPP049	Deposition and In-Situ Characterization of Alkali Antimonide Photocathodes	670
	X. Liang SBU, Stony Brook, New York, USA K. Attenkofer ANL, Argonne, USA I. Ben-Zvi, M. Ruiz-Osés Stony Brook University, Stony Brook, USA H.A. Padmore, T. Vecchione LBNL, Berkeley, California, USA S.G. Schubert HZB, Berlin, Germany J. Smedley BNL, Upton, Long Island, New York, USA
	Funding: This work was supported by the Director, Office of Science, Office of Basic Energy Sciences of the U. S. Department of Energy, under Contract No. KC0407-ALSJNT-I0013, and DE-SC0005713. Alkali antimonide cathodes have the potential to provide high quantum efficiency for visible light, and are significantly more tolerant of vacuum contaminants than GaAs, so they are attractive for high-average-current photoinjectors to generate high quality electron beams. These cathodes are crystalline; however, standard growth recipes used today do not produce large crystals. We have grown multi-alkali cathodes on silicon and molybdenum substrates with in-situ X-ray diffraction (XRD) and X-ray reflection (XRR) analysis. The correlation of the cathode structure to the growth parameters and quantum efficiency was explored. During the deposition and evaporation of Sb and K layers, the possibility of selective growth of specific crystalline orientation was observed via X-ray diffraction.

TUPPD050	Investigation of Laser-cleaning Process on Lead Photocathodes	1515
	S.G. Schubert, R. Barday, T. Kamps, T. Quast, A. Varykhalov HZB, Berlin, Germany R. Nietubyć The Andrzej Soltan Institute for Nuclear Studies, Centre Świerk, Świerk/Otwock, Poland F. Siewert BESSY GmbH, Berlin, Germany J. Smedley BNL, Upton, Long Island, New York, USA G. Weinberg FHI, Berlin, Germany
	Funding: Work supported by Bundesministerium für Bildung und Forschung and Land Berlin. Metal photocathodes are widely used in electron injectors due to their stability and long life time; unfortunately they exhibit low quantum efficiency. Due to adsorption of contaminants the work function increases and thus the quantum efficiency is further reduced. In order to increase the quantum efficiency of our Pb cathode we performed a cleaning procedure by means of a high power excimer laser as suggested by Smedley. The process was studied on witness samples in a combined photo emission, SEM and quantum efficiency measurement study. Thin Lead films were arc-deposited on optical polished Mo-substrates. Before and after irradiation the sample was analyzed at 140 eV photon energy at a XPS/ARPES end station at the synchrotron radiation source Bessy II. We followed the change of the Pb 5d signals. In the initial situation we observed signals originating from metallic Pb and Pb in the oxidized state, respectively. Since the surface roughness is of concern for the injector performance it was examined before and after the irradiation procedure with white-light-interferometry and the surface morphology by means of SEM. J. Smedley et al, PRST-AB 11, 013502 (2008). ** Rao, T. et al., IPAC 2010, THPEC020 (2010).

TUPPD062	The Source of Emittance Dilution and photoemission tunneling effect in Photocathode RF Guns	1542
	V. Volkov BINP SB RAS, Novosibirsk, Russia R. Barday, T. Kamps, J. Knobloch, A.N. Matveenko, S.G. Schubert, J. Völker HZB, Berlin, Germany J.K. Sekutowicz DESY, Hamburg, Germany
	Funding: Work supported by Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association VH-NG-636 and HRJRG-214. Experimental data on HoBiCaT SRF photoinjector give an emittance which is much larger than the predicted thermal emittance. Modeling of photocathode RF gun beams with the different imperfections of experimental setup (alignment errors, inhomogeneity of quantum efficiency and laser power distributions on the cathode) is given. The main reason for the beam emittance dilution is photocathode field imperfections induced by field emitters that change the local electric field. Some field models of such photocathodes are tested in the simulations. The dependence of photocathode beam currents on the surface electric field was measured with the HoBiCaT SRF Photoinjector. The dependence can be explained by the tunneling effect described by Fowler-Nordheim like equation and is difficult to explain by usually applying Schottky effect.

MOPPP015	Status of the BERLinPro Energy Recovery Linac Project	601
	J. Knobloch, M. Abo-Bakr, W. Anders, R. Barday, K. Bürkmann-Gehrlein, V. Dürr, S.C. Heßler, A. Jankowiak, T. Kamps, O. Kugeler, B.C. Kuske, P. Kuske, A.N. Matveenko, G. Meyer, R. Müller, A. Neumann, K. Ott, Y. Petenev, D. Pflückhahn, T. Quast, J. Rahn, S.G. Schubert HZB, Berlin, Germany
	Funding: Funding provided by the BMBF and the State of Berlin In October 2010 Helmholtz Zentrum Berlin received funding to design and build the Berlin Energy Recovery Linac Project BERLinPro. The goal of this compact ERL is to develop the accelerator physics and technology required to generate and accelerate a 100-mA, 1 mm·mrad emittance beam. Given the flexibility of ERLs, other operation modes such as short-bunch operation will also be investigated. The BERLinPro technology and know-how can then be transferred to a variety of ERL-based applications. Presently, BERLinPro is in the design phase and the optics has been settled. Furthermore, first beam has been achieved with a superconducting RF photoinjector, which represents an important step towards realizing a CW injector for BERLinPro. An overview of the present status and the conceptual design report is presented.

TUPPD051	Operational Experience with the Nb/Pb SRF Photoelectron Gun	1518
	T. Kamps, W. Anders, R. Barday, A. Jankowiak, J. Knobloch, O. Kugeler, A.N. Matveenko, A. Neumann, T. Quast, J. Rudolph, S.G. Schubert, J. Völker HZB, Berlin, Germany P. Kneisel JLAB, Newport News, Virginia, USA R. Nietubyć The Andrzej Soltan Institute for Nuclear Studies, Centre Świerk, Świerk/Otwock, Poland J.K. Sekutowicz DESY, Hamburg, Germany J. Smedley BNL, Upton, Long Island, New York, USA J. Teichert HZDR, Dresden, Germany V. Volkov BINP SB RAS, Novosibirsk, Russia I. Will MBI, Berlin, Germany
	SRF photoelectron guns offer the promise of high brightness, high average current beam production for the next generation of accelerator driven light sources such as free electron lasers, THz radiation sources or energy-recovery linac driven synchrotron radiation sources. In a first step a fully superconducting RF (SRF) photoelectron gun is under development by a collaboration between HZB, DESY, JLAB, BNL and NCBJ. The aim of the experiment is to understand and improve the performance of a Nb SRF gun cavity coated with a small metallic Pb cathode film on the cavity backplane. This paper describes the highlights from the commissioning and beam parameter measurements. The main focus is on lessons learned from operation of the SRF gun.

Paper

Title

Page

Deposition and In-Situ Characterization of Alkali Antimonide Photocathodes

670

X. Liang
SBU, Stony Brook, New York, USA
K. Attenkofer
ANL, Argonne, USA
I. Ben-Zvi, M. Ruiz-Osés
Stony Brook University, Stony Brook, USA
H.A. Padmore, T. Vecchione
LBNL, Berkeley, California, USA
S.G. Schubert
HZB, Berlin, Germany
J. Smedley
BNL, Upton, Long Island, New York, USA

Funding: This work was supported by the Director, Office of Science, Office of Basic Energy Sciences of the U. S. Department of Energy, under Contract No. KC0407-ALSJNT-I0013, and DE-SC0005713.
Alkali antimonide cathodes have the potential to provide high quantum efficiency for visible light, and are significantly more tolerant of vacuum contaminants than GaAs, so they are attractive for high-average-current photoinjectors to generate high quality electron beams. These cathodes are crystalline; however, standard growth recipes used today do not produce large crystals. We have grown multi-alkali cathodes on silicon and molybdenum substrates with in-situ X-ray diffraction (XRD) and X-ray reflection (XRR) analysis. The correlation of the cathode structure to the growth parameters and quantum efficiency was explored. During the deposition and evaporation of Sb and K layers, the possibility of selective growth of specific crystalline orientation was observed via X-ray diffraction.

TUPPD050

Investigation of Laser-cleaning Process on Lead Photocathodes

1515

S.G. Schubert, R. Barday, T. Kamps, T. Quast, A. Varykhalov
HZB, Berlin, Germany
R. Nietubyć
The Andrzej Soltan Institute for Nuclear Studies, Centre Świerk, Świerk/Otwock, Poland
F. Siewert
BESSY GmbH, Berlin, Germany
J. Smedley
BNL, Upton, Long Island, New York, USA
G. Weinberg
FHI, Berlin, Germany

Funding: Work supported by Bundesministerium für Bildung und Forschung and Land Berlin.
Metal photocathodes are widely used in electron injectors due to their stability and long life time; unfortunately they exhibit low quantum efficiency. Due to adsorption of contaminants the work function increases and thus the quantum efficiency is further reduced. In order to increase the quantum efficiency of our Pb cathode we performed a cleaning procedure by means of a high power excimer laser as suggested by Smedley*. The process was studied on witness samples in a combined photo emission, SEM and quantum efficiency measurement study. Thin Lead films were arc-deposited on optical polished Mo-substrates**. Before and after irradiation the sample was analyzed at 140 eV photon energy at a XPS/ARPES end station at the synchrotron radiation source Bessy II. We followed the change of the Pb 5d signals. In the initial situation we observed signals originating from metallic Pb and Pb in the oxidized state, respectively. Since the surface roughness is of concern for the injector performance it was examined before and after the irradiation procedure with white-light-interferometry and the surface morphology by means of SEM.
*J. Smedley et al, PRST-AB 11, 013502 (2008).
** Rao, T. et al., IPAC 2010, THPEC020 (2010).

TUPPD062

The Source of Emittance Dilution and photoemission tunneling effect in Photocathode RF Guns

1542

V. Volkov
BINP SB RAS, Novosibirsk, Russia
R. Barday, T. Kamps, J. Knobloch, A.N. Matveenko, S.G. Schubert, J. Völker
HZB, Berlin, Germany
J.K. Sekutowicz
DESY, Hamburg, Germany

Funding: Work supported by Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association VH-NG-636 and HRJRG-214.
Experimental data on HoBiCaT SRF photoinjector give an emittance which is much larger than the predicted thermal emittance. Modeling of photocathode RF gun beams with the different imperfections of experimental setup (alignment errors, inhomogeneity of quantum efficiency and laser power distributions on the cathode) is given. The main reason for the beam emittance dilution is photocathode field imperfections induced by field emitters that change the local electric field. Some field models of such photocathodes are tested in the simulations. The dependence of photocathode beam currents on the surface electric field was measured with the HoBiCaT SRF Photoinjector. The dependence can be explained by the tunneling effect described by Fowler-Nordheim like equation and is difficult to explain by usually applying Schottky effect.

MOPPP015

Status of the BERLinPro Energy Recovery Linac Project

601

J. Knobloch, M. Abo-Bakr, W. Anders, R. Barday, K. Bürkmann-Gehrlein, V. Dürr, S.C. Heßler, A. Jankowiak, T. Kamps, O. Kugeler, B.C. Kuske, P. Kuske, A.N. Matveenko, G. Meyer, R. Müller, A. Neumann, K. Ott, Y. Petenev, D. Pflückhahn, T. Quast, J. Rahn, S.G. Schubert
HZB, Berlin, Germany

Funding: Funding provided by the BMBF and the State of Berlin
In October 2010 Helmholtz Zentrum Berlin received funding to design and build the Berlin Energy Recovery Linac Project BERLinPro. The goal of this compact ERL is to develop the accelerator physics and technology required to generate and accelerate a 100-mA, 1 mm·mrad emittance beam. Given the flexibility of ERLs, other operation modes such as short-bunch operation will also be investigated. The BERLinPro technology and know-how can then be transferred to a variety of ERL-based applications. Presently, BERLinPro is in the design phase and the optics has been settled. Furthermore, first beam has been achieved with a superconducting RF photoinjector, which represents an important step towards realizing a CW injector for BERLinPro. An overview of the present status and the conceptual design report is presented.

TUPPD051

Operational Experience with the Nb/Pb SRF Photoelectron Gun

1518

T. Kamps, W. Anders, R. Barday, A. Jankowiak, J. Knobloch, O. Kugeler, A.N. Matveenko, A. Neumann, T. Quast, J. Rudolph, S.G. Schubert, J. Völker
HZB, Berlin, Germany
P. Kneisel
JLAB, Newport News, Virginia, USA
R. Nietubyć
The Andrzej Soltan Institute for Nuclear Studies, Centre Świerk, Świerk/Otwock, Poland
J.K. Sekutowicz
DESY, Hamburg, Germany
J. Smedley
BNL, Upton, Long Island, New York, USA
J. Teichert
HZDR, Dresden, Germany
V. Volkov
BINP SB RAS, Novosibirsk, Russia
I. Will
MBI, Berlin, Germany

SRF photoelectron guns offer the promise of high brightness, high average current beam production for the next generation of accelerator driven light sources such as free electron lasers, THz radiation sources or energy-recovery linac driven synchrotron radiation sources. In a first step a fully superconducting RF (SRF) photoelectron gun is under development by a collaboration between HZB, DESY, JLAB, BNL and NCBJ. The aim of the experiment is to understand and improve the performance of a Nb SRF gun cavity coated with a small metallic Pb cathode film on the cavity backplane. This paper describes the highlights from the commissioning and beam parameter measurements. The main focus is on lessons learned from operation of the SRF gun.