IPAC2018 - List of Authors (Karkare, S.S.)

Paper	Title	Page
TUPMF065	The Role of Electron-Phonon Scattering in Transverse Momentum Conservation in PbTe(111) Photocathodes	1414
SUSPF027	use link to see paper's listing under its alternate paper code
	J. K. Nangoi, T.A. Arias Cornell University, Ithaca, New York, USA S.S. Karkare, H.A. Padmore LBNL, Berkeley, California, USA W.A. Schroeder UIC, Chicago, Illinois, USA
	Funding: The U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams. The state of the art in creating high quality electron beams for particle accelerator applications and next generation ultrafast electron diffraction and microscopy involves laser-generated photoemission. A high quality beam requires that electrons emerge from the surface with low mean transverse energy (MTE). Recent density-functional theory calculations by T. Li and W. A. S. [arXiv:1704.00194v1 [physics.acc-ph] (2017)] suggest that PbTe(111) will produce low-MTE photoelectrons due to the low effective electron mass associated with its electronic band structure. Based on this, we measured the distribution of photoelectrons from PbTe(111) and found the MTE to be about 20x larger than expected. To explain the apparent lack of transverse momentum conservation, we carried out many-body photoemission calculations including electron-phonon scattering. Our results are in far better agreement with the experiment, underscoring the importance of electron-phonon scattering in photoemission from PbTe(111), and suggest that cooling could mitigate the phonon effects on the MTE for this material.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-TUPMF065
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TUPML026	Multi-photon Photoemission and Ultrafast Electron Heating in Cu Photocathodes at Threshold	1593
	J. Bae, L. Cultrera Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA I.V. Bazarov, J.M. Maxson Cornell University, Ithaca, New York, USA S.S. Karkare, H.A. Padmore LBNL, Berkeley, California, USA P. Musumeci, X.L. Shen UCLA, Los Angeles, California, USA
	Funding: U.S. National Science Foundation under award PHY-1549132, the Center for Bright Beams. Operating photocathodes near the photoemission threshold holds the promise of yielding small intrinsic emittance, at the cost of significantly reduced quantum efficiency. In modern femtosecond photoemission electron sources, this requires a very high intensity (10s of GW/cm²) to extract a useful quantity of electrons. At this intensity, the electron occupation function is far from equilibrium and evolves rapidly on sub-ps timescales. Thus, ultrafast laser heating and multiphoton photoemission effects may play a significant role in emission, thereby increasing the minimum achievable emittance. In this work, we use a Boltzmann equation approach to calculate the non-equilibrium occupation function evolution in time for a copper photocathode, yielding a prediction of quantum efficiency and mean transverse energy as a function of input intensity.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-TUPML026
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THPAK116	Modeling Surface Roughness Effects and Emission Properties of Bulk and Layered Metallic Photocathode	3515
	D.A. Dimitrov, G.I. Bell Tech-X, Boulder, Colorado, USA I. Ben-Zvi, J. Smedley BNL, Upton, Long Island, New York, USA J. Feng, S.S. Karkare, H.A. Padmore LBNL, Berkeley, California, USA
	Funding: This work was supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract Nos. DE-SC0013190, DE-AC02-05CH11231, and KC0407-ALSJNT-I0013. The thermal limit of the intrinsic emittance of photocathodes represents an important property to measure experimentally and to understand theoretically. Detailed measurements of intrinsic emittance have become possible in momentatron experiments. Moreover, recent developments in material design have allowed growing photoemissive layers with controlled surface roughness. Although analytical formulations of the effects of roughness have been developed, a full theoretical model and experimental verification are lacking. We aim to bridge this gap by developing realistic models for different materials in the three-dimensional VSim particle-in-cell code. We have recently implemented modeling of electron photo-excitation, transport, and emission from photoemissive layers grown on a substrate. We report results from simulations with these models on electron emission from antimony and gold. We consider effects due to density of states, photoemissive layer thickness, surface roughness and how they affect the spectral response of quantum yield and intrinsic emittance.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAK116
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THPMF080	Physical and Chemical Roughness of Alkali-Animonide Cathodes	4259
	S.S. Karkare, S. Emamian, G. Gevorkyan, H.A. Padmore, A.K. Schmid LBNL, Berkeley, California, USA I.V. Bazarov Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA A. Galdi Cornell University, Ithaca, New York, USA
	Over the last decade, alkali-antimonides have been investigated as high QE cathodes in green light and more recently as ultra-low intrinsic emittance cathodes in near-threshold red wavelengths at cryogenic temperatures. Nano-meter scale surface non-uniformities (physical roughness and chemical roughness or work function variations) are thought to limit the smallest possible emittance from these materials at the photoemission threshold under cryogenic conditions. Despite this, the surfaces of alkali-antimonides have not been well characterized in terms of the surface non-uniformities. Here, we present measurements of both the physical and chemical roughness of alkali-antimonide surfaces using several surface characterization techniques like atomic force microscopy, kelvin probe force microscopy, low energy electron microscopy and near-threshold photoemission electron microscopy and show how such non-uniformities limit the intrinsic emittance. L. Cultrera et al Phys. Rev. ST Accel. Beams 18, 113401 (2015) **J. Feng et al, J. of Appl. Phys. 121, 044904 (2017)
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPMF080
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THPMF081	Intrinsic Emittance of Single Crystal Cathodes	4263
	S.S. Karkare, H.A. Padmore LBNL, Berkeley, California, USA G. Adhikari, W.A. Schroeder UIC, Chicago, Illinois, USA
	The transverse momentum of electrons is conserved during photoemission from atomically ordered surfaces of single crystal materials. Photocathodes used in all photoinjectors today have disordered surfaces and do not exploit this phenomenon. Recently, using this conservation of transverse momentum, significant reduction in intrinsic emittance was demonstrated from the (111) surface of silver. Here, we present measurements of transverse momentum distributions of electrons photoemitted from the ordered surfaces of Ag and Cu single crystals at several photon energies. These measurements will help in understanding the photoemission process and show how band-structure and the conservation of transverse momentum can be used to obtain further reduction in intrinsic emittance from photocathodes. Karkare et al., Phys. Rev. Lett. 118, 164802 (2017)
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPMF081
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THPML053	Computational Screening for Low Emittance Photocathodes	4755
	J.T. Paul, R.G. Hennig University of Florida, Gainesville, Florida, USA I.V. Bazarov, A. Galdi Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA S.S. Karkare, H.A. Padmore LBNL, Berkeley, California, USA
	The majority of photocathode materials in use in accelerator applications have been discovered empirically through trial and error with little guidance from material science calculations. Alternatively, one can envision a process which is heavily guided by computational search using latest advances in density functional theory (DFT). In this work, the MaterialsProject database is searched for potential single crystal photocathodes that would be suitable for ultralow emittance beam production. The materials in the database are initially screened on the basis of experimental practicality. Following this, the expected emittance is calculated from the DFT computed band structures for the pre-screened materials using the conservation of energy and transverse momentum during photoemission. Based on such computational screening, we provide a list of potential low emittance photocathode materials which can be investigated experimentally as high brightness electron sources.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPML053
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

Paper

Title

Page

TUPMF065

The Role of Electron-Phonon Scattering in Transverse Momentum Conservation in PbTe(111) Photocathodes

1414

SUSPF027

use link to see paper's listing under its alternate paper code

J. K. Nangoi, T.A. Arias
Cornell University, Ithaca, New York, USA
S.S. Karkare, H.A. Padmore
LBNL, Berkeley, California, USA
W.A. Schroeder
UIC, Chicago, Illinois, USA

Funding: The U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams.
The state of the art in creating high quality electron beams for particle accelerator applications and next generation ultrafast electron diffraction and microscopy involves laser-generated photoemission. A high quality beam requires that electrons emerge from the surface with low mean transverse energy (MTE). Recent density-functional theory calculations by T. Li and W. A. S. [arXiv:1704.00194v1 [physics.acc-ph] (2017)] suggest that PbTe(111) will produce low-MTE photoelectrons due to the low effective electron mass associated with its electronic band structure. Based on this, we measured the distribution of photoelectrons from PbTe(111) and found the MTE to be about 20x larger than expected. To explain the apparent lack of transverse momentum conservation, we carried out many-body photoemission calculations including electron-phonon scattering. Our results are in far better agreement with the experiment, underscoring the importance of electron-phonon scattering in photoemission from PbTe(111), and suggest that cooling could mitigate the phonon effects on the MTE for this material.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-TUPMF065

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUPML026

Multi-photon Photoemission and Ultrafast Electron Heating in Cu Photocathodes at Threshold

1593

J. Bae, L. Cultrera
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
I.V. Bazarov, J.M. Maxson
Cornell University, Ithaca, New York, USA
S.S. Karkare, H.A. Padmore
LBNL, Berkeley, California, USA
P. Musumeci, X.L. Shen
UCLA, Los Angeles, California, USA

Funding: U.S. National Science Foundation under award PHY-1549132, the Center for Bright Beams.
Operating photocathodes near the photoemission threshold holds the promise of yielding small intrinsic emittance, at the cost of significantly reduced quantum efficiency. In modern femtosecond photoemission electron sources, this requires a very high intensity (10s of GW/cm²) to extract a useful quantity of electrons. At this intensity, the electron occupation function is far from equilibrium and evolves rapidly on sub-ps timescales. Thus, ultrafast laser heating and multiphoton photoemission effects may play a significant role in emission, thereby increasing the minimum achievable emittance. In this work, we use a Boltzmann equation approach to calculate the non-equilibrium occupation function evolution in time for a copper photocathode, yielding a prediction of quantum efficiency and mean transverse energy as a function of input intensity.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-TUPML026

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPAK116

Modeling Surface Roughness Effects and Emission Properties of Bulk and Layered Metallic Photocathode

3515

D.A. Dimitrov, G.I. Bell
Tech-X, Boulder, Colorado, USA
I. Ben-Zvi, J. Smedley
BNL, Upton, Long Island, New York, USA
J. Feng, S.S. Karkare, H.A. Padmore
LBNL, Berkeley, California, USA

Funding: This work was supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract Nos. DE-SC0013190, DE-AC02-05CH11231, and KC0407-ALSJNT-I0013.
The thermal limit of the intrinsic emittance of photocathodes represents an important property to measure experimentally and to understand theoretically. Detailed measurements of intrinsic emittance have become possible in momentatron experiments. Moreover, recent developments in material design have allowed growing photoemissive layers with controlled surface roughness. Although analytical formulations of the effects of roughness have been developed, a full theoretical model and experimental verification are lacking. We aim to bridge this gap by developing realistic models for different materials in the three-dimensional VSim particle-in-cell code. We have recently implemented modeling of electron photo-excitation, transport, and emission from photoemissive layers grown on a substrate. We report results from simulations with these models on electron emission from antimony and gold. We consider effects due to density of states, photoemissive layer thickness, surface roughness and how they affect the spectral response of quantum yield and intrinsic emittance.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAK116

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPMF080

Physical and Chemical Roughness of Alkali-Animonide Cathodes

4259

S.S. Karkare, S. Emamian, G. Gevorkyan, H.A. Padmore, A.K. Schmid
LBNL, Berkeley, California, USA
I.V. Bazarov
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
A. Galdi
Cornell University, Ithaca, New York, USA

Over the last decade, alkali-antimonides have been investigated as high QE cathodes in green light and more recently as ultra-low intrinsic emittance cathodes in near-threshold red wavelengths at cryogenic temperatures*. Nano-meter scale surface non-uniformities (physical roughness and chemical roughness or work function variations) are thought to limit the smallest possible emittance from these materials at the photoemission threshold under cryogenic conditions**. Despite this, the surfaces of alkali-antimonides have not been well characterized in terms of the surface non-uniformities. Here, we present measurements of both the physical and chemical roughness of alkali-antimonide surfaces using several surface characterization techniques like atomic force microscopy, kelvin probe force microscopy, low energy electron microscopy and near-threshold photoemission electron microscopy and show how such non-uniformities limit the intrinsic emittance.
*L. Cultrera et al Phys. Rev. ST Accel. Beams 18, 113401 (2015)
**J. Feng et al, J. of Appl. Phys. 121, 044904 (2017)

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPMF080

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPMF081

Intrinsic Emittance of Single Crystal Cathodes

4263

S.S. Karkare, H.A. Padmore
LBNL, Berkeley, California, USA
G. Adhikari, W.A. Schroeder
UIC, Chicago, Illinois, USA

The transverse momentum of electrons is conserved during photoemission from atomically ordered surfaces of single crystal materials. Photocathodes used in all photoinjectors today have disordered surfaces and do not exploit this phenomenon. Recently, using this conservation of transverse momentum, significant reduction in intrinsic emittance was demonstrated from the (111) surface of silver*. Here, we present measurements of transverse momentum distributions of electrons photoemitted from the ordered surfaces of Ag and Cu single crystals at several photon energies. These measurements will help in understanding the photoemission process and show how band-structure and the conservation of transverse momentum can be used to obtain further reduction in intrinsic emittance from photocathodes.
*Karkare et al., Phys. Rev. Lett. 118, 164802 (2017)

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPMF081

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPML053

Computational Screening for Low Emittance Photocathodes

4755

J.T. Paul, R.G. Hennig
University of Florida, Gainesville, Florida, USA
I.V. Bazarov, A. Galdi
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
S.S. Karkare, H.A. Padmore
LBNL, Berkeley, California, USA

The majority of photocathode materials in use in accelerator applications have been discovered empirically through trial and error with little guidance from material science calculations. Alternatively, one can envision a process which is heavily guided by computational search using latest advances in density functional theory (DFT). In this work, the MaterialsProject database is searched for potential single crystal photocathodes that would be suitable for ultralow emittance beam production. The materials in the database are initially screened on the basis of experimental practicality. Following this, the expected emittance is calculated from the DFT computed band structures for the pre-screened materials using the conservation of energy and transverse momentum during photoemission. Based on such computational screening, we provide a list of potential low emittance photocathode materials which can be investigated experimentally as high brightness electron sources.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPML053

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)