2011 Particle Accelerator Conference - Classification: Light Sources and FELs / Tech 02: Lepton Sources

Paper	Title	Page
THP107	Source of Microbunching at BNL NSLS Source Development Laboratory	2324
	S. Seletskiy, Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, Y. Shen, X.J. Wang, X. Yang BNL, Upton, Long Island, New York, USA
	We report experimental studies of the origins of electron beam microbunching instability at BNL Source Development Laboratory (SDL). We eliminated laser-induced microbunching by utilizing an ultra-short photocathode laser. The measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level. Our results demonstrated that the only source of microbunching instability at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain.

THP196	High Power Beam Test of a 1.6-cell Photocathode RF Gun at PAL	2486
	M.S. Chae, J.H. Hong, I.S. Ko, Y.W. Parc POSTECH, Pohang, Kyungbuk, Republic of Korea C. Kim, S.J. Park PAL, Pohang, Kyungbuk, Republic of Korea
	Funding: This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (grant No. 2008-0059842) The photocathode RF gun with four holes at the side of the full cell will be tested soon at the gun test stand which consists of a 1.6 cell cavity, a solenoid magnet, beam diagnostic components and auxiliary systems such as ICT, spectrometer, YAG scintillator and screens, Faraday cup, etc. Basic diagnostics such as the measurements of charge, energy and its spread, transverse emittance will be performed. It is expected that these diagnostics will confirm a successful fabrication of the RF gun. In this presentation, we will show the status of the RF gun aging in PAL and detail plan of measurements on various beam parameters. The results with the simulation code PARMELA will be presented to prepare measurement devices properly.

THP198	Upgrade of the RF Photo-Injector for the Duke Storage Ring	2489
	V. Popov, J.Y. Li, S.F. Mikhailov, P.W. Wallace, P. Wang, Y.K. Wu FEL/Duke University, Durham, North Carolina, USA
	Funding: This work is supported in part by the US DOE grant no. DE-FG02-97ER41033. The accelerator facility for the Duke FEL and High Intensity Gamma-ray Source (HIGS) consists of a linac pre-injector, a top-off booster injector, and the storage ring. The S-band RF gun with the LaB6 cathode was initially operated in the thermionic mode, producing a long electron beam pulse and a large radiation background. In 1997, the thermionic RF gun was converted to a photo-cathode operation using a nitrogen drive laser for single bunch injection into the storage ring. The photo-cathode operation typically delivers 0.1 nC of charge in a 1 ns long pulse to the linac. Since 2006, substantial improvements have been made to the photo-cathode and the linac, including improvements of the nitrogen drive laser, development of driver laser optical transport and beam monitoring system, and optimization of the cathode heater current to minimize the thermionic emission. In addition, two electron beam charge measurement systems using Faraday cup detectors and sample and hold electronics have been developed. In this work, we will present these new developments and discuss the impact of these upgrades on everyday operation of the linac pre-injector.

THP199	Raising Photoemission Efficiency with Surface Acoustic Waves	2492
	A. Afanasev Hampton University, Hampton, Virginia, USA R.P. Johnson Muons, Inc, Batavia, USA
	Funding: Supported in part by Muons, Inc. Current and future synchrotron radiation light sources and free electron laser facilities are in need of improvements in Electron Gun Technology, especially regarding the cost and efficiency of photoinjectors. The generation of Surface Acoustic Waves (SAW) on piezoelectric substrates is known to produce strong piezoelectric fields that propagate on the surface of the material. These fields significantly reduce the recombination probability of electrons and holes which can result in enhanced quantum efficiency of photoemission. Additional advantages are provided by the mobility of charge carriers that can be controlled by SAW. It is expected that this novel feature will result in enhanced efficiency of photocathode operation, leading to the production of intense, low emittance electron bunches at a high repetition rate using laser excitation.

THP200	Photoinjector Beam Dynamics for a Next Generation X-Ray FEL	2495
	C. F. Papadopoulos, J.N. Corlett, D. Filippetto, G. Penn, J. Qiang, F. Sannibale, J.W. Staples, M. Venturini, R.P. Wells, M.S. Zolotorev LBNL, Berkeley, California, USA
	Funding: This work was supported by the Director of the Office of Science of the US Department of Energy under Contract no. DEAC02-05CH11231. In this paper, we will present the status of the beam dynamics simulations for a Next Generation Light Source (NGLS) injector, based on a high repetition rate (1 MHz), high brightness design. A multi-stage beam compression scheme is proposed, based on the concepts of velocity bunching and emittance compensation. For the optimization of the design parameters we use a genetic algorithm approach, and we focus on a mode providing charges of 300 pC, with normalized transverse emittance less than 0.6 microns, suitable to operate a next generation light source based on an X-ray FEL. In addition, we discuss the effects of bunch compression and linearity of the transverse and longitudinal phase space of the beam.

THP202	First Operation of the LANL/AES Normal Conducting Radio Frequency Photoinjector	2498
	N.A. Moody, H.L. Andrews, G.O. Bolme, L.J. Castellano, C.E. Heath, F.L. Krawczyk, S. Kwon, D. C. Lizon, P.S. Marroquin, F.A. Martinez, D.C. Nguyen, M.S. Prokop, R.M. Renneke, W. Roybal, P.A. Torrez, W.M. Tuzel, T. Zaugg LANL, Los Alamos, New Mexico, USA L. Roybal TechSource, Santa Fe, New Mexico, USA
	Funding: We gratefully acknowledge funding from the Office of Naval Research (ONR) and the High Energy Laser Joint Technology Office (HEL-JTO). The LANL/AES normal-conducting radio-frequency (NCRF) injector has undergone high power testing, confirming field gradients of up to 10 MV/m at the cathode. Most NCRF designs are limited to low-duty-factor operation to constrain rf power consumption and limit ohmic heat generation. This cavity structure utilizes high density micro-channel cooling to successfully remove heat with the option of dynamic temperature control to actively adjust cavity resonance. This first high power rf test demonstrated stable cw (100% duty cycle) operation using resonant frequency tracking and produced intentional dark current emission from a roughened cathode blank. Resulting end-point x-ray measurements confirm the cathode gradient of 9.8 ± 0.2 MV/m required for acceleration of nC bunches to a beam energy of 2.5 MeV.

THP203	Improved Inverted DC Gun Insulator Assembly	2501
	M.L. Neubauer, A. Dudas, R. Sah Muons, Inc, Batavia, USA
	Funding: Work supported in part by USDOE Contract No. DE-AC05-84-ER-40150. High gradient DC guns are currently being developed with inverted ceramic insulators in order to avoid failure of the insulators from field emission and charge build-up. Our goal is to increase the DC voltages from 250 kV to 500 kV in these inverted ceramic DC Gun insulator assemblies. To achieve reliability, the arc-path gradient along the length of the insulator ceramic at the interface with the dielectric material should be lower than 500 kV/m (13 V/mil). In order to achieve this low arc-path gradient, a novel extended inverted insulator ceramic is being developed. Novel assembly processes are being developed for the high voltage connector, so that the interface between the connector dielectric and the surface of the extended inverted ceramic insulator will be void free. A complete DC Gun Inverted Ceramic Insulator Assembly will be designed and fabricated for reliable 500 kV DC operation.

THP204	Corrections to Quantum Efficiency Predictions for Low Work Function Electron Sources	2504
	K. L. Jensen NRL, Washington, DC, USA D.W. Feldman, E.J. Montgomery, P.G. O'Shea UMD, College Park, Maryland, USA J.J. Petillo SAIC, Billerica, Massachusetts, USA
	Funding: Funding by the Joint Technology Office and the Office of Naval Research. The Three-Step Model of Spicer, or the analogous Moments-based models, can be used to predict photoemission from metals and cesiated metals. In either, it is a convenient approximation to neglect electrons that have undergone scattering. Using Monte Carlo to follow scattered electrons, we assess the utility of the approximation particularly for low work function (cesiated) surfaces.

THP205	Modeling the Performance of a Diamond Current Amplifier for FELs	2507
	K. L. Jensen, B. Pate, J.L. Shaw, J.E. Yater NRL, Washington, DC, USA J.J. Petillo SAIC, Billerica, Massachusetts, USA
	Funding: We gratefully acknowledge funding by the Joint Technology Office and the Office of Naval Research. A diamond current amplifier concept can reduce demands made of photocathodes under development for high performance Free Electron Lasers (FELs) by augmenting the charge per bunch (i.e., increasing the apparent QE of the photocathode) by employing secondary emission amplification in a diamond flake. The characteristics of the bunch that emerges from the diamond flake is dependent on properties of the diamond (e.g., impurity concentrations) and the conditions under which it is operated (e.g., voltage drop, space charge, temperature). A study of the electron bunches produced by an incident 3-5 keV beam striking a very thin diamond and its transport under bias subject to scattering and space charge forces is considered. The quantities of greatest interest are then the yield, the transit time, emittance, and the rise/fall characteristics of the emerging bunch. These are simulated using Monte Carlo techniques, the application of which shall be described as it applies to the initial generation of the secondary electrons followed by their scattering and transport in the presence of band bending and space charge. J.E. Yater, et al., IEEE IVNC (2009); J. L. Shaw, et al., ibid. **K.L. Jensen, et al. J. Appl. Phys. 108, 044509 (2010).

THP208	Development of Alkali-Based High Quantum Efficiency Semiconductors for Dispenser Photocathodes	2510
	E.J. Montgomery, D.W. Feldman, S.A. Khan, P.G. O'Shea, P.Z. Pan, B.C. Riddick UMD, College Park, Maryland, USA K. L. Jensen NRL, Washington, DC, USA
	Funding: This work is supported by the Office of Naval Research. Photocathodes as electron beam sources can meet the stringent requirements of high performance FELs, but exhibit a lifetime-efficiency tradeoff. High quantum efficiency (QE) cathodes are typically semiconductors, well described by recently enhanced theory. Cesium dispenser technology, proven to extend lifetime of tungsten cathodes, can be extended to high QE via the development of semiconductor coatings which are suitable for rejuvenation. Rejuvenation occurs via controlled cesium diffusion through a sintered substrate to resupply the surface (as described by models of pore and surface diffusion). Compatible coatings must be thermally stable materials with a cesium-based surface layer. Following standard fabrication processes**, we discuss alkali antimonides and alkali aurides as cesium dispenser photocathode coatings and analyze future prospects. We also describe improvements to experimental techniques. K.L. Jensen et al., (this conference) Moody et al., J. Appl. Phys. 102(10), 2010 B.C. Riddick et al., (this conference) *P.Z. Pan et al., (this conference) ***S.A. Khan et al., (this conference)

Paper

Title

Page

Source of Microbunching at BNL NSLS Source Development Laboratory

2324

S. Seletskiy, Y. Hidaka, J.B. Murphy, B. Podobedov, H.J. Qian, Y. Shen, X.J. Wang, X. Yang
BNL, Upton, Long Island, New York, USA

We report experimental studies of the origins of electron beam microbunching instability at BNL Source Development Laboratory (SDL). We eliminated laser-induced microbunching by utilizing an ultra-short photocathode laser. The measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level. Our results demonstrated that the only source of microbunching instability at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain.

THP196

High Power Beam Test of a 1.6-cell Photocathode RF Gun at PAL

2486

M.S. Chae, J.H. Hong, I.S. Ko, Y.W. Parc
POSTECH, Pohang, Kyungbuk, Republic of Korea
C. Kim, S.J. Park
PAL, Pohang, Kyungbuk, Republic of Korea

Funding: This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (grant No. 2008-0059842)
The photocathode RF gun with four holes at the side of the full cell will be tested soon at the gun test stand which consists of a 1.6 cell cavity, a solenoid magnet, beam diagnostic components and auxiliary systems such as ICT, spectrometer, YAG scintillator and screens, Faraday cup, etc. Basic diagnostics such as the measurements of charge, energy and its spread, transverse emittance will be performed. It is expected that these diagnostics will confirm a successful fabrication of the RF gun. In this presentation, we will show the status of the RF gun aging in PAL and detail plan of measurements on various beam parameters. The results with the simulation code PARMELA will be presented to prepare measurement devices properly.

THP198

Upgrade of the RF Photo-Injector for the Duke Storage Ring

2489

V. Popov, J.Y. Li, S.F. Mikhailov, P.W. Wallace, P. Wang, Y.K. Wu
FEL/Duke University, Durham, North Carolina, USA

Funding: This work is supported in part by the US DOE grant no. DE-FG02-97ER41033.
The accelerator facility for the Duke FEL and High Intensity Gamma-ray Source (HIGS) consists of a linac pre-injector, a top-off booster injector, and the storage ring. The S-band RF gun with the LaB6 cathode was initially operated in the thermionic mode, producing a long electron beam pulse and a large radiation background. In 1997, the thermionic RF gun was converted to a photo-cathode operation using a nitrogen drive laser for single bunch injection into the storage ring. The photo-cathode operation typically delivers 0.1 nC of charge in a 1 ns long pulse to the linac. Since 2006, substantial improvements have been made to the photo-cathode and the linac, including improvements of the nitrogen drive laser, development of driver laser optical transport and beam monitoring system, and optimization of the cathode heater current to minimize the thermionic emission. In addition, two electron beam charge measurement systems using Faraday cup detectors and sample and hold electronics have been developed. In this work, we will present these new developments and discuss the impact of these upgrades on everyday operation of the linac pre-injector.

THP199

Raising Photoemission Efficiency with Surface Acoustic Waves

2492

A. Afanasev
Hampton University, Hampton, Virginia, USA
R.P. Johnson
Muons, Inc, Batavia, USA

Funding: Supported in part by Muons, Inc.
Current and future synchrotron radiation light sources and free electron laser facilities are in need of improvements in Electron Gun Technology, especially regarding the cost and efficiency of photoinjectors. The generation of Surface Acoustic Waves (SAW) on piezoelectric substrates is known to produce strong piezoelectric fields that propagate on the surface of the material. These fields significantly reduce the recombination probability of electrons and holes which can result in enhanced quantum efficiency of photoemission. Additional advantages are provided by the mobility of charge carriers that can be controlled by SAW. It is expected that this novel feature will result in enhanced efficiency of photocathode operation, leading to the production of intense, low emittance electron bunches at a high repetition rate using laser excitation.

THP200

Photoinjector Beam Dynamics for a Next Generation X-Ray FEL

2495

C. F. Papadopoulos, J.N. Corlett, D. Filippetto, G. Penn, J. Qiang, F. Sannibale, J.W. Staples, M. Venturini, R.P. Wells, M.S. Zolotorev
LBNL, Berkeley, California, USA

Funding: This work was supported by the Director of the Office of Science of the US Department of Energy under Contract no. DEAC02-05CH11231.
In this paper, we will present the status of the beam dynamics simulations for a Next Generation Light Source (NGLS) injector, based on a high repetition rate (1 MHz), high brightness design. A multi-stage beam compression scheme is proposed, based on the concepts of velocity bunching and emittance compensation. For the optimization of the design parameters we use a genetic algorithm approach, and we focus on a mode providing charges of 300 pC, with normalized transverse emittance less than 0.6 microns, suitable to operate a next generation light source based on an X-ray FEL. In addition, we discuss the effects of bunch compression and linearity of the transverse and longitudinal phase space of the beam.

THP202

First Operation of the LANL/AES Normal Conducting Radio Frequency Photoinjector

2498

N.A. Moody, H.L. Andrews, G.O. Bolme, L.J. Castellano, C.E. Heath, F.L. Krawczyk, S. Kwon, D. C. Lizon, P.S. Marroquin, F.A. Martinez, D.C. Nguyen, M.S. Prokop, R.M. Renneke, W. Roybal, P.A. Torrez, W.M. Tuzel, T. Zaugg
LANL, Los Alamos, New Mexico, USA
L. Roybal
TechSource, Santa Fe, New Mexico, USA

Funding: We gratefully acknowledge funding from the Office of Naval Research (ONR) and the High Energy Laser Joint Technology Office (HEL-JTO).
The LANL/AES normal-conducting radio-frequency (NCRF) injector has undergone high power testing, confirming field gradients of up to 10 MV/m at the cathode. Most NCRF designs are limited to low-duty-factor operation to constrain rf power consumption and limit ohmic heat generation. This cavity structure utilizes high density micro-channel cooling to successfully remove heat with the option of dynamic temperature control to actively adjust cavity resonance. This first high power rf test demonstrated stable cw (100% duty cycle) operation using resonant frequency tracking and produced intentional dark current emission from a roughened cathode blank. Resulting end-point x-ray measurements confirm the cathode gradient of 9.8 ± 0.2 MV/m required for acceleration of nC bunches to a beam energy of 2.5 MeV.

THP203

Improved Inverted DC Gun Insulator Assembly

2501

M.L. Neubauer, A. Dudas, R. Sah
Muons, Inc, Batavia, USA

Funding: Work supported in part by USDOE Contract No. DE-AC05-84-ER-40150.
High gradient DC guns are currently being developed with inverted ceramic insulators in order to avoid failure of the insulators from field emission and charge build-up. Our goal is to increase the DC voltages from 250 kV to 500 kV in these inverted ceramic DC Gun insulator assemblies. To achieve reliability, the arc-path gradient along the length of the insulator ceramic at the interface with the dielectric material should be lower than 500 kV/m (13 V/mil). In order to achieve this low arc-path gradient, a novel extended inverted insulator ceramic is being developed. Novel assembly processes are being developed for the high voltage connector, so that the interface between the connector dielectric and the surface of the extended inverted ceramic insulator will be void free. A complete DC Gun Inverted Ceramic Insulator Assembly will be designed and fabricated for reliable 500 kV DC operation.

THP204

Corrections to Quantum Efficiency Predictions for Low Work Function Electron Sources

2504

K. L. Jensen
NRL, Washington, DC, USA
D.W. Feldman, E.J. Montgomery, P.G. O'Shea
UMD, College Park, Maryland, USA
J.J. Petillo
SAIC, Billerica, Massachusetts, USA

Funding: Funding by the Joint Technology Office and the Office of Naval Research.
The Three-Step Model of Spicer, or the analogous Moments-based models, can be used to predict photoemission from metals and cesiated metals. In either, it is a convenient approximation to neglect electrons that have undergone scattering. Using Monte Carlo to follow scattered electrons, we assess the utility of the approximation particularly for low work function (cesiated) surfaces.

THP205

Modeling the Performance of a Diamond Current Amplifier for FELs

2507

K. L. Jensen, B. Pate, J.L. Shaw, J.E. Yater
NRL, Washington, DC, USA
J.J. Petillo
SAIC, Billerica, Massachusetts, USA

Funding: We gratefully acknowledge funding by the Joint Technology Office and the Office of Naval Research.
A diamond current amplifier concept can reduce demands made of photocathodes under development for high performance Free Electron Lasers (FELs) by augmenting the charge per bunch (i.e., increasing the apparent QE of the photocathode) by employing secondary emission amplification in a diamond flake*. The characteristics of the bunch that emerges from the diamond flake is dependent on properties of the diamond (e.g., impurity concentrations) and the conditions under which it is operated (e.g., voltage drop, space charge, temperature)**. A study of the electron bunches produced by an incident 3-5 keV beam striking a very thin diamond and its transport under bias subject to scattering and space charge forces is considered. The quantities of greatest interest are then the yield, the transit time, emittance, and the rise/fall characteristics of the emerging bunch. These are simulated using Monte Carlo techniques, the application of which shall be described as it applies to the initial generation of the secondary electrons followed by their scattering and transport in the presence of band bending and space charge.
*J.E. Yater, et al., IEEE IVNC (2009); J. L. Shaw, et al., ibid.
**K.L. Jensen, et al. J. Appl. Phys. 108, 044509 (2010).

THP208

Development of Alkali-Based High Quantum Efficiency Semiconductors for Dispenser Photocathodes

2510

E.J. Montgomery, D.W. Feldman, S.A. Khan, P.G. O'Shea, P.Z. Pan, B.C. Riddick
UMD, College Park, Maryland, USA
K. L. Jensen
NRL, Washington, DC, USA

Funding: This work is supported by the Office of Naval Research.
Photocathodes as electron beam sources can meet the stringent requirements of high performance FELs, but exhibit a lifetime-efficiency tradeoff. High quantum efficiency (QE) cathodes are typically semiconductors, well described by recently enhanced theory*. Cesium dispenser technology, proven to extend lifetime of tungsten cathodes**, can be extended to high QE via the development of semiconductor coatings which are suitable for rejuvenation. Rejuvenation occurs via controlled cesium diffusion through a sintered substrate to resupply the surface (as described by models of pore*** and surface**** diffusion). Compatible coatings must be thermally stable materials with a cesium-based surface layer. Following standard fabrication processes*****, we discuss alkali antimonides and alkali aurides as cesium dispenser photocathode coatings and analyze future prospects. We also describe improvements to experimental techniques.
*K.L. Jensen et al., (this conference)
**Moody et al., J. Appl. Phys. 102(10), 2010
***B.C. Riddick et al., (this conference)
****P.Z. Pan et al., (this conference)
*****S.A. Khan et al., (this conference)