FEL2013 - List of Keywords (cathode)

Paper	Title	Other Keywords	Page
MOPSO44	Laser Cooling to Counteract Back-Bombardment Heating in Microwave Thermionic Electron Guns	laser, electron, gun, simulation	79
	J.M.D. Kowalczyk, M.R. Hadmack, J. Madey University of Hawaii, Honolulu, HI, USA
	Funding: This work was funded by the Department of Homeland Security through grant #2011-DN-077-ARI055-03. A theoretical study of the use of laser cooling to counteract electron back-bombardment heating (BB) in thermionic electron guns is presented. Electron beams with short bunches, minimum energy spread, and maximum length pulse trains are required for many applications, including the inverse-Compton X-ray source being developed at UH. Currently, these three electron beam parameters are limited by BB which causes the cathode temperature and emission current to increase leading to beam loading. Beam loading elongates the bunches by shifting the electrons’ relative phases, introduces energy spread by reducing the energy of electrons emitted later in the macropulse, and forces the use of shorter macropulses to minimize energy spread. Irradiation of the electron gun cathode with a short laser pulse prior to beam acceleration allows the laser heat to diffuse into the cathode bulk effectively cooling the surface and counteracting the BB. Calculation of the the cooling produced by laser pulses of various duration and energy is presented.

MOPSO77	Timing Jitter Measurements of the SwissFEL Test Injector	laser, gun, electron, feedback	140
	C. Vicario, B. Beutner, M.C. Divall, C.P. Hauri, S. Hunziker, M.G. Kaiser, M. Luethi, M. Pedrozzi, T. Schietinger PSI, Villigen PSI, Switzerland C.P. Hauri EPFL, Lausanne, Switzerland
	To reach nominal bunch compression and FEL performance of SwissFEL with stable beam conditions for the users, less than 40fs relative rms jitter is required from the injector. Phase noise measurement of the gun laser oscillator shows an exceptional 30fs integrated rms jitter. We present these measurements and analyze the contribution to the timing jitter and drift from the rest of the laser chain. These studies were performed at the SwissFEL injector test facility, using the rising edge of the Schottky-scan curve and on the laser system using fast digital signal analyzer and photodiode, revealing a residual jitter of 150fs at the cathode from the pulsed laser amplifier and beam transport, measured at 10Hz. Spectrally resolved cross-correlation technique will also be reviewed here as a future solution of measuring timing jitter at 100Hz directly against the pulsed optical timing link with an expected resolution in the order of 50fs. This device will provide the signal for feedback systems compensating for long term timing drift of the laser for the gun as well as for the pulsed lasers at the experimental stations.

TUOANO04	PITZ Experience on the Experimental Optimization of the RF Photo Injector for the European XFEL	emittance, laser, electron, brightness	160
	M. Krasilnikov, H.-J. Grabosch, M. Groß, L. Hakobyan, I.I. Isaev, L. Jachmann, M. Khojoyan, W. Köhler, M. Mahgoub, D. Malyutin, A. Oppelt, M. Otevřel, B. Petrosyan, A. Shapovalov, F. Stephan, G. Vashchenko, S. Weidinger, R.W. Wenndorff DESY Zeuthen, Zeuthen, Germany G. Asova INRNE, Sofia, Bulgaria K. Flöttmann, M. Hoffmann, G. Klemz, S. Lederer, H. Schlarb, S. Schreiber DESY, Hamburg, Germany Ye. Ivanisenko PSI, Villigen PSI, Switzerland M.A. Nozdrin JINR, Dubna, Moscow Region, Russia V.V. Paramonov RAS/INR, Moscow, Russia D. Richter HZB, Berlin, Germany S. Rimjaem Chiang Mai University, Chiang Mai, Thailand I.H. Templin, I. Will MBI, Berlin, Germany
	The Photo Injector Test facility at DESY, Zeuthen site (PITZ), develops high brightness electron sources for modern free electron lasers. A continuous experimental optimization of the L-band photo injector for such FEL facilities like FLASH and the European XFEL has been performed for a wide range of electron bunch charges – from 20 pC to 2 nC – yielding very small emittance values for all charge levels. Experience and results of the experimental optimization will be presented in comparison with beam dynamics simulations. The influence of various parameters onto the photo injector performance will be discussed. Phys. Rev. ST Accel. Beams 15, 100701 (2012)
	Slides TUOANO04 [3.126 MB]

TUICNO01	Progress in SRF Guns	gun, SRF, electron, cavity	176
	S.A. Belomestnykh BNL, Upton, Long Island, New York, USA
	Funding: Work is supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. DOE In the last couple of years great progress has been made in the commissioning and operation of SRF electron beam sources. Both elliptical cavity designs and reentrant cavities have been developed. This talk will review recent progress in SRF guns.
	Slides TUICNO01 [12.748 MB]

TUOCNO03	Progress in a Photocathode DC Gun at the Compact ERL	gun, vacuum, acceleration, high-voltage	184
	N. Nishimori, R. Hajima, S.M. Matsuba, R. Nagai JAEA, Ibaraki-ken, Japan Y. Honda, T. Miyajima, M. Yamamoto KEK, Ibaraki, Japan H. Iijima, M. Kuriki HU/AdSM, Higashi-Hiroshima, Japan M. Kuwahara Nagoya University, Nagoya, Japan
	Photocathode DC gun to produce a train of electron bunch at high-average current and small emittance is a key component of advanced accelerators for high-power beams. However, DC guns operated at a voltage above 350 kV have suffered from field emitted electrons from a support rod since the development of Lasertron in 1980's. This critical issue has been resolved by a novel configuration, segmented insulator and guard rings, adopted in a DC gun at JAEA and stable application of high voltage at 550 kV has been demonstrated. The gun has been installed at the Compact ERL at KEK and ready for the beam generation. Similar type of DC guns are under development at KEK, Cornell, JLAB and IHEP. In this talk, we present progress in photocathode DC gun for high voltage and small emittance.
	Slides TUOCNO03 [4.946 MB]

TUPSO03	Dark Current Transport and Collimation Studies for SwissFEL	gun, simulation, emittance, wakefield	209
	S. Bettoni, P. Craievich, M. Pedrozzi, S. Reiche, L. Stingelin PSI, Villigen PSI, Switzerland
	In all accelerating cavities a non negligible background of electrons can be generated by field emission (dark current), transported and further accelerated. A careful estimate of the transport of the dark current is crucial in order to minimize radiation damage to the components and activation of the machine. This paper describes the generation and the transport of dark current from the SwissFEL photo injector downstream of the accelerator. The analysis is based on numerical simulations and experimental measurements performed at the SwissFEL Injector Test Facility (SITF). In the simulations the charge distribution is generated by an emission model based on the Fowler-Nordheim equation taking into account the filling time of the cavity and then tracked through the machine. This model has been used to analyze the impact of a low energy collimation system upstream of the first travelling wave accelerating structure on the dark current transport. A slit with several apertures has been installed in the SITF to benchmark the simulations and to verify the impact of the wakefields on the nominal beam.

TUPSO19	The Photocathode Laser System for the APEX High Repetition Rate Photoinjector	laser, electron, controls, feedback	255
	D. Filippetto, L.R. Doolittle, G. Huang, G. Marcus, H.J. Qian, F. Sannibale LBNL, Berkeley, California, USA
	Funding: DOE grants No. DE-AC02-05CH11231. The APEX injector has been built and commissioned at LBNL. A CW-RF Gun accelerates electron bunches to up 750 keV at MHz repetition rate. Different high efficiency photocathodes with different work functions are being tested with the help of a load lock system. The photocathode drive laser is thus conceived to provide up to 40 nJ per pulse in the UV and 200 nJ per pulse in the green at 1 MHz, with transverse and longitudinal shaping (flat top, up to 60 ps) for electron beam creation. A transfer line of about 15 meters has been designed and optimized for minimal jitters. Remote control of repetition rate, energy and position have been implemented on the system, together with offline and online diagnostic for beam monitoring. Here we present the laser system setup as well as the first measurements on longitudinal pulse shaping and jitter characterization.

TUPSO21	SwissFEL Cathode Load-lock System	vacuum, gun, laser, extraction	259
	R. Ganter, M. Bopp, N. Gaiffi, T. Le Quang, M. Pedrozzi, M. Schaer, T. Schietinger, L. Schulz, L. Stingelin, A. Trisorio PSI, Villigen PSI, Switzerland
	The SwissFEL electron source is an RF photo-injector in which the photo-cathode plug can be exchanged. Without load-lock, the cathode exchange takes about one week and cathode surface gets contaminated in the atmosphere during installation, leading to unpredictable quantum efficiency (QE) fluctuations. This motivated the construction of a load lock system to prepare and insert cathodes in the photo-injector. This load lock system consists of three parts: the preparation chamber, the transportable vacuum suitcase and the gun load lock chamber. This three parts system gives the possibility to prepare the cathode surface with methods like vacuum firing and plasma cleaning. The QE can be checked and the plug can be inserted in the gun without breaking vacuum. This will allow establishing an optimized a reproducible cathode preparation procedure. Since several cathodes can be loaded in advance, the exchange procedure reduces the machine shutdown to a few hours (shorter RF conditioning). The system is described and first experience with its use is reported.

TUPSO30	Conditioning Status of the First XFEL Gun at PITZ	gun, vacuum, solenoid, cavity	282
	I.I. Isaev, J.D. Good, M. Groß, L. Hakobyan, L. Jachmann, M. Khojoyan, W. Köhler, G. Kourkafas, M. Krasilnikov, D. Malyutin, B. Marchetti, R. Martin, A. Oppelt, M. Otevřel, B. Petrosyan, D. Richter, A. Shapovalov, F. Stephan, G. Vashchenko, R.W. Wenndorff DESY Zeuthen, Zeuthen, Germany G. Asova INRNE, Sofia, Bulgaria P. Boonpornprasert, S. Rimjaem Chiang Mai University, Chiang Mai, Thailand M.A. Nozdrin JINR, Dubna, Moscow Region, Russia G. Pathak Uni HH, Hamburg, Germany
	The paper describes the recent results of conditioning and dark current measurements for the photocathode RF gun at the photoinjector test facility at DESY, Zeuthen site (PITZ). The aim of PITZ is to develop and operate an optimized photo injector for free electron lasers and linear accelerators which require high quality beams. In order to get high gradients in the RF gun extensive conditioning is required. A data analysis of the conditioning process is based on data saved by a Data Acquisition system (DAQ). Conditioning results of the first gun cavity for the XFEL is presented. The events which occurred during the conditioning are briefly described.

TUPSO33	The Commissioning of Tess: An Experimental Facility for Measuring the Electron Energy Distribution From Photocathodes	electron, laser, brightness, vacuum	290
	L.B. Jones, R.J. Cash, B.D. Fell, K.J. Middleman, B.L. Militsyn, T.C.Q. Noakes STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom D.V. Gorshkov, H.E. Scheibler, A.S. Terekhov ISP, Novosibirsk, Russia
	ASTeC have developed a Transverse Energy Spread Spectrometer (TESS) – an experimental facility to characterise the energy distribution of electrons emitted by a photocathode. Electron injector brightness is fundamentally limited by the width of this distribution or energy spread, and brightness will be increased significantly by reducing the longitudinal and transverse energy spread at source. TESS supports photocathode performance measurements at room and LN2-temperature under illumination from a range of fixed- and variable-wavelength light sources, allowing characterisation of both metal and semiconductor photocathodes. Preliminary work with GaAs* has shown that electron energy spread is dependent on the quantum efficiency (Q.E.) of the photocathode source, and TESS includes a piezo-electric leak valve to allow controlled degradation of the photocathode Q.E. whilst monitoring the energy spread of emitted electrons. This system offers huge potential to support future photocathode R&D work into a range of photocathode materials. Using GaAs photocathodes activated to high levels of Q.E. in our photocathode preparation facility*, we present commissioning results for TESS. Proc. IPAC ’12, TUPPD067, 1557-1559 ** Proc. IPAC ’11, THPC129, 3185-3187

TUPSO36	Beam Dynamics Optimization for the High Brightness PITZ Photo Injector Using 3D Ellipsoidal Cathode Laser Pulses	laser, emittance, electron, simulation	298
	M. Khojoyan, M. Krasilnikov, F. Stephan, G. Vashchenko DESY Zeuthen, Zeuthen, Germany
	Funding: The work is funded by the German Federal Ministry of education and Research, project 05K10CHE “Development and experimental test of a laser system for producing quasi 3D ellipsoidal laser pulses”. The Photo Injector Test facility at DESY, Zeuthen Site (PITZ) is one of the leading producers of high brightness electron beams for linac based Free Electron Lasers (FELs) with a specific focus on the requirements of FLASH and the European XFEL. The main activities at PITZ are devoted to the detailed characterization and optimization of electron sources yielding to an extremely small transverse beam emittance. The cathode laser pulse shaping is considered as one of the key issues for the high brightness photo injector. Beam dynamics simulations show that the injector performance could be further improved by replacing the typical cylindrically shaped PITZ bunches by uniformly filled 3D ellipsoidal shaped electron beams. A set of numerical simulations were performed to study the beam dynamics of uniformly filled 3D ellipsoidal bunches with 1 nC charge in order to find an optimum PITZ machine setup which will yield the best transverse emittance. Simulation results comparing both options of cylindrical and 3D ellipsoidal beams are also presented and discussed.

TUPSO39	Development of a Photo Cathode Laser System for Quasi Ellipsoidal Bunches at PITZ	laser, diagnostics, electron, polarization	303
	M. Krasilnikov, M. Khojoyan, F. Stephan DESY Zeuthen, Zeuthen, Germany A. Andrianov, E. Gacheva, E. Khazanov, S. Mironov, A. Poteomkin, V. Zelenogorsky IAP/RAS, Nizhny Novgorod, Russia E. Syresin JINR, Dubna, Moscow Region, Russia
	Funding: The work is funded by the German Federal Ministry of education and Research, project 05K10CHE “Development and experimental test of a laser system for producing quasi 3D ellipsoidal laser pulses”. Cathode laser pulse shaping is one of the key issues for high brightness photo injector optimization. A flat-top temporal profile of the cylindrical pulses reduces significantly the transverse emittance of space charge dominated beams. As a next step towards further improvement in photo injector performance a 3D pulse shaping is considered. An ellipsoid with uniform photon density is the goal of studies in the frame of a Joint German-Russian Research Group, including the Institute of Applied Physics (Nizhny Novgorod), Joint Institute of Nuclear Research (Dubna) and the Photo Injector test facility at DESY, Zeuthen site (PITZ). The major purpose of the project is the development of a laser system capable of producing 3D quasi ellipsoidal bunches and supporting a bunch train structure close to the European XFEL specifications. The laser pulse shaping is realized using the spatial light modulator technique. The laser pulse shape diagnostics based on a cross-correlator is under development as well. Experimental tests of the new laser system with electron beam production are foreseen at PITZ. First results on the quasi ellipsoidal laser pulse shaping will be reported.

TUPSO49	Electric Field Dependence of Photoemission From n- and p- Type SI Crystals	FEL, lattice, free-electron-laser, laser	339
	S. Mingels, B. Bornmann, D. Lützenkirchen-Hecht, G. Müller Bergische Universität Wuppertal, Wuppertal, Germany C. Langer, C. Prommesberger, R. Schreiner Regensburg University of Applied Sciences, Regensburg, Germany
	Funding: Funding Agency: German Federal Ministry of Education and Research BMBF (contract number 05K10PXA) The performance of free electron lasers depends on the brilliance of the electron source. Nowadays photocathodes (e.g. Cs2Te) are used despite of their high emittance. To develop robust and more brilliant cathodes we have built up an UHV system which enables systematic photoemission (PE) measurements with a tunable pulsed laser (W=0.5-5.9 eV) at high electric fields (E<400 MV/m). First results on Au and Ag crystals revealed only low quantum efficiency (QE) due to fast electron relaxation. Hence, we have started QE(W,E) investigations on n- and p-Si wafers. Resonant PE was observed above as well as below the work function F, which can be allocated to optical transitions in the band structure of Si or explained by thermally excited states at the bottom of the conduction band. As expected, only low QE values were achieved even for n-Si probably due to surface oxide. Moreover, a significant rise of the QE peaks above F were obtained for n-Si already at E=8-9 MV/m, which was limited by discharges due to surface pollution. Detailed results and a discussion on the potential of semiconductors as highly brilliant photo-induced field emission cathodes will be presented at the conference. D.H. Dowell et al., Nucl. Instr. And Meth. Phys. A 622, 685-697 (2010) **B. Bornmann et al., Rev. Sci. Instrum. 83, 013302 (2012)

TUPSO50	Numerical Study on Electron Beam Properties in Triode Type Thermionic RF Gun	cavity, gun, electron, FEL	344
	M. Mishima, M. Inukai, T. Kii, K. Masuda, H. Negm, H. Ohgaki, K. Okumura, M. Omer, K. Torgasin, K. Yoshida, H. Zen Kyoto University, Institute for Advanced Energy, Kyoto, Japan
	The KU-FEL(Kyoto University- Free Electron Laser) facility uses a thermionic 4.5 cell S-band RF gun for electron beam generation because of such advantages over photocathode rf guns as lower cost, higher average current, longer cathode lifetime, and less vacuum requirement. The main disadvantage of using a thermionic RF gun is the back bombardment effect, which causes energy drop in macro pulse of FEL. A triode structure for RF gun was designed in order to minimize the inherent back-bombardment effect. The 2D-simulation has shown significant reduction of back-bombardment power, longitudinal emittance, and an increase of peak current. A coaxial RF cavity was fabricated based on the design for modification of the existing RF gun to a triode type one. The coaxial RF cavity is equipped with gasket tuning system in order to adjust the cavity resonance frequency. However the frequency adjustment by variation of gasket thickness changes the coaxial cavity geometry and might affect the predicted beam optics. Another parameter influencing beam optics is the position of thermionic cathode to be installed in the coaxial cavity, which might vary due to misalignment. K. Masuda, et al., Proceedings of FEL 2009, Liverpool, Pages 281-284 (2009). **K. Torgasin, et al., Proceedings of FEL 2012, Nara(2012).

TUPSO57	Generation of Ultrafast, High-brightness Electron Beams	gun, electron, cavity, brightness	355
	J.H. Park, H. Bluem, J. Rathke, T. Schultheiss, A.M.M. Todd AES, Princeton, New Jersey, USA
	Funding: This work was supported by the U.S. Department of Energy, under Contract No. DE-SC0009556. The production and preservation of ultrafast, high-brightness electron beams is a major R&D challenge for free electron laser (FEL) and ultrafast electron diffraction (UED) because transverse and longitudinal space charge forces drive emittance dilution and bunch lengthening in such beams. Several approaches, such as velocity bunching and magnetic compression, have been considered to solve this problem but each has drawbacks. We present a concept that uses radial bunch compression in an X-band photocathode radio frequency electron gun. By compensating for the path length differential with a curved cathode in an extremely high acceleration gradient cavity, we have demonstrated numerically the possibility of achieving more than an order of magnitude increase in beam brightness over existing electron guns. The initial thermo-structural analysis and mechanical conceptual design of this electron source are presented.

TUPSO58	Developments of a High-average-current Thermionic RF Gun for ERLs and FELs	gun, cavity, electron, FEL	359
	J.H. Park, H. Bluem, J. Rathke, T. Schultheiss, A.M.M. Todd AES, Princeton, New Jersey, USA
	Funding: Supported by ONR under Contract No. N00014-10-C-0191. The development of a high-average-current thermionic RF gun with the required beam performance for lasing would provide significant cost of ownership and reliability gains for high-average-power energy recovery linac (ERL) and free electron laser (FEL) devices. The beam for these applications requires high quality and high performance, specifically: low transverse emittance, short pulse duration and high average current. We are developing a gridded thermionic cathode embedded in a copper one-and-half cell UHF cavity to generate the electron beam. The fundamental RF and higher harmonics are combined on the grid and a gated DC voltage controls the beam emission from the cathode. Simulations indicate that short pulse ~ 10 psec, < 1 MeV electron beams with low-emittance ~ 15 mm-mrad at currents ≥ 100 mA can be generated. The elimination of sensitive photocathodes and their drive laser systems would provide significant capital cost saving, improved reliability and uptime due to increased robustness and hence operating and lifecycle cost savings as well. We will present the gun design and performance simulations and the progress achieved to date in optimizing the device.

TUPSO63	High Average Brightness Photocathode Development for FEL Applications	gun, laser, SRF, electron	376
	T. Rao, I. Ben-Zvi, J. Skaritka, E. Wang BNL, Upton, Long Island, New York, USA
	Next generation, high average flux, light sources call for electron beams with high average current as well as high peak brightness. Alkali antimonide cathodes, especially K2CsSb show great promise in delivering electron beams to meet these requirements. In the past few years, there have been a number of experiments geared towards understanding the stoichiometry, crystalline structure, surface properties and sensitivity of these cathodes. At BNL, we have used the x-ray beams from NSLS, CFN and CHESS for in-situ characterization of K-Cs-Sb cathode growth. We have also designed and built several load-lock systems for ex-situ cathode fabrication and quick cathode exchange, to be used with a number of guns. One load-lock system/cathode combination has been tested with a DC gun and the others will be tested with SRF guns operating at 112 and 704 MHz. In this paper we will present the results on improving the QE with excimer laser and the performance of the load-lock/cathode combination in the guns.

TUPSO67	Design Optimization of 100 Kv DC Gun Wehnelt Electrode for FEL Linac at LEBRA	electron, gun, simulation, extraction	387
	T. Sakai, K. Hayakawa, Y. Hayakawa, M. Inagaki, K. Nakao, K. Nogami, T. Tanaka LEBRA, Funabashi, Japan
	The 125-MeV electron linac at the Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University has been used for generation of the near infrared FEL and the Parametric X-ray Radiation. In addition, the THz beam generated in a bending magnet became available in the FEL experimental rooms in 2012 by transporting in the FEL optical beam line. The electron gun system for the LEBRA linac can extract the electron beam in three modes, the full bunch, the superimposed and the burst modes. However, the shape of the electron gun wehnelt electrode has not been optimized for the operation with the superimposed or the burst modes; the wehnelt was designed for use in the full bunch operation. The beam trace simulation suggested that the beam extracted from the cathode in the superimposed and the burst modes was slightly lost at the anode due to the strong space charge effect resulted from a high peak extraction current. Therefore, simulation of the beam trace was carried out to optimize the wehnelt shape for the maximum beam extraction efficiency for all the beam operation modes. The present paper reports the result of the simulation on the optimized electron gun design.

TUPSO69	Injector Design Studies for NGLS	gun, emittance, simulation, electron	391
	C. F. Papadopoulos, P. Emma, D. Filippetto, H.J. Qian, F. Sannibale, M. Venturini, R.P. Wells 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 The APEX project at LBNL is developing an electron injector to operate a high repetition rate x-ray FEL. The injector is based on the VHF gun, a high-brightness, high-repetition-rate photocathode electron gun presently under test at LBNL. The design of the injector is particularly critical because it has to take the relatively low energy beam from the VHF gun, accelerate it at more relativistic energies while simultaneously preserving high-brightness and performing longitudinal compression. The present status of the APEX injector design studies is presented.

TUPSO76	In Situ Characterization of ALKALI Antimonide Photocathodes	scattering, emittance, synchrotron, brightness	403
	J. Smedley, K. Attenkofer, S.G. Schubert BNL, Upton, Long Island, New York, USA I. Ben-Zvi, X. Liang, E.M. Muller, M. Ruiz-Osés Stony Brook University, Stony Brook, USA M. DeMarteau Fermilab, Batavia, USA H.A. Padmore, J.J. Wong LBNL, Berkeley, California, USA A.R. Woll Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA J. Xie ANL, Argonne, USA
	Funding: The authors wish to acknowledge the support of the US DOE, under Contract No. KC0407-ALSJNT-I0013, DE-AC02-98CH10886 and DE-SC0005713. Use of CHESS is supported by NSF award DMR-0936384. Alkali antimonide photocathodes are a prime candidate for use in high-brightness photoinjectors of free electron lasers and 4th generation light sources. These materials have complex growth kinetics - many methods exist for forming the compounds, each with different grain size, roughness, and crystalline texture. These parameters impact the performance of the cathodes, including efficiency, intrinsic emittance and lifetime. In situ analysis of the growth of these materials has allowed investigation of correlations between cathode structure and growth parameters and the resulting quantum efficiency (QE). The best cathodes have a QE at 532 nm in excess of 6% and are structurally textured K2CsSb with grain sizes in excess of 20 nm. X-ray reflection (XRR) has been used to characterize the roughness evolution of the cathode, while X-ray Diffraction (XRD) has been used to characterize the texture, grain size and stoichometry.

TUPSO82	Spectroscopy System for LCLS Photocathodes	electron, vacuum, gun, emittance	421
	P. Stefan, A. Brachmann, T. Vecchione SLAC, Menlo Park, California, USA
	Funding: Work supported by US DOE contract DE-AC02-76SF00515. Photocathode reliability is important from an operational standpoint. Unfortunately LCLS copper photocathodes have not always been reliable. Some have operated well for long periods of time while others have required continual maintenance. It is believed that the observed variations in quantum efficiency, emittance and lifetimes are inherently surface related, corresponding to changes in composition or morphology. The RF Electron-gun Cathode, Electron Surface Spectrometer, or RECESS, system has been commissioned to study this by making essential measurements that could not be obtained otherwise. These involve photocathode surface chemical characterization. The system is designed to use a combination of angle-resolved ultraviolet and x-ray photoelectron spectroscopy and is capable of either stand-alone operation or interoperability with a beam line at SSRL. Here we report on the first commissioning spectra and the direction of the project going forward.

TUPSO85	High Brightness Electron Beams from a Multi-filamentary Niobium-tin Photocathode	electron, emittance, laser, space-charge	431
	C. Vicario, A. Anghel, F. Ardana-Lamas, C.P. Hauri, F. Le Pimpec PSI, Villigen PSI, Switzerland C.P. Hauri EPFL, Lausanne, Switzerland
	High-brightness electron sources are of fundamental interest for modern FELs. Inspired by the micro-structure of field emitter arrays, we propose a new type of metallic photo-cathode consisting of thousands of Nb3Sn micro-columns. With this metallic photo-cathode quantum efficiencies up to 0.5% are achieved under stable operation, and preliminary emittance measurements are presented.

TUPSO86	Photocathode Laser Wavelength-tuning for Thermal Emittance and Quantum Efficiency Studies	laser, emittance, photon, electron	434
	C. Vicario, S. Bettoni, B. Beutner, M.C. Divall, C.P. Hauri, E. Prat, T. Schietinger, A. Trisorio PSI, Villigen PSI, Switzerland
	SwissFEL compact design is based on extremely low emittance electron beam from an RF photoinjector. Proper temporal and spatial shaping of the photocathode drive laser is employed to reduce the space charge emittance contribution. However, the ultimate limit for the beam emittance is the thermal emittance, which depends on the excess energy of the emitted photoelectrons. By varying the photocathode laser wavelength it is possible to reduce the thermal emittance. For this purpose, we developed a tunable Ti:sapphire laser and an optical parametric amplifier which allow to scan the wavelength between 250 and 305 nm. The system permits to study the thermal emittance and the quantum efficiency evolution as function of the laser wavelength for the copper photocathode in the RF gun of the SwissFEL injector test facility. The results are presented and discussed.

TUPSO88	New Concept for the SwissFEL Gun Laser	laser, gun, FEL, electron	442
	A. Trisorio, M.C. Divall, C.P. Hauri, C. Vicario PSI, Villigen PSI, Switzerland A. Courjaud Amplitude Systemes, Pessac, France
	The operation of Swiss FEL put very stringent constrains on the gun laser system. First the parameters, such as energy stability, timing jitter, double pulse operation, temporal and spatial pulse shape of the ultra-violet laser pulses used to generate the photo-electrons are challenging even for the state of the art laser technologies. Second, the laser system must be extremely stable, reliable and its maintenance cost as low as possible. In this perspective, we prospected for alternative technologies to the well known, commonly used but costly Ti:sapphire laser systems. We show that a hybrid Yb fiber and solid state Yb:CaF2 amplifier system can be a very interesting approach. This gain medium allows the production of sub-500 fs, high fidelity, high stability, high energy pulses in the ultra-violet with low timing jitter. The system profits of the mature, stable direct diode pumping technology and optimized design. It delivers the two high-energy, shaped UV pulses separated by 28 ns to produce the photo-electrons, a short IR probe (<100 fs FWHM) to temporally characterize those pulses and the two stretched IR pulses ( 50 ps FWHM) necessary for the laser heater.

TUPSO92	Dark Current Measurements at the Rossendorf SRF Gun	cavity, gun, SRF, electron	455
	R. Xiang, A. Arnold, P.N. Lu, P. Murcek, J. Teichert, H. Vennekate HZDR, Dresden, Germany R. Barday, T. Kamps HZB, Berlin, Germany V. Volkov BINP SB RAS, Novosibirsk, Russia
	Funding: the European Community-Research Infrastructure Activity (EuCARD, contract number 227579) and the German Federal Ministry of Education and Research grant 05 ES4BR1/8 In high gradient photo injectors electron field emission creates so-called dark current. The dark current produces beam loss that increases the radiation level, causes damages to the accelerator components, and produces additional background for the users. Field emitted electrons which stay inside the gun, increases RF power consumption and heat load for the superconducting cavities. It is also believed that dark current is the source of local outgassing and plasma formation which can damage sensitive photocathodes. Thus, to understand and control the dark current has become increasingly important for accelerators. In this presentation, we report on dark current measurement at the ELBE SRF Gun at HZDR. The measurements were carried out with the 3.5 cell-cavity SRF gun and Cs2Te photocathodes. We discuss the dark current behavior for different cavity gradients and various solenoid fields. Simulations have been done to understand the experimental results.

WEPSO24	Compact XFEL Light Source	electron, emittance, laser, FEL	757
	W.S. Graves, K.K. Berggren, F.X. Kaertner, D.E. Moncton MIT, Cambridge, Massachusetts, USA P. Piot Northern Illinois University, DeKalb, Illinois, USA
	Funding: This work was supported by DARPA grant N66001-11-1-4192, CFEL DESY, DOE grants DE-FG02-10ER46745, and NSF grant DMR-1042342. X-ray free electron laser studies are presented that rely on a nanostructured electron beam interacting with a “laser undulator” configured in the head-on inverse Compton scattering geometry. The structure in the electron beam is created by a nanoengineered cathode that produces a transversely modulated electron beam. Electron optics demagnify the modulation period and then an emittance exchange line translates the modulation to the longitudinal direction resulting in coherent bunching at x-ray wavelength. The predicted output radiation at 1 keV from a 7 MeV electron beam reaches 10 nJ or 6X10⁸ photons per shot and is fully coherent in all dimensions, a result of the dominant mode growth transversely and the longitudinal coherence imposed by the electron beam nanostructure. This output is several orders of magnitude higher than incoherent inverse Compton scattering and occupies a much smaller phase space volume, reaching peak brilliance of 10²⁷ and average brilliance of 10¹⁷ photons/(mm² mrad² 0.1% sec).

Paper

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Other Keywords

Page

MOPSO44

Laser Cooling to Counteract Back-Bombardment Heating in Microwave Thermionic Electron Guns

laser, electron, gun, simulation

79

J.M.D. Kowalczyk, M.R. Hadmack, J. Madey
University of Hawaii, Honolulu, HI, USA

Funding: This work was funded by the Department of Homeland Security through grant #2011-DN-077-ARI055-03.
A theoretical study of the use of laser cooling to counteract electron back-bombardment heating (BB) in thermionic electron guns is presented. Electron beams with short bunches, minimum energy spread, and maximum length pulse trains are required for many applications, including the inverse-Compton X-ray source being developed at UH. Currently, these three electron beam parameters are limited by BB which causes the cathode temperature and emission current to increase leading to beam loading. Beam loading elongates the bunches by shifting the electrons’ relative phases, introduces energy spread by reducing the energy of electrons emitted later in the macropulse, and forces the use of shorter macropulses to minimize energy spread. Irradiation of the electron gun cathode with a short laser pulse prior to beam acceleration allows the laser heat to diffuse into the cathode bulk effectively cooling the surface and counteracting the BB. Calculation of the the cooling produced by laser pulses of various duration and energy is presented.

MOPSO77

Timing Jitter Measurements of the SwissFEL Test Injector

laser, gun, electron, feedback

140

C. Vicario, B. Beutner, M.C. Divall, C.P. Hauri, S. Hunziker, M.G. Kaiser, M. Luethi, M. Pedrozzi, T. Schietinger
PSI, Villigen PSI, Switzerland
C.P. Hauri
EPFL, Lausanne, Switzerland

To reach nominal bunch compression and FEL performance of SwissFEL with stable beam conditions for the users, less than 40fs relative rms jitter is required from the injector. Phase noise measurement of the gun laser oscillator shows an exceptional 30fs integrated rms jitter. We present these measurements and analyze the contribution to the timing jitter and drift from the rest of the laser chain. These studies were performed at the SwissFEL injector test facility, using the rising edge of the Schottky-scan curve and on the laser system using fast digital signal analyzer and photodiode, revealing a residual jitter of 150fs at the cathode from the pulsed laser amplifier and beam transport, measured at 10Hz. Spectrally resolved cross-correlation technique will also be reviewed here as a future solution of measuring timing jitter at 100Hz directly against the pulsed optical timing link with an expected resolution in the order of 50fs. This device will provide the signal for feedback systems compensating for long term timing drift of the laser for the gun as well as for the pulsed lasers at the experimental stations.

TUOANO04

PITZ Experience on the Experimental Optimization of the RF Photo Injector for the European XFEL

emittance, laser, electron, brightness

160

M. Krasilnikov, H.-J. Grabosch, M. Groß, L. Hakobyan, I.I. Isaev, L. Jachmann, M. Khojoyan, W. Köhler, M. Mahgoub, D. Malyutin, A. Oppelt, M. Otevřel, B. Petrosyan, A. Shapovalov, F. Stephan, G. Vashchenko, S. Weidinger, R.W. Wenndorff
DESY Zeuthen, Zeuthen, Germany
G. Asova
INRNE, Sofia, Bulgaria
K. Flöttmann, M. Hoffmann, G. Klemz, S. Lederer, H. Schlarb, S. Schreiber
DESY, Hamburg, Germany
Ye. Ivanisenko
PSI, Villigen PSI, Switzerland
M.A. Nozdrin
JINR, Dubna, Moscow Region, Russia
V.V. Paramonov
RAS/INR, Moscow, Russia
D. Richter
HZB, Berlin, Germany
S. Rimjaem
Chiang Mai University, Chiang Mai, Thailand
I.H. Templin, I. Will
MBI, Berlin, Germany

The Photo Injector Test facility at DESY, Zeuthen site (PITZ), develops high brightness electron sources for modern free electron lasers. A continuous experimental optimization of the L-band photo injector for such FEL facilities like FLASH and the European XFEL has been performed for a wide range of electron bunch charges – from 20 pC to 2 nC – yielding very small emittance values for all charge levels. Experience and results of the experimental optimization will be presented in comparison with beam dynamics simulations. The influence of various parameters onto the photo injector performance will be discussed.
Phys. Rev. ST Accel. Beams 15, 100701 (2012)

Slides TUOANO04 [3.126 MB]

TUICNO01

Progress in SRF Guns

gun, SRF, electron, cavity

176

S.A. Belomestnykh
BNL, Upton, Long Island, New York, USA

Funding: Work is supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. DOE
In the last couple of years great progress has been made in the commissioning and operation of SRF electron beam sources. Both elliptical cavity designs and reentrant cavities have been developed. This talk will review recent progress in SRF guns.

Slides TUICNO01 [12.748 MB]

TUOCNO03

Progress in a Photocathode DC Gun at the Compact ERL

gun, vacuum, acceleration, high-voltage

184

N. Nishimori, R. Hajima, S.M. Matsuba, R. Nagai
JAEA, Ibaraki-ken, Japan
Y. Honda, T. Miyajima, M. Yamamoto
KEK, Ibaraki, Japan
H. Iijima, M. Kuriki
HU/AdSM, Higashi-Hiroshima, Japan
M. Kuwahara
Nagoya University, Nagoya, Japan

Photocathode DC gun to produce a train of electron bunch at high-average current and small emittance is a key component of advanced accelerators for high-power beams. However, DC guns operated at a voltage above 350 kV have suffered from field emitted electrons from a support rod since the development of Lasertron in 1980's. This critical issue has been resolved by a novel configuration, segmented insulator and guard rings, adopted in a DC gun at JAEA and stable application of high voltage at 550 kV has been demonstrated. The gun has been installed at the Compact ERL at KEK and ready for the beam generation. Similar type of DC guns are under development at KEK, Cornell, JLAB and IHEP. In this talk, we present progress in photocathode DC gun for high voltage and small emittance.

Slides TUOCNO03 [4.946 MB]

TUPSO03

Dark Current Transport and Collimation Studies for SwissFEL

gun, simulation, emittance, wakefield

209

S. Bettoni, P. Craievich, M. Pedrozzi, S. Reiche, L. Stingelin
PSI, Villigen PSI, Switzerland

In all accelerating cavities a non negligible background of electrons can be generated by field emission (dark current), transported and further accelerated. A careful estimate of the transport of the dark current is crucial in order to minimize radiation damage to the components and activation of the machine. This paper describes the generation and the transport of dark current from the SwissFEL photo injector downstream of the accelerator. The analysis is based on numerical simulations and experimental measurements performed at the SwissFEL Injector Test Facility (SITF). In the simulations the charge distribution is generated by an emission model based on the Fowler-Nordheim equation taking into account the filling time of the cavity and then tracked through the machine. This model has been used to analyze the impact of a low energy collimation system upstream of the first travelling wave accelerating structure on the dark current transport. A slit with several apertures has been installed in the SITF to benchmark the simulations and to verify the impact of the wakefields on the nominal beam.

TUPSO19

The Photocathode Laser System for the APEX High Repetition Rate Photoinjector

laser, electron, controls, feedback

255

D. Filippetto, L.R. Doolittle, G. Huang, G. Marcus, H.J. Qian, F. Sannibale
LBNL, Berkeley, California, USA

Funding: DOE grants No. DE-AC02-05CH11231.
The APEX injector has been built and commissioned at LBNL. A CW-RF Gun accelerates electron bunches to up 750 keV at MHz repetition rate. Different high efficiency photocathodes with different work functions are being tested with the help of a load lock system. The photocathode drive laser is thus conceived to provide up to 40 nJ per pulse in the UV and 200 nJ per pulse in the green at 1 MHz, with transverse and longitudinal shaping (flat top, up to 60 ps) for electron beam creation. A transfer line of about 15 meters has been designed and optimized for minimal jitters. Remote control of repetition rate, energy and position have been implemented on the system, together with offline and online diagnostic for beam monitoring. Here we present the laser system setup as well as the first measurements on longitudinal pulse shaping and jitter characterization.

TUPSO21

SwissFEL Cathode Load-lock System

vacuum, gun, laser, extraction

259

R. Ganter, M. Bopp, N. Gaiffi, T. Le Quang, M. Pedrozzi, M. Schaer, T. Schietinger, L. Schulz, L. Stingelin, A. Trisorio
PSI, Villigen PSI, Switzerland

The SwissFEL electron source is an RF photo-injector in which the photo-cathode plug can be exchanged. Without load-lock, the cathode exchange takes about one week and cathode surface gets contaminated in the atmosphere during installation, leading to unpredictable quantum efficiency (QE) fluctuations. This motivated the construction of a load lock system to prepare and insert cathodes in the photo-injector. This load lock system consists of three parts: the preparation chamber, the transportable vacuum suitcase and the gun load lock chamber. This three parts system gives the possibility to prepare the cathode surface with methods like vacuum firing and plasma cleaning. The QE can be checked and the plug can be inserted in the gun without breaking vacuum. This will allow establishing an optimized a reproducible cathode preparation procedure. Since several cathodes can be loaded in advance, the exchange procedure reduces the machine shutdown to a few hours (shorter RF conditioning). The system is described and first experience with its use is reported.

TUPSO30

Conditioning Status of the First XFEL Gun at PITZ

gun, vacuum, solenoid, cavity

282

I.I. Isaev, J.D. Good, M. Groß, L. Hakobyan, L. Jachmann, M. Khojoyan, W. Köhler, G. Kourkafas, M. Krasilnikov, D. Malyutin, B. Marchetti, R. Martin, A. Oppelt, M. Otevřel, B. Petrosyan, D. Richter, A. Shapovalov, F. Stephan, G. Vashchenko, R.W. Wenndorff
DESY Zeuthen, Zeuthen, Germany
G. Asova
INRNE, Sofia, Bulgaria
P. Boonpornprasert, S. Rimjaem
Chiang Mai University, Chiang Mai, Thailand
M.A. Nozdrin
JINR, Dubna, Moscow Region, Russia
G. Pathak
Uni HH, Hamburg, Germany

The paper describes the recent results of conditioning and dark current measurements for the photocathode RF gun at the photoinjector test facility at DESY, Zeuthen site (PITZ). The aim of PITZ is to develop and operate an optimized photo injector for free electron lasers and linear accelerators which require high quality beams. In order to get high gradients in the RF gun extensive conditioning is required. A data analysis of the conditioning process is based on data saved by a Data Acquisition system (DAQ). Conditioning results of the first gun cavity for the XFEL is presented. The events which occurred during the conditioning are briefly described.

TUPSO33

The Commissioning of Tess: An Experimental Facility for Measuring the Electron Energy Distribution From Photocathodes

electron, laser, brightness, vacuum

290

L.B. Jones, R.J. Cash, B.D. Fell, K.J. Middleman, B.L. Militsyn, T.C.Q. Noakes
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
D.V. Gorshkov, H.E. Scheibler, A.S. Terekhov
ISP, Novosibirsk, Russia

ASTeC have developed a Transverse Energy Spread Spectrometer (TESS) – an experimental facility to characterise the energy distribution of electrons emitted by a photocathode. Electron injector brightness is fundamentally limited by the width of this distribution or energy spread, and brightness will be increased significantly by reducing the longitudinal and transverse energy spread at source. TESS supports photocathode performance measurements at room and LN2-temperature under illumination from a range of fixed- and variable-wavelength light sources, allowing characterisation of both metal and semiconductor photocathodes. Preliminary work with GaAs* has shown that electron energy spread is dependent on the quantum efficiency (Q.E.) of the photocathode source, and TESS includes a piezo-electric leak valve to allow controlled degradation of the photocathode Q.E. whilst monitoring the energy spread of emitted electrons. This system offers huge potential to support future photocathode R&D work into a range of photocathode materials. Using GaAs photocathodes activated to high levels of Q.E. in our photocathode preparation facility**, we present commissioning results for TESS.
* Proc. IPAC ’12, TUPPD067, 1557-1559
** Proc. IPAC ’11, THPC129, 3185-3187

TUPSO36

Beam Dynamics Optimization for the High Brightness PITZ Photo Injector Using 3D Ellipsoidal Cathode Laser Pulses

laser, emittance, electron, simulation

298

M. Khojoyan, M. Krasilnikov, F. Stephan, G. Vashchenko
DESY Zeuthen, Zeuthen, Germany

Funding: The work is funded by the German Federal Ministry of education and Research, project 05K10CHE “Development and experimental test of a laser system for producing quasi 3D ellipsoidal laser pulses”.
The Photo Injector Test facility at DESY, Zeuthen Site (PITZ) is one of the leading producers of high brightness electron beams for linac based Free Electron Lasers (FELs) with a specific focus on the requirements of FLASH and the European XFEL. The main activities at PITZ are devoted to the detailed characterization and optimization of electron sources yielding to an extremely small transverse beam emittance. The cathode laser pulse shaping is considered as one of the key issues for the high brightness photo injector. Beam dynamics simulations show that the injector performance could be further improved by replacing the typical cylindrically shaped PITZ bunches by uniformly filled 3D ellipsoidal shaped electron beams. A set of numerical simulations were performed to study the beam dynamics of uniformly filled 3D ellipsoidal bunches with 1 nC charge in order to find an optimum PITZ machine setup which will yield the best transverse emittance. Simulation results comparing both options of cylindrical and 3D ellipsoidal beams are also presented and discussed.

TUPSO39

Development of a Photo Cathode Laser System for Quasi Ellipsoidal Bunches at PITZ

laser, diagnostics, electron, polarization

303

M. Krasilnikov, M. Khojoyan, F. Stephan
DESY Zeuthen, Zeuthen, Germany
A. Andrianov, E. Gacheva, E. Khazanov, S. Mironov, A. Poteomkin, V. Zelenogorsky
IAP/RAS, Nizhny Novgorod, Russia
E. Syresin
JINR, Dubna, Moscow Region, Russia

Funding: The work is funded by the German Federal Ministry of education and Research, project 05K10CHE “Development and experimental test of a laser system for producing quasi 3D ellipsoidal laser pulses”.
Cathode laser pulse shaping is one of the key issues for high brightness photo injector optimization. A flat-top temporal profile of the cylindrical pulses reduces significantly the transverse emittance of space charge dominated beams. As a next step towards further improvement in photo injector performance a 3D pulse shaping is considered. An ellipsoid with uniform photon density is the goal of studies in the frame of a Joint German-Russian Research Group, including the Institute of Applied Physics (Nizhny Novgorod), Joint Institute of Nuclear Research (Dubna) and the Photo Injector test facility at DESY, Zeuthen site (PITZ). The major purpose of the project is the development of a laser system capable of producing 3D quasi ellipsoidal bunches and supporting a bunch train structure close to the European XFEL specifications. The laser pulse shaping is realized using the spatial light modulator technique. The laser pulse shape diagnostics based on a cross-correlator is under development as well. Experimental tests of the new laser system with electron beam production are foreseen at PITZ. First results on the quasi ellipsoidal laser pulse shaping will be reported.

TUPSO49

Electric Field Dependence of Photoemission From n- and p- Type SI Crystals

FEL, lattice, free-electron-laser, laser

339

S. Mingels, B. Bornmann, D. Lützenkirchen-Hecht, G. Müller
Bergische Universität Wuppertal, Wuppertal, Germany
C. Langer, C. Prommesberger, R. Schreiner
Regensburg University of Applied Sciences, Regensburg, Germany

Funding: Funding Agency: German Federal Ministry of Education and Research BMBF (contract number 05K10PXA)
The performance of free electron lasers depends on the brilliance of the electron source*. Nowadays photocathodes (e.g. Cs2Te) are used despite of their high emittance. To develop robust and more brilliant cathodes we have built up an UHV system which enables systematic photoemission (PE) measurements with a tunable pulsed laser (W=0.5-5.9 eV) at high electric fields (E<400 MV/m)**. First results on Au and Ag crystals revealed only low quantum efficiency (QE) due to fast electron relaxation. Hence, we have started QE(W,E) investigations on n- and p-Si wafers. Resonant PE was observed above as well as below the work function F, which can be allocated to optical transitions in the band structure of Si or explained by thermally excited states at the bottom of the conduction band. As expected, only low QE values were achieved even for n-Si probably due to surface oxide. Moreover, a significant rise of the QE peaks above F were obtained for n-Si already at E=8-9 MV/m, which was limited by discharges due to surface pollution. Detailed results and a discussion on the potential of semiconductors as highly brilliant photo-induced field emission cathodes will be presented at the conference.
*D.H. Dowell et al., Nucl. Instr. And Meth. Phys. A 622, 685-697 (2010)
**B. Bornmann et al., Rev. Sci. Instrum. 83, 013302 (2012)

TUPSO50

Numerical Study on Electron Beam Properties in Triode Type Thermionic RF Gun

cavity, gun, electron, FEL

344

M. Mishima, M. Inukai, T. Kii, K. Masuda, H. Negm, H. Ohgaki, K. Okumura, M. Omer, K. Torgasin, K. Yoshida, H. Zen
Kyoto University, Institute for Advanced Energy, Kyoto, Japan

The KU-FEL(Kyoto University- Free Electron Laser) facility uses a thermionic 4.5 cell S-band RF gun for electron beam generation because of such advantages over photocathode rf guns as lower cost, higher average current, longer cathode lifetime, and less vacuum requirement. The main disadvantage of using a thermionic RF gun is the back bombardment effect, which causes energy drop in macro pulse of FEL. A triode structure for RF gun was designed in order to minimize the inherent back-bombardment effect. The 2D-simulation has shown significant reduction of back-bombardment power, longitudinal emittance, and an increase of peak current*. A coaxial RF cavity was fabricated based on the design for modification of the existing RF gun to a triode type one. The coaxial RF cavity is equipped with gasket tuning system in order to adjust the cavity resonance frequency**. However the frequency adjustment by variation of gasket thickness changes the coaxial cavity geometry and might affect the predicted beam optics. Another parameter influencing beam optics is the position of thermionic cathode to be installed in the coaxial cavity, which might vary due to misalignment.
*K. Masuda, et al., Proceedings of FEL 2009, Liverpool, Pages 281-284 (2009).
**K. Torgasin, et al., Proceedings of FEL 2012, Nara(2012).

TUPSO57

Generation of Ultrafast, High-brightness Electron Beams

gun, electron, cavity, brightness

355

J.H. Park, H. Bluem, J. Rathke, T. Schultheiss, A.M.M. Todd
AES, Princeton, New Jersey, USA

Funding: This work was supported by the U.S. Department of Energy, under Contract No. DE-SC0009556.
The production and preservation of ultrafast, high-brightness electron beams is a major R&D challenge for free electron laser (FEL) and ultrafast electron diffraction (UED) because transverse and longitudinal space charge forces drive emittance dilution and bunch lengthening in such beams. Several approaches, such as velocity bunching and magnetic compression, have been considered to solve this problem but each has drawbacks. We present a concept that uses radial bunch compression in an X-band photocathode radio frequency electron gun. By compensating for the path length differential with a curved cathode in an extremely high acceleration gradient cavity, we have demonstrated numerically the possibility of achieving more than an order of magnitude increase in beam brightness over existing electron guns. The initial thermo-structural analysis and mechanical conceptual design of this electron source are presented.

TUPSO58

Developments of a High-average-current Thermionic RF Gun for ERLs and FELs

gun, cavity, electron, FEL

359

J.H. Park, H. Bluem, J. Rathke, T. Schultheiss, A.M.M. Todd
AES, Princeton, New Jersey, USA

Funding: Supported by ONR under Contract No. N00014-10-C-0191.
The development of a high-average-current thermionic RF gun with the required beam performance for lasing would provide significant cost of ownership and reliability gains for high-average-power energy recovery linac (ERL) and free electron laser (FEL) devices. The beam for these applications requires high quality and high performance, specifically: low transverse emittance, short pulse duration and high average current. We are developing a gridded thermionic cathode embedded in a copper one-and-half cell UHF cavity to generate the electron beam. The fundamental RF and higher harmonics are combined on the grid and a gated DC voltage controls the beam emission from the cathode. Simulations indicate that short pulse ~ 10 psec, < 1 MeV electron beams with low-emittance ~ 15 mm-mrad at currents ≥ 100 mA can be generated. The elimination of sensitive photocathodes and their drive laser systems would provide significant capital cost saving, improved reliability and uptime due to increased robustness and hence operating and lifecycle cost savings as well. We will present the gun design and performance simulations and the progress achieved to date in optimizing the device.

TUPSO63

High Average Brightness Photocathode Development for FEL Applications

gun, laser, SRF, electron

376

T. Rao, I. Ben-Zvi, J. Skaritka, E. Wang
BNL, Upton, Long Island, New York, USA

Next generation, high average flux, light sources call for electron beams with high average current as well as high peak brightness. Alkali antimonide cathodes, especially K2CsSb show great promise in delivering electron beams to meet these requirements. In the past few years, there have been a number of experiments geared towards understanding the stoichiometry, crystalline structure, surface properties and sensitivity of these cathodes. At BNL, we have used the x-ray beams from NSLS, CFN and CHESS for in-situ characterization of K-Cs-Sb cathode growth. We have also designed and built several load-lock systems for ex-situ cathode fabrication and quick cathode exchange, to be used with a number of guns. One load-lock system/cathode combination has been tested with a DC gun and the others will be tested with SRF guns operating at 112 and 704 MHz. In this paper we will present the results on improving the QE with excimer laser and the performance of the load-lock/cathode combination in the guns.

TUPSO67

Design Optimization of 100 Kv DC Gun Wehnelt Electrode for FEL Linac at LEBRA

electron, gun, simulation, extraction

387

T. Sakai, K. Hayakawa, Y. Hayakawa, M. Inagaki, K. Nakao, K. Nogami, T. Tanaka
LEBRA, Funabashi, Japan

The 125-MeV electron linac at the Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University has been used for generation of the near infrared FEL and the Parametric X-ray Radiation. In addition, the THz beam generated in a bending magnet became available in the FEL experimental rooms in 2012 by transporting in the FEL optical beam line. The electron gun system for the LEBRA linac can extract the electron beam in three modes, the full bunch, the superimposed and the burst modes. However, the shape of the electron gun wehnelt electrode has not been optimized for the operation with the superimposed or the burst modes; the wehnelt was designed for use in the full bunch operation. The beam trace simulation suggested that the beam extracted from the cathode in the superimposed and the burst modes was slightly lost at the anode due to the strong space charge effect resulted from a high peak extraction current. Therefore, simulation of the beam trace was carried out to optimize the wehnelt shape for the maximum beam extraction efficiency for all the beam operation modes. The present paper reports the result of the simulation on the optimized electron gun design.

TUPSO69

Injector Design Studies for NGLS

gun, emittance, simulation, electron

391

C. F. Papadopoulos, P. Emma, D. Filippetto, H.J. Qian, F. Sannibale, M. Venturini, R.P. Wells
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
The APEX project at LBNL is developing an electron injector to operate a high repetition rate x-ray FEL. The injector is based on the VHF gun, a high-brightness, high-repetition-rate photocathode electron gun presently under test at LBNL. The design of the injector is particularly critical because it has to take the relatively low energy beam from the VHF gun, accelerate it at more relativistic energies while simultaneously preserving high-brightness and performing longitudinal compression. The present status of the APEX injector design studies is presented.

TUPSO76

In Situ Characterization of ALKALI Antimonide Photocathodes

scattering, emittance, synchrotron, brightness

403

J. Smedley, K. Attenkofer, S.G. Schubert
BNL, Upton, Long Island, New York, USA
I. Ben-Zvi, X. Liang, E.M. Muller, M. Ruiz-Osés
Stony Brook University, Stony Brook, USA
M. DeMarteau
Fermilab, Batavia, USA
H.A. Padmore, J.J. Wong
LBNL, Berkeley, California, USA
A.R. Woll
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
J. Xie
ANL, Argonne, USA

Funding: The authors wish to acknowledge the support of the US DOE, under Contract No. KC0407-ALSJNT-I0013, DE-AC02-98CH10886 and DE-SC0005713. Use of CHESS is supported by NSF award DMR-0936384.
Alkali antimonide photocathodes are a prime candidate for use in high-brightness photoinjectors of free electron lasers and 4th generation light sources. These materials have complex growth kinetics - many methods exist for forming the compounds, each with different grain size, roughness, and crystalline texture. These parameters impact the performance of the cathodes, including efficiency, intrinsic emittance and lifetime. In situ analysis of the growth of these materials has allowed investigation of correlations between cathode structure and growth parameters and the resulting quantum efficiency (QE). The best cathodes have a QE at 532 nm in excess of 6% and are structurally textured K2CsSb with grain sizes in excess of 20 nm. X-ray reflection (XRR) has been used to characterize the roughness evolution of the cathode, while X-ray Diffraction (XRD) has been used to characterize the texture, grain size and stoichometry.

TUPSO82

Spectroscopy System for LCLS Photocathodes

electron, vacuum, gun, emittance

421

P. Stefan, A. Brachmann, T. Vecchione
SLAC, Menlo Park, California, USA

Funding: Work supported by US DOE contract DE-AC02-76SF00515.
Photocathode reliability is important from an operational standpoint. Unfortunately LCLS copper photocathodes have not always been reliable. Some have operated well for long periods of time while others have required continual maintenance. It is believed that the observed variations in quantum efficiency, emittance and lifetimes are inherently surface related, corresponding to changes in composition or morphology. The RF Electron-gun Cathode, Electron Surface Spectrometer, or RECESS, system has been commissioned to study this by making essential measurements that could not be obtained otherwise. These involve photocathode surface chemical characterization. The system is designed to use a combination of angle-resolved ultraviolet and x-ray photoelectron spectroscopy and is capable of either stand-alone operation or interoperability with a beam line at SSRL. Here we report on the first commissioning spectra and the direction of the project going forward.

TUPSO85

High Brightness Electron Beams from a Multi-filamentary Niobium-tin Photocathode

electron, emittance, laser, space-charge

431

C. Vicario, A. Anghel, F. Ardana-Lamas, C.P. Hauri, F. Le Pimpec
PSI, Villigen PSI, Switzerland
C.P. Hauri
EPFL, Lausanne, Switzerland

High-brightness electron sources are of fundamental interest for modern FELs. Inspired by the micro-structure of field emitter arrays, we propose a new type of metallic photo-cathode consisting of thousands of Nb3Sn micro-columns. With this metallic photo-cathode quantum efficiencies up to 0.5% are achieved under stable operation, and preliminary emittance measurements are presented.

TUPSO86

Photocathode Laser Wavelength-tuning for Thermal Emittance and Quantum Efficiency Studies

laser, emittance, photon, electron

434

C. Vicario, S. Bettoni, B. Beutner, M.C. Divall, C.P. Hauri, E. Prat, T. Schietinger, A. Trisorio
PSI, Villigen PSI, Switzerland

SwissFEL compact design is based on extremely low emittance electron beam from an RF photoinjector. Proper temporal and spatial shaping of the photocathode drive laser is employed to reduce the space charge emittance contribution. However, the ultimate limit for the beam emittance is the thermal emittance, which depends on the excess energy of the emitted photoelectrons. By varying the photocathode laser wavelength it is possible to reduce the thermal emittance. For this purpose, we developed a tunable Ti:sapphire laser and an optical parametric amplifier which allow to scan the wavelength between 250 and 305 nm. The system permits to study the thermal emittance and the quantum efficiency evolution as function of the laser wavelength for the copper photocathode in the RF gun of the SwissFEL injector test facility. The results are presented and discussed.

TUPSO88

New Concept for the SwissFEL Gun Laser

laser, gun, FEL, electron

442

A. Trisorio, M.C. Divall, C.P. Hauri, C. Vicario
PSI, Villigen PSI, Switzerland
A. Courjaud
Amplitude Systemes, Pessac, France

The operation of Swiss FEL put very stringent constrains on the gun laser system. First the parameters, such as energy stability, timing jitter, double pulse operation, temporal and spatial pulse shape of the ultra-violet laser pulses used to generate the photo-electrons are challenging even for the state of the art laser technologies. Second, the laser system must be extremely stable, reliable and its maintenance cost as low as possible. In this perspective, we prospected for alternative technologies to the well known, commonly used but costly Ti:sapphire laser systems. We show that a hybrid Yb fiber and solid state Yb:CaF2 amplifier system can be a very interesting approach. This gain medium allows the production of sub-500 fs, high fidelity, high stability, high energy pulses in the ultra-violet with low timing jitter. The system profits of the mature, stable direct diode pumping technology and optimized design. It delivers the two high-energy, shaped UV pulses separated by 28 ns to produce the photo-electrons, a short IR probe (<100 fs FWHM) to temporally characterize those pulses and the two stretched IR pulses ( 50 ps FWHM) necessary for the laser heater.

TUPSO92

Dark Current Measurements at the Rossendorf SRF Gun

cavity, gun, SRF, electron

455

R. Xiang, A. Arnold, P.N. Lu, P. Murcek, J. Teichert, H. Vennekate
HZDR, Dresden, Germany
R. Barday, T. Kamps
HZB, Berlin, Germany
V. Volkov
BINP SB RAS, Novosibirsk, Russia

Funding: the European Community-Research Infrastructure Activity (EuCARD, contract number 227579) and the German Federal Ministry of Education and Research grant 05 ES4BR1/8
In high gradient photo injectors electron field emission creates so-called dark current. The dark current produces beam loss that increases the radiation level, causes damages to the accelerator components, and produces additional background for the users. Field emitted electrons which stay inside the gun, increases RF power consumption and heat load for the superconducting cavities. It is also believed that dark current is the source of local outgassing and plasma formation which can damage sensitive photocathodes. Thus, to understand and control the dark current has become increasingly important for accelerators. In this presentation, we report on dark current measurement at the ELBE SRF Gun at HZDR. The measurements were carried out with the 3.5 cell-cavity SRF gun and Cs2Te photocathodes. We discuss the dark current behavior for different cavity gradients and various solenoid fields. Simulations have been done to understand the experimental results.

WEPSO24

Compact XFEL Light Source

electron, emittance, laser, FEL

757

W.S. Graves, K.K. Berggren, F.X. Kaertner, D.E. Moncton
MIT, Cambridge, Massachusetts, USA
P. Piot
Northern Illinois University, DeKalb, Illinois, USA

Funding: This work was supported by DARPA grant N66001-11-1-4192, CFEL DESY, DOE grants DE-FG02-10ER46745, and NSF grant DMR-1042342.
X-ray free electron laser studies are presented that rely on a nanostructured electron beam interacting with a “laser undulator” configured in the head-on inverse Compton scattering geometry. The structure in the electron beam is created by a nanoengineered cathode that produces a transversely modulated electron beam. Electron optics demagnify the modulation period and then an emittance exchange line translates the modulation to the longitudinal direction resulting in coherent bunching at x-ray wavelength. The predicted output radiation at 1 keV from a 7 MeV electron beam reaches 10 nJ or 6X10⁸ photons per shot and is fully coherent in all dimensions, a result of the dominant mode growth transversely and the longitudinal coherence imposed by the electron beam nanostructure. This output is several orders of magnitude higher than incoherent inverse Compton scattering and occupies a much smaller phase space volume, reaching peak brilliance of 10²⁷ and average brilliance of 10¹⁷ photons/(mm² mrad² 0.1% sec).