FEL2011 - Table of Session: THPB (Accelerator Technology)

Paper	Title	Page
THPB01	Optical Comb and Interferometer Development for Laser Synchronization in Femtosecond FELs	561
	R.B. Wilcox, J.M. Byrd LBNL, Berkeley, California, USA R. Holzwarth Menlo Systems GmbH, Martinsried, Germany
	Funding: This work supported by the U.S. Department of Energy under contract DE-AC02-05CH11231 We describe a method of synchronizing lasers in FELs to potential sub-femtosecond precision using interferometry and optical clock techniques, and show supporting experimental results. This precision is needed for pump/probe experiments in ultrafast FELs. The proposed system consists of carrier/offset phase stabilized, pulsed lasers synchronized via a single optical frequency delivered over fiber, analogous to RF oscillators synchronized with a reference frequency, but at 200 to 400THz. Our tests of modelocked lasers, interferometers and stabilized CW lasers show that subsystems can perform to the required precision. We have synchronized fiber lasers to less than 10fs jitter using two different frequency comb line locking schemes, and demonstrated interferometers in a working FEL with less than 100as jitter over 150m fiber. Based on these tests and published work by others, we calculate the performance of an optimized, integrated timing system to be less than 1fs in the short term. Long term stability is maintained by feedback from X-ray/optical cross-correlation at the experiment.

THPB02	Implementation of 2D-emittance Compensation Scheme in the BERLinPro Injector	564
	A.V. Bondarenko, A.N. Matveenko HZB, Berlin, Germany
	Helmholtz-Zentrum Berlin officially started Jan. 2011 the design and construction of the Berlin Energy Recovery Linac Project BERLinPro. The initial goal of this compact ERL is to develop the ERL accelerator physics and technology required to accelerate a high-current (100 mA) low emittance beam (1 mm•mrad normalized), as required for future ERL-based synchrotron light sources. High power ERL based FELs demand low emittance, high peak and average current beams. The injection energy in an ERL is usually rather low to decrease power consumption and avoid activation of the beam dump. Therefore, the space charge is the main reason of the emittance degradation in the injector. The implementation of an emittance compensation scheme in the injector is necessary to achieve a low emittance. Since injector’s optics is axially non-symmetric, the 2D-emittance compensation scheme [1] should be used. The implementation of the 2D-emittance compensation scheme at BERLinPro injector is presented in this contribution. Other sources of emittance growth in ERL injectors are also discussed. [1] S.V. Miginsky, "Emittance compensation of elliptical beam", NIM A 603 (2009), pp 32-34.

THPB05	Modeling of the Beam Break Up Instability in Berlin Energy Recovery Linac Project	568
	Y. Petenev, A.V. Bondarenko, A.N. Matveenko HZB, Berlin, Germany
	Helmholtz-Zentrum Berlin officially started Jan. 2011 the design and construction of the Berlin Energy Recovery Linac Project BERLinPro. The initial goal of this compact ERL is to develop the ERL accelerator physics and technology required to accelerate a high-current low emittance beam. The conversion efficiency of an FEL is about 1% therefore superconducting ERL-based FEL machines look promising. One of the problems of superconducting ERL machines is the Beam Break Up (BBU) instability which limits the current. In this work the threshold current of the BBU instability was calculated for the BERLinPro. The comparison of two 100 MeV linacs based on different type of superconducting cavities is made. Different methods of BBU suppression are investigated (e.g. the influence of solenoid, pseudo-reflector and quadruple triplets in the linac structure on the BBU threshold).

THPB06	Coherent Terahertz Radiation Monitors for Multiple Spectral Bands	572
	R. Ischebeck, G.L. Orlandi, P. Peier, V. Schlott, B. Smit, C. Vicario, C. Zimmerli Paul Scherrer Institut, Villigen, Switzerland C. Gerth DESY, Hamburg, Germany
	The SwissFEL Injector Test Facility is destined for demonstrating electron beam parameters that are suitable for FEL operation. Of particular interest is the on-line measurement of longitudinal phase space properties, as this provides insight into the bunch compression process. The spectral distribution of diffraction radiation offers a robust way to assess bunch length and longitudinal profile. The bunch length at the SwissFEL Injector Test Facility can be varied by changing the photocathode laser. Diffraction radiation is emitted as the electron bunches pass through a hole in a titanium foil. The emitted Terahertz radiation has been simulated by the code THz Transport, and the propagation to the detectors has been modeled.

THPB08	Study of Reflective Optics for LFC-Camera	576
	K. Nanbu Tohoku University, School of Science, Sendai, Japan H. Hama, F. Hinode, S. Kashiwagi, M. Kawai, X. Li, T. Muto, Y. Tanaka Tohoku University, Research Center for Electron Photon Science, Sendai, Japan
	Funding: This work is partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (S), Contract #20226003. A test accelerator for the terahertz source project (t-ACTS) employing isochronous ring and bunched free electron laser has been under development at Tohoku University [1,2]. Stable production of very short electron bunches is a key issue for the t-ACTS project. We have chosen thermionic RF gun for the injector of t-ACTS because of stability, multi-bunch operation and cheaper cost. The longitudinal phase space distribution of the beam extracted from the rf-gun is crucial for the final bunch length of electron beam passing through bunch compression process. Therefore, measurement of the longitudinal phase space of the beam is indispensable for efficient bunch compression. In order to measure the electron distribution in the longitudinal phase space of relatively lower energy beam, we have been developing a novel method to observe the energy spectrum employing a velocity dependence of opening angle of Cherenkov radiation, namely Linear Focal Cherenkov (LFC) ring camera. We describe principle of LFC camera and discuss relations between surface roughness of Cherenkov radiator and energy resolution in this conference. [1] H. Hama et al., New J. Phys. 8 (2006) 292, [2] H. Hama and M. Yasuda, Proc. of FEL2009, (2009) 394

THPB09	Study of the Microbunching Instablity in the LINAC of the Future Shanghai Soft X-ray FEL Facility (SXFEL)	579
	D. Huang IIT, Chicago, Illinois, USA Q. Gu SINAP, Shanghai, People's Republic of China
	The microbunching instability in the LINAC of a FEL facility has always been an issue which may degrade the quality of the electron beam. As the result, the whole facility may not be working properly. Therefore, learning how to control and reduce the instability is the key to the success of a FEL project. Shanghai soft X-ray FEL project (SXFEL) has just been granted, once it is built, it will be the first X-ray FEL facility in China. In this article, detailed study will be given based on the design parameters of the facility to gain better understanding and control over the possible microbunching instability in SXFEL, which is important to the success of the project.

THPB14	APEX Project Phase 0 and I Status and Plans and Activities for Phase II	582
	F. Sannibale, B.J. Bailey, K.M. Baptiste, J.M. Byrd, A.L. Catalano, D. Colomb, C.W. Cork, J.N. Corlett, S. De Santis, L.R. Doolittle, J. Feng, D. Filippetto, G. Huang, S. Kwiatkowski, W.E. Norum, H.A. Padmore, C. F. Papadopoulos, G. Penn, G.J. Portmann, S. Prestemon, J. Qiang, D.G. Quintas, J.W. Staples, M.E. Stuart, T. Vecchione, M. Venturini, M. Vinco, W. Wan, R.P. Wells, M.S. Zolotorev, F.A. Zucca LBNL, Berkeley, California, USA M. J. Messerly, M.A. Prantil LLNL, Livermore, California, USA C. Pellegrini UCLA, Los Angeles, California, USA M. Yoon POSTECH, Pohang, Kyungbuk, Republic of Korea
	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 the Lawrence Berkeley National Laboratory is devoted to the development of a high repetition rate (MHz-class) electron injector for X-ray FEL applications. The injector is based on a new concept photo-gun, utilizing a normal conducting 187 MHz RF cavity operating in CW mode in conjunction with high quantum efficiency photocathodes able to deliver the required repetition rates with available laser technology. The APEX activities are staged in two phases. In Phase I, the electron photo-gun is constructed, tested and several different photo-cathodes, such as alkali antimonides, Cs₂Te [1], diamond amplifiers [2], and metals, are tested at full repetition rate. In Phase II, a pulsed linac is added for accelerating the beam at several tens of MeV to prove the high brightness performance of the gun when integrated in an injector scheme. Based on funding availability, after Phase II, the program could also include testing of new undulator technologies and FEL studies. The status of Phase I, in its initial experimental phase, is described together with plans and activities for Phase II and beyond. [1] In collaboration with INFN-LASA, Milano, Italy. [2] In collaboration with Brookhaven National Laboratory, Upton NY, USA

THPB15	Metal Cathodes with Reduced Emittance and Enhanced Quantum Efficiency	586
	C.M.R. Greaves, J. Feng, H.A. Padmore, W. Wan LBNL, Berkeley, California, USA D. Dowell AES, Princeton, New Jersey, USA
	In this paper, we report experimental results on photoemission from copper and silver surfaces. Using the technique of angle resolved photoemission spectroscopy (ARPES), we demonstrate that, for excess energy around 0.5 eV, the photoelectrons from the Cu(111) and Ag(111) surfaces generated by p-polarized light originate primarily from the well-known surface state with normalized emittance only a fraction of that of the polycrystalline copper cathode presently used in the RF guns. Meanwhile, we demonstrate that the enhancement of the quantum efficiency (QE) at grazing angle is closely related to the surface state as well. Furthermore, we show that the surface state can be easily restored by a simple anneal process, thus pointing to a practical way to reducing the emittance and QE of a metal cathode simultaniously.

THPB16	Beam Profile Measurements Using a Fast Gated CCD Camera and a Scintillation Screen to Suppress COTR	590
	M. Yan Uni HH, Hamburg, Germany C. Behrens, C. Gerth, G. Kube, B. Schmidt, S. Wesch DESY, Hamburg, Germany
	For standard beam profile measurements of high-brightness electron beams using optical transition radiation (OTR) screens, coherence effects induced by microbunching instabilities render direct imaging of the beam impossible. A technique of using a scintillation screen with a fast gated CCD camera has been demonstrated to successfully suppress coherent OTR (COTR) in transverse beam diagnostics at FLASH. The fast gated CCD camera has been installed next to a standard CCD camera setup and images the same viewing screens. The results of transverse beam profile measurements under operating conditions without COTR are compared for both setups. The fast gated camera has also been employed for longitudinal bunch profile measurements with a transverse deflecting structure (TDS). Results obtained under operating conditions with COTR are compared to those from longitudinal phase space measurements in a dispersive arm, where no coherence effects have been observed so far. In this paper, we examine the performance of the fast gated CCD camera for beam profile measurements and present further studies on the use of scintillation screens for high-energy electron beam diagnostics.

THPB19	Investigations of OTR Polarization Effects in Beam-profile Monitors	594
	A.H. Lumpkin, A.S. Johnson, J. Ruan, R.M. Thurman-Keup Fermilab, Batavia, USA
	Funding: Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. The characterization of transverse beam size using optical transition radiation (OTR) imaging is a well-established technique at many accelerators including the Fermilab A0 photoinjector (A0PI) facility. However, there is growing empirical evidence that the utilization of the polarization component orthogonal to the dimension of interest results in a smaller observed projected image profile. We have continued investigations of this phenomenon with a more controlled experiment where the linear polarizers are selectable in a filter wheel which also included a blank glass position to compensate for the optical path. The aperture for light collection is thus kept fixed compared to our previous tests. We also have balanced the digital camera gain to present similar signal levels to the data analysis program for both the total OTR and the polarized components. At the relatively low Lorentz factor (gamma) of 30, we observed 10-15% projected profile size reductions on a 65-micron beam size case with the perpendicularly polarized components. This anomalous effect in magnitude is compared to results from a standard OTR point-spread-function model.

THPB20	DC High Voltage Photoemission Electron Gun for CAEP FEL	598
	H.B. Wang, M. Li, X. Yang CAEP/IAE, Mianyang, Sichuan, People's Republic of China
	The research on high average power Terahertz free electron laser requires more demanding specifications of electron source. DC high voltage electron guns with photoemission cathodes are a natural choice for generating the critical beams considering the condition of technology. Field emission from the electrode structures limits the operating voltage and cathode field gradient in these guns. A ceramic insulator determines the level of operating voltage. The photocathode operational lifetime is limited by the gun vacuum and by ion back bombardment. The designing thought and the technical solution to aforementioned issues are presented. The results of the beam dynamic simulation based on the design are displayed, normalized emittance at the location 120 cm far from the cathode surface: x=1.335 πmmmrad, y=1.364 πmmmrad, z=4.81 π*keV-deg, using the following initial beam parameters: the laser spot 4 mm in diameter, the laser pulse length FWHM 12 ps, the charge per bunch 35 pC and the accelerating voltage 350 kV. Now the DC photoemission gun is conditioning.

THPB21	Extraction Arc for FLASH2	601
	M. Scholz, W. Decking, B. Faatz, T. Limberg DESY, Hamburg, Germany B. Liu SINAP, Shanghai, People's Republic of China
	FLASH2 is an extension of the existing FEL FLASH at DESY, Hamburg. It uses the same linear accelerator. A separate tunnel and a new experimental hall will be built next to the existing FLASH facilities. First constructions started in spring 2011. A fast kicker and a septum to be installed behind the last superconducting acceleration module give the possibility to distribute the beam to the existing beam line and to the new extraction arc. Within this arc a pulsed bending magnet allows to send the beam into two separate beam lines: One hosting undulators for SASE and space for HHG seeding (FLASH2), the other serving a proposed plasma wake field experiment or later on another FEL beam line (FLASH3). The extraction arc design has to fulfill specific requirements such as small emittance and energy spread growth. Furthermore, constrains are given by the existing FLASH buildings and by the space required for the in-coupling of the seed laser. Beam quality impairment has been mitigated by designing the beam optics with horizontal beam waists in all bending magnets. To optimize the extraction arc, simulations for different layouts were carried out using the programs ELEGANT and CSRTRACK.

THPB24	Generation and Acceleration of Uniformly-filled Ellipsoidal Bunches Obtained via Space-charge Expansion from a Semiconductor Photocathode	605
	P. Piot, J. Ruan, Y.-E. Sun, J.C.T. Thangaraj Fermilab, Batavia, USA T.J. Maxwell Northern Illinois University, DeKalb, Illinois, USA
	We report on the experimental generation, acceleration and characterization of a uniformly-filled electron bunch obtained via space-charge-driven expansion (so called "blow-out regime") at the A0 photoinjector at Fermilab. The beam is photoemitted from a CsTe photocathode using a short (<~200 fs) ultraviolet pulse obtained via frequency-tripling of an amplified Ti:Sp infrared pulse. The produced electron bunches are characterized with conventional diagnostics and the measurements are bench-marked against numerical simulations performed with ASTRA and GPT.

THPB25	EXPERIMENT AND SIMULATIONS OF SUB-PS ELECTRON BUNCH TRAIN GENERATION AT FERMILAB PHOTOINJECTORS	609
	Y.-E. Sun, M.D. Church Fermilab, Batavia, USA P. Piot, C.R. Prokop Northern Illinois University, DeKalb, Illinois, USA
	Funding: The work was supported by the US DOE Contracts No. DE-AC02-07CH11359 with the Fermi Research Alliance, LLC. and No. DE-FG02-08ER41532 with Northern Illinois University. Recently the generation of electron bunch trains with sub-picosecond time structure has been experimentally demonstrated at the A0 photoinjector of Fermilab using a transverse-longitudinal phase-space exchange beamline. The temporal profile of the bunch train can be easily tuned to meet the requirements of the applications of modern accelerator beams. In this paper we report the A0 bunch-train experiment and explore numerically the possible extension of this technique to shorter time scales at the Fermilab SRF Accelerator Test Facility, a superconducting linear electron accelerator currently under construction in the NML building.

THPB27	Application and Design of the Streak and TV Readout Systems at PITZ	613
	M. Mahgoub, H.-J. Grabosch, M. Groß, L. Hakobyan, I.I. Isaev, Ye. Ivanisenko, M.A. Khojoyan, G. Klemz, M. Krasilnikov, D. Malyutin, A. Oppelt, M. Otevřel, B. Petrosyan, D. Richter, S. Rimjaem, F. Stephan, G. Vashchenko, S. Weidinger DESY Zeuthen, Zeuthen, Germany G. Asova, J. Bähr DESY, Hamburg, Germany J. Rönsch-Schulenburg Uni HH, Hamburg, Germany K. Rosbach Humboldt University Berlin, Institut für Physik, Berlin, Germany
	Funding: Deutsches Elektronen-Synchrotron DESY, Germany The Photo Injector Test facility at DESY in Zeuthen (PITZ) was built to develop and optimize photoelectron injectors for FELs like FLASH and the European XFEL. In PITZ electrons can be accelerated to momenta up to 20 MeV/c. Optimization of all injector parameters such as the longitudinal properties of the electron bunch is needed. A streak system is used to measure the complete longitudinal phase space distribution of the bunch with an accuracy of few ps. In this system the electron beam penetrates Aerogel radiators or Optical Transition Radiation screens OTR and produces Cherenkov light, which is transported by an optical line to a streak camera. The emitted light presents the charge distribution in the electron bunch. Some modifications of the streak beamline, such as using a Hybrid of lenses and mirrors to improve resolution and using quartz lenses to overcome the radiation damage are foreseen. A TV system is used to observe the electron beam directly, where screens of Yttrium Aluminum Garnet YAG and OTR are used to produce a direct image of the beam. An overview of the existing systems, the measurements, the difficulties and future modifications will be presented.

THPB28	The High Power Test Model of C-band Accelerating Structure for Compact XFEL at SINAP	617
	W. Fang, Q. Gu, Z.T. Zhao SINAP, Shanghai, People's Republic of China D.C. Tong TUB, Beijing, People's Republic of China
	R&D of a C-band (5712 MHz) high gradient traveling-wave accelerating structure is being in progress at Shanghai Institute of Applied Physics (SINAP). Conceptual design of the accelerating structure has been accomplished, and verified by the cold test of the experimental model. Now the first prototype structure is ready for high RF power test and the optimization of a new operating mode is proposed for developing a robust high gradient C-band struture. In this paper, the results of the cold test of the first prototype structure and the optimization details are introduced.

THPB29	Design of a Low Emittance and High Repetition Rate S-band Photoinjector	621
	J.H. Han Diamond, Oxfordshire, United Kingdom
	One of key components for the success of X-ray free-electron lasers (FELs) is the electron injector. Injectors starting with photocathode RF guns provide exceptionally high brightness electron beams and therefore they are being adopted as injectors of X-ray FELs. In this paper we show how to improve the photoinjector performance in terms of emittance and repetition rate by means of components optimization based on mature technologies. Transverse emittance at an injector is reduced by optimizing the RF gun cavity design, gun solenoid position, and accelerating section position. The repetition rate of an injector mainly depends on the cooling capability of the gun cavity. By adopting the coaxial RF gun coupler and improving cooling-water channels of the gun, a maximum repetition rate of 1 kHz for the injector will be achieved.

THPB30	SwissFEL Injector Test Facility – Test and Plans	625
	M. Pedrozzi, M. Aiba, S. Bettoni, B. Beutner, A. Falone, R. Ganter, R. Ischebeck, F. Le Pimpec, G.L. Orlandi, E. Prat, S. Reiche, T. Schietinger, A. Trisorio, C. Vicario Paul Scherrer Institut, Villigen, Switzerland
	In August 2010 the Paul Scherrer Institute inaugurated the SwissFEL Injector test facility as a first step toward the Swiss hard X-ray FEL planned at PSI. The main purpose of the facility is to demonstrate and consolidate the generation of high-brightness beam as required to drive the 6 GeV SwissFEL accelerator. Additionally the injector serves as a platform supporting development and test of accelerator components/systems and optimization procedures foreseen for SwissFEL. In this paper we report on the present status of the commissioning with some emphasis on emittance measurements and component performances. The scientific program and long-term plans will be discussed as well.

THPB31	Multiple FELs from the One LCLS Undulator	629
	F.-J. Decker, P. Emma, J.C. Frisch, K. Horovitz, Z. Huang, R.H. Iverson, J. Krzywinski, H. Loos, S.P. Moeller, H.-D. Nuhn, J.L. Turner, J.J. Welch, J. Wu SLAC, Menlo Park, California, USA
	Funding: Work supported by U.S. Department of Energy, Office of Basic Energy Science, under Contract DE-AC02-76SF00515. The FEL of the Linac Coherent Light Source (LCLS) at SLAC is generated in a 132 m long undulator. By introducing a kink in the undulator setup and launching different electron pulses with a small kick, we achieved two FEL beams with a separation of about 10 σ. These beams were separated at down stream mirrors and brought to the entrances of the soft and hard X-ray hutches. This was done at low energy creating soft X-rays which require only a shorter length to get to saturation. At high energy the whole undulator has to be "re-pointed" pulse by pulse. This can be done using 33 undulator correctors creating two straight lines for the photons with small angle to point the FEL to different mirrors pulse by pulse even at high energy. Experiments will be presented and further ideas discussed to get different energy photons created and sent to the soft and hard X-ray mirrors and experiments.

Paper

Title

Page

Optical Comb and Interferometer Development for Laser Synchronization in Femtosecond FELs

561

R.B. Wilcox, J.M. Byrd
LBNL, Berkeley, California, USA
R. Holzwarth
Menlo Systems GmbH, Martinsried, Germany

Funding: This work supported by the U.S. Department of Energy under contract DE-AC02-05CH11231
We describe a method of synchronizing lasers in FELs to potential sub-femtosecond precision using interferometry and optical clock techniques, and show supporting experimental results. This precision is needed for pump/probe experiments in ultrafast FELs. The proposed system consists of carrier/offset phase stabilized, pulsed lasers synchronized via a single optical frequency delivered over fiber, analogous to RF oscillators synchronized with a reference frequency, but at 200 to 400THz. Our tests of modelocked lasers, interferometers and stabilized CW lasers show that subsystems can perform to the required precision. We have synchronized fiber lasers to less than 10fs jitter using two different frequency comb line locking schemes, and demonstrated interferometers in a working FEL with less than 100as jitter over 150m fiber. Based on these tests and published work by others, we calculate the performance of an optimized, integrated timing system to be less than 1fs in the short term. Long term stability is maintained by feedback from X-ray/optical cross-correlation at the experiment.

THPB02

Implementation of 2D-emittance Compensation Scheme in the BERLinPro Injector

564

A.V. Bondarenko, A.N. Matveenko
HZB, Berlin, Germany

Helmholtz-Zentrum Berlin officially started Jan. 2011 the design and construction of the Berlin Energy Recovery Linac Project BERLinPro. The initial goal of this compact ERL is to develop the ERL accelerator physics and technology required to accelerate a high-current (100 mA) low emittance beam (1 mm•mrad normalized), as required for future ERL-based synchrotron light sources. High power ERL based FELs demand low emittance, high peak and average current beams. The injection energy in an ERL is usually rather low to decrease power consumption and avoid activation of the beam dump. Therefore, the space charge is the main reason of the emittance degradation in the injector. The implementation of an emittance compensation scheme in the injector is necessary to achieve a low emittance. Since injector’s optics is axially non-symmetric, the 2D-emittance compensation scheme [1] should be used. The implementation of the 2D-emittance compensation scheme at BERLinPro injector is presented in this contribution. Other sources of emittance growth in ERL injectors are also discussed.
[1] S.V. Miginsky, "Emittance compensation of elliptical beam", NIM A 603 (2009), pp 32-34.

THPB05

Modeling of the Beam Break Up Instability in Berlin Energy Recovery Linac Project

568

Y. Petenev, A.V. Bondarenko, A.N. Matveenko
HZB, Berlin, Germany

Helmholtz-Zentrum Berlin officially started Jan. 2011 the design and construction of the Berlin Energy Recovery Linac Project BERLinPro. The initial goal of this compact ERL is to develop the ERL accelerator physics and technology required to accelerate a high-current low emittance beam. The conversion efficiency of an FEL is about 1% therefore superconducting ERL-based FEL machines look promising. One of the problems of superconducting ERL machines is the Beam Break Up (BBU) instability which limits the current. In this work the threshold current of the BBU instability was calculated for the BERLinPro. The comparison of two 100 MeV linacs based on different type of superconducting cavities is made. Different methods of BBU suppression are investigated (e.g. the influence of solenoid, pseudo-reflector and quadruple triplets in the linac structure on the BBU threshold).

THPB06

Coherent Terahertz Radiation Monitors for Multiple Spectral Bands

572

R. Ischebeck, G.L. Orlandi, P. Peier, V. Schlott, B. Smit, C. Vicario, C. Zimmerli
Paul Scherrer Institut, Villigen, Switzerland
C. Gerth
DESY, Hamburg, Germany

The SwissFEL Injector Test Facility is destined for demonstrating electron beam parameters that are suitable for FEL operation. Of particular interest is the on-line measurement of longitudinal phase space properties, as this provides insight into the bunch compression process. The spectral distribution of diffraction radiation offers a robust way to assess bunch length and longitudinal profile. The bunch length at the SwissFEL Injector Test Facility can be varied by changing the photocathode laser. Diffraction radiation is emitted as the electron bunches pass through a hole in a titanium foil. The emitted Terahertz radiation has been simulated by the code THz Transport, and the propagation to the detectors has been modeled.

THPB08

Study of Reflective Optics for LFC-Camera

576

K. Nanbu
Tohoku University, School of Science, Sendai, Japan
H. Hama, F. Hinode, S. Kashiwagi, M. Kawai, X. Li, T. Muto, Y. Tanaka
Tohoku University, Research Center for Electron Photon Science, Sendai, Japan

Funding: This work is partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (S), Contract #20226003.
A test accelerator for the terahertz source project (t-ACTS) employing isochronous ring and bunched free electron laser has been under development at Tohoku University [1,2]. Stable production of very short electron bunches is a key issue for the t-ACTS project. We have chosen thermionic RF gun for the injector of t-ACTS because of stability, multi-bunch operation and cheaper cost. The longitudinal phase space distribution of the beam extracted from the rf-gun is crucial for the final bunch length of electron beam passing through bunch compression process. Therefore, measurement of the longitudinal phase space of the beam is indispensable for efficient bunch compression. In order to measure the electron distribution in the longitudinal phase space of relatively lower energy beam, we have been developing a novel method to observe the energy spectrum employing a velocity dependence of opening angle of Cherenkov radiation, namely Linear Focal Cherenkov (LFC) ring camera. We describe principle of LFC camera and discuss relations between surface roughness of Cherenkov radiator and energy resolution in this conference.
[1] H. Hama et al., New J. Phys. 8 (2006) 292,
[2] H. Hama and M. Yasuda, Proc. of FEL2009, (2009) 394

THPB09

Study of the Microbunching Instablity in the LINAC of the Future Shanghai Soft X-ray FEL Facility (SXFEL)

579

D. Huang
IIT, Chicago, Illinois, USA
Q. Gu
SINAP, Shanghai, People's Republic of China

The microbunching instability in the LINAC of a FEL facility has always been an issue which may degrade the quality of the electron beam. As the result, the whole facility may not be working properly. Therefore, learning how to control and reduce the instability is the key to the success of a FEL project. Shanghai soft X-ray FEL project (SXFEL) has just been granted, once it is built, it will be the first X-ray FEL facility in China. In this article, detailed study will be given based on the design parameters of the facility to gain better understanding and control over the possible microbunching instability in SXFEL, which is important to the success of the project.

THPB14

APEX Project Phase 0 and I Status and Plans and Activities for Phase II

582

F. Sannibale, B.J. Bailey, K.M. Baptiste, J.M. Byrd, A.L. Catalano, D. Colomb, C.W. Cork, J.N. Corlett, S. De Santis, L.R. Doolittle, J. Feng, D. Filippetto, G. Huang, S. Kwiatkowski, W.E. Norum, H.A. Padmore, C. F. Papadopoulos, G. Penn, G.J. Portmann, S. Prestemon, J. Qiang, D.G. Quintas, J.W. Staples, M.E. Stuart, T. Vecchione, M. Venturini, M. Vinco, W. Wan, R.P. Wells, M.S. Zolotorev, F.A. Zucca
LBNL, Berkeley, California, USA
M. J. Messerly, M.A. Prantil
LLNL, Livermore, California, USA
C. Pellegrini
UCLA, Los Angeles, California, USA
M. Yoon
POSTECH, Pohang, Kyungbuk, Republic of Korea

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 the Lawrence Berkeley National Laboratory is devoted to the development of a high repetition rate (MHz-class) electron injector for X-ray FEL applications. The injector is based on a new concept photo-gun, utilizing a normal conducting 187 MHz RF cavity operating in CW mode in conjunction with high quantum efficiency photocathodes able to deliver the required repetition rates with available laser technology. The APEX activities are staged in two phases. In Phase I, the electron photo-gun is constructed, tested and several different photo-cathodes, such as alkali antimonides, Cs₂Te [1], diamond amplifiers [2], and metals, are tested at full repetition rate. In Phase II, a pulsed linac is added for accelerating the beam at several tens of MeV to prove the high brightness performance of the gun when integrated in an injector scheme. Based on funding availability, after Phase II, the program could also include testing of new undulator technologies and FEL studies. The status of Phase I, in its initial experimental phase, is described together with plans and activities for Phase II and beyond.
[1] In collaboration with INFN-LASA, Milano, Italy.
[2] In collaboration with Brookhaven National Laboratory, Upton NY, USA

THPB15

Metal Cathodes with Reduced Emittance and Enhanced Quantum Efficiency

586

C.M.R. Greaves, J. Feng, H.A. Padmore, W. Wan
LBNL, Berkeley, California, USA
D. Dowell
AES, Princeton, New Jersey, USA

In this paper, we report experimental results on photoemission from copper and silver surfaces. Using the technique of angle resolved photoemission spectroscopy (ARPES), we demonstrate that, for excess energy around 0.5 eV, the photoelectrons from the Cu(111) and Ag(111) surfaces generated by p-polarized light originate primarily from the well-known surface state with normalized emittance only a fraction of that of the polycrystalline copper cathode presently used in the RF guns. Meanwhile, we demonstrate that the enhancement of the quantum efficiency (QE) at grazing angle is closely related to the surface state as well. Furthermore, we show that the surface state can be easily restored by a simple anneal process, thus pointing to a practical way to reducing the emittance and QE of a metal cathode simultaniously.

THPB16

Beam Profile Measurements Using a Fast Gated CCD Camera and a Scintillation Screen to Suppress COTR

590

M. Yan
Uni HH, Hamburg, Germany
C. Behrens, C. Gerth, G. Kube, B. Schmidt, S. Wesch
DESY, Hamburg, Germany

For standard beam profile measurements of high-brightness electron beams using optical transition radiation (OTR) screens, coherence effects induced by microbunching instabilities render direct imaging of the beam impossible. A technique of using a scintillation screen with a fast gated CCD camera has been demonstrated to successfully suppress coherent OTR (COTR) in transverse beam diagnostics at FLASH. The fast gated CCD camera has been installed next to a standard CCD camera setup and images the same viewing screens. The results of transverse beam profile measurements under operating conditions without COTR are compared for both setups. The fast gated camera has also been employed for longitudinal bunch profile measurements with a transverse deflecting structure (TDS). Results obtained under operating conditions with COTR are compared to those from longitudinal phase space measurements in a dispersive arm, where no coherence effects have been observed so far. In this paper, we examine the performance of the fast gated CCD camera for beam profile measurements and present further studies on the use of scintillation screens for high-energy electron beam diagnostics.

THPB19

Investigations of OTR Polarization Effects in Beam-profile Monitors

594

A.H. Lumpkin, A.S. Johnson, J. Ruan, R.M. Thurman-Keup
Fermilab, Batavia, USA

Funding: Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.
The characterization of transverse beam size using optical transition radiation (OTR) imaging is a well-established technique at many accelerators including the Fermilab A0 photoinjector (A0PI) facility. However, there is growing empirical evidence that the utilization of the polarization component orthogonal to the dimension of interest results in a smaller observed projected image profile. We have continued investigations of this phenomenon with a more controlled experiment where the linear polarizers are selectable in a filter wheel which also included a blank glass position to compensate for the optical path. The aperture for light collection is thus kept fixed compared to our previous tests. We also have balanced the digital camera gain to present similar signal levels to the data analysis program for both the total OTR and the polarized components. At the relatively low Lorentz factor (gamma) of 30, we observed 10-15% projected profile size reductions on a 65-micron beam size case with the perpendicularly polarized components. This anomalous effect in magnitude is compared to results from a standard OTR point-spread-function model.

THPB20

DC High Voltage Photoemission Electron Gun for CAEP FEL

598

H.B. Wang, M. Li, X. Yang
CAEP/IAE, Mianyang, Sichuan, People's Republic of China

The research on high average power Terahertz free electron laser requires more demanding specifications of electron source. DC high voltage electron guns with photoemission cathodes are a natural choice for generating the critical beams considering the condition of technology. Field emission from the electrode structures limits the operating voltage and cathode field gradient in these guns. A ceramic insulator determines the level of operating voltage. The photocathode operational lifetime is limited by the gun vacuum and by ion back bombardment. The designing thought and the technical solution to aforementioned issues are presented. The results of the beam dynamic simulation based on the design are displayed, normalized emittance at the location 120 cm far from the cathode surface: x=1.335 π*mm*mrad, y=1.364 π*mm*mrad, z=4.81 π*keV-deg, using the following initial beam parameters: the laser spot 4 mm in diameter, the laser pulse length FWHM 12 ps, the charge per bunch 35 pC and the accelerating voltage 350 kV. Now the DC photoemission gun is conditioning.

THPB21

Extraction Arc for FLASH2

601

M. Scholz, W. Decking, B. Faatz, T. Limberg
DESY, Hamburg, Germany
B. Liu
SINAP, Shanghai, People's Republic of China

FLASH2 is an extension of the existing FEL FLASH at DESY, Hamburg. It uses the same linear accelerator. A separate tunnel and a new experimental hall will be built next to the existing FLASH facilities. First constructions started in spring 2011. A fast kicker and a septum to be installed behind the last superconducting acceleration module give the possibility to distribute the beam to the existing beam line and to the new extraction arc. Within this arc a pulsed bending magnet allows to send the beam into two separate beam lines: One hosting undulators for SASE and space for HHG seeding (FLASH2), the other serving a proposed plasma wake field experiment or later on another FEL beam line (FLASH3). The extraction arc design has to fulfill specific requirements such as small emittance and energy spread growth. Furthermore, constrains are given by the existing FLASH buildings and by the space required for the in-coupling of the seed laser. Beam quality impairment has been mitigated by designing the beam optics with horizontal beam waists in all bending magnets. To optimize the extraction arc, simulations for different layouts were carried out using the programs ELEGANT and CSRTRACK.

THPB24

Generation and Acceleration of Uniformly-filled Ellipsoidal Bunches Obtained via Space-charge Expansion from a Semiconductor Photocathode

605

P. Piot, J. Ruan, Y.-E. Sun, J.C.T. Thangaraj
Fermilab, Batavia, USA
T.J. Maxwell
Northern Illinois University, DeKalb, Illinois, USA

We report on the experimental generation, acceleration and characterization of a uniformly-filled electron bunch obtained via space-charge-driven expansion (so called "blow-out regime") at the A0 photoinjector at Fermilab. The beam is photoemitted from a CsTe photocathode using a short (<~200 fs) ultraviolet pulse obtained via frequency-tripling of an amplified Ti:Sp infrared pulse. The produced electron bunches are characterized with conventional diagnostics and the measurements are bench-marked against numerical simulations performed with ASTRA and GPT.

THPB25

EXPERIMENT AND SIMULATIONS OF SUB-PS ELECTRON BUNCH TRAIN GENERATION AT FERMILAB PHOTOINJECTORS

609

Y.-E. Sun, M.D. Church
Fermilab, Batavia, USA
P. Piot, C.R. Prokop
Northern Illinois University, DeKalb, Illinois, USA

Funding: The work was supported by the US DOE Contracts No. DE-AC02-07CH11359 with the Fermi Research Alliance, LLC. and No. DE-FG02-08ER41532 with Northern Illinois University.
Recently the generation of electron bunch trains with sub-picosecond time structure has been experimentally demonstrated at the A0 photoinjector of Fermilab using a transverse-longitudinal phase-space exchange beamline. The temporal profile of the bunch train can be easily tuned to meet the requirements of the applications of modern accelerator beams. In this paper we report the A0 bunch-train experiment and explore numerically the possible extension of this technique to shorter time scales at the Fermilab SRF Accelerator Test Facility, a superconducting linear electron accelerator currently under construction in the NML building.

THPB27

Application and Design of the Streak and TV Readout Systems at PITZ

613

M. Mahgoub, H.-J. Grabosch, M. Groß, L. Hakobyan, I.I. Isaev, Ye. Ivanisenko, M.A. Khojoyan, G. Klemz, M. Krasilnikov, D. Malyutin, A. Oppelt, M. Otevřel, B. Petrosyan, D. Richter, S. Rimjaem, F. Stephan, G. Vashchenko, S. Weidinger
DESY Zeuthen, Zeuthen, Germany
G. Asova, J. Bähr
DESY, Hamburg, Germany
J. Rönsch-Schulenburg
Uni HH, Hamburg, Germany
K. Rosbach
Humboldt University Berlin, Institut für Physik, Berlin, Germany

Funding: Deutsches Elektronen-Synchrotron DESY, Germany
The Photo Injector Test facility at DESY in Zeuthen (PITZ) was built to develop and optimize photoelectron injectors for FELs like FLASH and the European XFEL. In PITZ electrons can be accelerated to momenta up to 20 MeV/c. Optimization of all injector parameters such as the longitudinal properties of the electron bunch is needed. A streak system is used to measure the complete longitudinal phase space distribution of the bunch with an accuracy of few ps. In this system the electron beam penetrates Aerogel radiators or Optical Transition Radiation screens OTR and produces Cherenkov light, which is transported by an optical line to a streak camera. The emitted light presents the charge distribution in the electron bunch. Some modifications of the streak beamline, such as using a Hybrid of lenses and mirrors to improve resolution and using quartz lenses to overcome the radiation damage are foreseen. A TV system is used to observe the electron beam directly, where screens of Yttrium Aluminum Garnet YAG and OTR are used to produce a direct image of the beam. An overview of the existing systems, the measurements, the difficulties and future modifications will be presented.

THPB28

The High Power Test Model of C-band Accelerating Structure for Compact XFEL at SINAP

617

W. Fang, Q. Gu, Z.T. Zhao
SINAP, Shanghai, People's Republic of China
D.C. Tong
TUB, Beijing, People's Republic of China

R&D of a C-band (5712 MHz) high gradient traveling-wave accelerating structure is being in progress at Shanghai Institute of Applied Physics (SINAP). Conceptual design of the accelerating structure has been accomplished, and verified by the cold test of the experimental model. Now the first prototype structure is ready for high RF power test and the optimization of a new operating mode is proposed for developing a robust high gradient C-band struture. In this paper, the results of the cold test of the first prototype structure and the optimization details are introduced.

THPB29

Design of a Low Emittance and High Repetition Rate S-band Photoinjector

621

J.H. Han
Diamond, Oxfordshire, United Kingdom

One of key components for the success of X-ray free-electron lasers (FELs) is the electron injector. Injectors starting with photocathode RF guns provide exceptionally high brightness electron beams and therefore they are being adopted as injectors of X-ray FELs. In this paper we show how to improve the photoinjector performance in terms of emittance and repetition rate by means of components optimization based on mature technologies. Transverse emittance at an injector is reduced by optimizing the RF gun cavity design, gun solenoid position, and accelerating section position. The repetition rate of an injector mainly depends on the cooling capability of the gun cavity. By adopting the coaxial RF gun coupler and improving cooling-water channels of the gun, a maximum repetition rate of 1 kHz for the injector will be achieved.

THPB30

SwissFEL Injector Test Facility – Test and Plans

625

M. Pedrozzi, M. Aiba, S. Bettoni, B. Beutner, A. Falone, R. Ganter, R. Ischebeck, F. Le Pimpec, G.L. Orlandi, E. Prat, S. Reiche, T. Schietinger, A. Trisorio, C. Vicario
Paul Scherrer Institut, Villigen, Switzerland

In August 2010 the Paul Scherrer Institute inaugurated the SwissFEL Injector test facility as a first step toward the Swiss hard X-ray FEL planned at PSI. The main purpose of the facility is to demonstrate and consolidate the generation of high-brightness beam as required to drive the 6 GeV SwissFEL accelerator. Additionally the injector serves as a platform supporting development and test of accelerator components/systems and optimization procedures foreseen for SwissFEL. In this paper we report on the present status of the commissioning with some emphasis on emittance measurements and component performances. The scientific program and long-term plans will be discussed as well.

THPB31

Multiple FELs from the One LCLS Undulator

629

F.-J. Decker, P. Emma, J.C. Frisch, K. Horovitz, Z. Huang, R.H. Iverson, J. Krzywinski, H. Loos, S.P. Moeller, H.-D. Nuhn, J.L. Turner, J.J. Welch, J. Wu
SLAC, Menlo Park, California, USA

Funding: Work supported by U.S. Department of Energy, Office of Basic Energy Science, under Contract DE-AC02-76SF00515.
The FEL of the Linac Coherent Light Source (LCLS) at SLAC is generated in a 132 m long undulator. By introducing a kink in the undulator setup and launching different electron pulses with a small kick, we achieved two FEL beams with a separation of about 10 σ. These beams were separated at down stream mirrors and brought to the entrances of the soft and hard X-ray hutches. This was done at low energy creating soft X-rays which require only a shorter length to get to saturation. At high energy the whole undulator has to be "re-pointed" pulse by pulse. This can be done using 33 undulator correctors creating two straight lines for the photons with small angle to point the FEL to different mirrors pulse by pulse even at high energy. Experiments will be presented and further ideas discussed to get different energy photons created and sent to the soft and hard X-ray mirrors and experiments.