Author: Qian, H.J.
Paper Title Page
TUP002 Progress in Preparing a Proof-of-Principle Experiment for THz SASE FEL at PITZ 41
 
  • X. Li, P. Boonpornprasert, Y. Chen, G.Z. Georgiev, J.D. Good, M. Groß, P.W. Huang, I.I. Isaev, C. Koschitzki, M. Krasilnikov, S. Lal, O. Lishilin, G. Loisch, D. Melkumyan, R. Niemczyk, A. Oppelt, H.J. Qian, H. Shaker, G. Shu, F. Stephan, G. Vashchenko
    DESY Zeuthen, Zeuthen, Germany
 
  A proof-of-princle experiment for a THz SASE FEL is undergoing preparation at the Photo Injector Test facility at DESY in Zeuthen (PITZ), as a prototype THz source for pump-probe experiments at the European XFEL, which could potentially provide up to mJ/pulse THz radiation while maintaining the identical pulse train structure as the XFEL pulses. In the proof-of-principle experiment, LCLS-I undulators will be installed to generate SASE radiation in the THz range of 3-5 THz from electron bunches of 16-22 MeV/c. One key design is to obtain the peak current of nearly 200 A from the heavily charged bunches of a few nC. In this paper, we report our simulation results on the optimization of the space charge dominated beam in the photo injector and the following transport line with two cathode laser setups. Experimental results based on a short Gaussian laser will also be discussed.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP002  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEA01 Overview of CW RF Guns for Short Wavelength FELs 290
 
  • H.J. Qian
    DESY Zeuthen, Zeuthen, Germany
  • E. Vogel
    DESY, Hamburg, Germany
 
  Hard X-ray FELs (XFELs) operating with pulsed RF provide unprecedented peak brilliance for scientific research. Operating the accelerators with CW RF improves the flexibility w.r.t. the available time structure for experiments and opens the next frontier of average brilliance. One of the challenges of CW XFELs is the electron source, which requires both CW operation and highest possible beam quality allowing lasing at shortest wavelengths. The CW mode technically constraints the gun acceleration gradient, which is one of the keys to electron source brightness, so R&D is devoted to CW gun improvements since decades. In this contribution, the worldwide development status of CW RF guns, both normal conducting and superconducting, is reviewed.  
slides icon Slides WEA01 [16.329 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEA01  
About • paper received ※ 25 August 2019       paper accepted ※ 07 November 2019       issue date ※ 05 November 2019  
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WEA03
Emittance Budget in the Transition Regime Between Linear Emission and Space Charge Dominated Photoemission  
 
  • Y. Chen, M. Krasilnikov, H.J. Qian, F. Stephan
    DESY Zeuthen, Zeuthen, Germany
 
  Free electron laser based X-ray facilities require high brightness accelerators which entails minimization of the beam emittance in all planes for a fixed bunch charge. Since the lowest achievable emittance of linac based accelerators is set at the injectors already, emittance optimization at the injector exit needs to carefully budget the contributions from the space charge and rf forces as well as the intrinsic cathode contribution. The optimization of normalized transverse emittance at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) has routinely found the smallest possible emittance for a fixed bunch charge in a so-called transition regime of photoemission. In such a regime, high space charge density of the beam significantly contributes to the phase space formation. Strong space charge fields during the emission process alter the cathode physics thereby changing the emittance budget distribution. Based on an advanced beam dynamics modeling approach, we analyze each decomposed contribution of the measured emittance for understanding the optimization scheme in the transition regime between linear and space charge dominated emission. Obtained results will be presented.  
slides icon Slides WEA03 [1.918 MB]  
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WEP051 PITZ Experimental Optimization for the Aimed Cathode Gradient of a Superconducting CW RF Gun 440
 
  • M. Krasilnikov, P. Boonpornprasert, Y. Chen, G.Z. Georgiev, J.D. Good, M. Groß, P.W. Huang, I.I. Isaev, C. Koschitzki, S. Lal, X. Li, O. Lishilin, G. Loisch, D. Melkumyan, R. Niemczyk, A. Oppelt, H.J. Qian, H. Shaker, G. Shu, F. Stephan, G. Vashchenko
    DESY Zeuthen, Zeuthen, Germany
  • M. Dohlus, E. Vogel
    DESY, Hamburg, Germany
 
  A continuous wave (CW) mode operation of the European X-ray Free-Electron Laser (XFEL) is under considerations for a future upgrade. Therefore, a superconducting radio frequency (SRF) CW gun is under experimental development at DESY in Hamburg. Beam dynamics simulations for this setup have been done assuming 100 pC bunch charge and a maximum electric field at the photocathode of 40 MV/m. Experimental studies for these parameters using a normal conducting RF photogun have been performed at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The beam transverse emittance was minimized by optimizing the main photo injector parameters in order to demonstrate the feasibility of generating electron beams with a beam quality required for successful CW operation of the European XFEL for conditions similar to the SRF gun setup.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP051  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP052 Simulation Studies on the Saturated Emission at PITZ 444
 
  • X. Li, P. Boonpornprasert, Y. Chen, J.D. Good, M. Groß, I.I. Isaev, C. Koschitzki, M. Krasilnikov, S. Lal, O. Lishilin, G. Loisch, D. Melkumyan, R. Niemczyk, A. Oppelt, H.J. Qian, H. Shaker, G. Shu, F. Stephan, G. Vashchenko
    DESY Zeuthen, Zeuthen, Germany
 
  In this paper we report our consideration and simulation on the space charge dominated emission in the L-band photocathode RF gun at the Photo Injector Test facility at DESY in Zeuthen (PITZ). It has been found that the emission curve, which relates the extracted and accelerated bunch charge after the gun to the laser energy, doesn’t agree very well with Astra simulations when the emission is nearly or fully saturated. Previous studies with a core-halo model for a better fit of the experimentally measured laser transverse profile as well as with an improved transient emission model have resulted in a better agreement between experimental data and simulation. A 3D FFT space charge solver including mirror charge and binned energy/momentum has been built, which also allows more emission mechanisms to be included in the future. In this paper, the energy spread during emission was preliminarily analyzed. Experimentally measured emission curves were compared with simulation, showing the effect of the inhomogeneity of the laser on the emission and beam parameters.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP052  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP054 Beam Dynamics Optimization of a Normal-Conducting Gun Based CW Injector for the European XFEL 452
 
  • H. Shaker, S. Lal, H.J. Qian, G. Shu, F. Stephan
    DESY Zeuthen, Zeuthen, Germany
 
  The European XFEL is operating up to 17.5 GeV electron energy with maximum 0.65% duty cycle. There is a prospect for continuous wave and long pulse mode (CW/LP) operation of the European XFEL, which enables more flexible bunch pattern time structure for experiments, higher average brightness and better stability. Due to engineering limitations, the maximum electron beam energy in the CW/LP mode is about 8.6/12.8 GeV, which puts more pressure on the injector beam quality for lasing at the shortest wavelength. This paper optimizes the beam dynamics of an injector based on a normal-conducting VHF gun.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP054  
About • paper received ※ 20 August 2019       paper accepted ※ 26 August 2019       issue date ※ 05 November 2019  
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WEP055 Multiphysics Analysis of a CW VHF Gun for European XFEL 456
 
  • G. Shu, Y. Chen, S. Lal, H.J. Qian, H. Shaker, F. Stephan
    DESY Zeuthen, Zeuthen, Germany
 
  R&D for a possible future CW mode operation of European XFEL started after the successful commissioning of the pulse mode operation. For the CW electron source upgrade, a fully superconducting CW gun is under experimental development at DESY in Hamburg, and a normal conducting (NC) CW gun is under physics design at the Photo Injector Test facility at DESY in Zeuthen (PITZ) as a backup option. Based on the experience of the LBNL on a 187 MHz gun, the DESY 217 MHz gun increased the cathode gradient and RF power to 28 MV/m and 100 kW, respectively, to further improve the beam brightness. In this paper, the multiphysics analysis investigating the RF, thermal and mechanical properties of the 217 MHz NC CW gun are presented.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP055  
About • paper received ※ 20 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEP062 Test of Cs2Te Thickness on Cathode Performance at PITZ 473
 
  • P.W. Huang
    TUB, Beijing, People’s Republic of China
  • Y. Chen, M. Groß, I.I. Isaev, P. Kitisri, C. Koschitzki, M. Krasilnikov, S. Lal, X. Li, O. Lishilin, D. Melkumyan, R. Niemczyk, A. Oppelt, H.J. Qian, H. Shaker, G. Shu, F. Stephan, G. Vashchenko, T. Weilbach
    DESY Zeuthen, Zeuthen, Germany
  • A. Grigoryan
    CANDLE, Yerevan, Armenia
  • S. Lederer
    DESY, Hamburg, Germany
  • P. Michelato, L. Monaco, D. Sertore
    INFN/LASA, Segrate (MI), Italy
 
  Cesium telluride is a widely used cathode in photo injectors, and its performance is one of the keys for not only emittance but also reliable operation. Over the years lots of experiences with Cs2Te photocathodes produced with the same recipe and thickness were gained at the DESY photo injectors, but cathode performance dependence on the cathode layer thickness were not investigated. In this paper, we test fresh Cs2Te cathodes with different thickness at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). The QE and thermal emittance of these cathodes inside the high gradient RF gun will be compared. Besides, the injector emittance under the operation conditions of the XFEL will also be measured with these cathodes.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP062  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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THP007 Frequency-Detuning Dependent Transient Coaxial RF Coupler Kick 599
 
  • Y. Chen, J.D. Good, M. Groß, P.W. Huang, I.I. Isaev, C. Koschitzki, M. Krasilnikov, S. Lal, X. Li, O. Lishilin, G. Loisch, D. Melkumyan, R. Niemczyk, A. Oppelt, H.J. Qian, H. Shaker, G. Shu, F. Stephan, G. Vashchenko
    DESY Zeuthen, Zeuthen, Germany
  • F. Brinker, W. Decking
    DESY, Hamburg, Germany
 
  We model and characterize a transverse kick which results from the coaxial RF coupler in the L-band RF gun at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). The RF pulse is typically 600 µs long and used to produce a train of up to 2700 electron bunches. The kick is transient and found to be dependent on the detuning of the resonance frequency of the gun cavity. The frequency detuning within the RF macro-pulse results in a variation in the kick strength along the pulse. This leads to a downstream orbit and size change of individual bunches within the train. Using 3D RF field distributions calculated at detuned frequencies of the cavity, particle tracking simulations are performed to simulate the transient kick onto the bunch train. Given a drift distance, the orbit and size change along a train of fixed length is estimated. Systematic measurements of the kick have meanwhile been carried out. The temperature of the cooling water for the gun is tuned allowing detailed characterization of the frequency detuning within the RF pulse, and thereby measurements of the kick under conditions of practical interest. Experimental findings and simulation results will be presented.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP007  
About • paper received ※ 13 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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