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At the electron accelerator for beams with high brilliance and low emittance (ELBE), the second version of a superconducting radio-frequency (SRF) photoinjector was brought into operation in 2014. After a period of commissioning, a gradual transfer to routine operation took place in 2017 and 2018, so that more than 3000h of user beam have already been generated since 2019. During this time, a total of 20 cathodes (2 Cu, 12 Mg, 6 Cs₂Te) were used, but no serious cavity degradation was observed. In this paper, we summarize the operational experience of the last 6 years of SRF gun operation, with special emphasis on the main RF properties of the cavity. This includes the evolution of QvsE, dark current, multipacting, but also mechanical properties such as Lorentz force detuning, helium pressure sensitivity as well as microphonics. The latter is closely connected to an active compensation by a so-called low-level RF feedback loop, which is also briefly presented., Proceedings of the 20th International Conference on RF Superconductivity, SRF2021, East Lansing, MI, USA

The first all-SRF accelerator driven THz source has been operated as a user facility since 2018 at ELBE radiation center. The CW electron beam is extracted from SRF gun II, accelerated to relativistic energies and compressed to sub-ps length in the ELBE SRF linac with a chicane. THz pulses are produced by pass-ing the short electron bunches through a diffraction radiator (CDR) and an undulator. The coherent THz power increases quadratically with bunch charge. The pulse energy up to 10 µJ at 0.3 THz with 100 kHz has been generated., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

The superconducting RF photoinjector (SRF Gun II) has successfully served for the ELBE user facility at HZDR. Nevertheless, the quality of photocathodes is one of the most critical issues in improving the stability and reliability for its application. Magnesium has a comparably low work function (3.6 eV) and shows a quantum efficiency up to 0.3% after laser cleaning. However, the present cleaning process with a high intensity laser beam is time consuming and produces unwanted surface roughness, which leads to a higher thermal emittance. Thermal treatment and Excimer laser cleaning for Mg cathodes are investigated as alternative methods. In this work, the new cleaning procedures are tested and optimized, and the quantum efficiency of Mg samples with different microstructure, composition and suppliers are compared., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

The multipurpose CW accelerator ELBE at HZDR, which is delivering a large set of secondary beams, is driven by a thermionic DC injector. In order to enhance the machine's beam quality, the development of a superconducting RF injector has been pursued since the early 2000ies. The corresponding ELBE SRF Gun I of 2007 and Gun II of 2014 already delivered beam for the operation of several user beamlines, such as the FEL, positron generation, and THz facility. Currently, the next version ' Gun III ' and its cryomodule are being assembled, characterized, and prepared for the final commissioning throughout late 2017/early 2018. The new module benefits from the experiences in terms of emittance compensation and surveillance of operation variables made with the two predecessors. Results of the latest Gun II operations as well as first commissioning data of the Gun III module will be presented., Proceedings of the 6th Int. Beam Instrumentation Conf., IBIC2017, Grand Rapids, MI, USA

The second version of the superconducting RF pho-toinjector (SRF Gun II) was successfully commissioned at the ELBE radiation source in 2014. The gun features an improved 3.5-cell niobium cavity combined with a super-conducting solenoid integrated in the cryostat. With a Mg photocathode the SRF Gun II is able to generate bunches with up to 200 pC and with sub-ps length in CW mode with 100 kHz pulse frequency for the THz radiation fa-cility at ELBE. In the ELBE linac, the beam is accelerat-ed, gets a proper correlated energy spread, and is com-pressed in a magnetic chicane. Sub-ps pulses are obtained producing coherent diffraction radiation and superradiant undulator radiation., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

As a new electron source with higher brilliance, the second version of the superconducting RF photoinjector (SRF Gun II) has been successfully commissioned at the ELBE Center for High-Power Radiation Sources since 2014. SRF Gun II features an improved 3.5-cell niobium cavity as well as a superconducting solenoid in the same cryomodule. For user operation the SRF Gun II with Mg photocathode successfully generated stable beam with bunch charges up to 200 pC in CW mode, and with sub-ps bunch length. In this presentation the gun's status and beam parameters will be presented., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

Since 2015, Mg photocathodes have been applied for the ELBE SRF gun. Recently, user experiments with a bunch charge of 100 pC and beam transport experiments with 200 pC have been performed. The electron beam diagnostics will be presented, including the slit-scan measurements of the transverse phase space, the quadrupole-scan emittance measurement and the phase-scan bunch length measurement. Principles and results of these diagnostics will be introduced, as well as the error analysis. A UV laser is used to emit electron, the pulse length of which directly impacts the electron beam. A commercial cross correlator has been installed to measure the temporal profile of the UV laser. The alignment of the beam measurement results for this will be presented. Bases on the operation experiences from the current SRF gun, a new gun-lab has been proposed to build and test the next SRF gun in a separated room from the ELBE accelerator. The preliminary design of a new corresponding diagnostics beamline will be presented, featuring a fast beam-scan transverse phase space measurement and a transverse deflecting structure based measurement of longitudinal phase space., Proceedings of the 6th Int. Beam Instrumentation Conf., IBIC2017, Grand Rapids, MI, USA

To improve the quality of photocathodes is one of the critical issues in enhancing the stability and reliability of photo-injector systems. Presently the primary choice is to use metallic photocathodes for the ELBE SRF Gun-II to reduce the risk of contamination of the superconducting cavity. Magnesium has a low work function (3.6 eV) and shows high quantum efficiency (QE) up to 0.3 % after laser cleaning. The SRF Gun II with an Mg photocathode has successfully provided electron beam for ELBE users. However, the present cleaning process with a high intensi-ty laser (activation) is time consuming and generates unwanted surface roughness. This paper presents the investigation of alternative surface cleaning procedures, such as thermal treatment. The QE and topography of Mg samples after treatment are reported., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

An SRF gun at the ELBE center has been operated with a magnesium cathode. Electron beams were produced with a maximum bunch charge of 200 pC and an emit-tance of 7.7 μm. Simulations have been conducted with ASTRA and Elegant for applying the SRF gun to ELBE user experiments, including neutron beam generation, positron beam generation, THz radiation and Compton backscattering experiment. Beam transport has been optimized to solve the best beam performance for these user stations at the bunch charge of 200 pC. Simulation results indicate that the SRF gun is potential to benefit the high bunch charge applications at ELBE., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

For the accelerator-based light sources and the electron colliders, the development of photoinjectors has become a key technology. Especially for the superconducting radio frequency cavity based injector (SRF Gun), the searching for better photocathodes is always a principal technical challenge. To use metallic photocathodes for ELBE SRF Gun is the primary choice to prevent cavity contamination. In this contribution, we will report the investigation of Magnesium (Mg) in ELBE SRF gun, including laser cleaning treatment and the measurement on quantum efficiency, Schottky effect, dark current and damage threshold., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

RF photoelectron sources with superconducting cavities provide the potential to generate high quality, high brightness electron beams for future accelerator applications. At Helmholtz-Zentrum Dresden Rossendorf, such an electron source was operated for many years. The commissioning of an improved SRF Gun with a new high-performance gun cavity with low field emission and a superconducting solenoid inside the gun cryomodule (SRF Gun II) has started in June 2014. Simulations of the experimental set-up with ASTRA and ELEGANT were performed. First low current measurements as well as studies of unwanted beam transport using SRF Gun II with Cu photocathode and an acceleration gradient up to 9 MV/m will be presented. First beam characterization of the SRF Gun in combination with ELBE, a two-stage superconducting linear accelerator will be discussed., Proceedings of the 4th International Beam Instrumentation Conference, IBIC2015, Melbourne, Australia

In order to achieve the CW electron beam with a high average current up to 1 mA and a very low emittance of 1 μm, an improved superconducting photo-injector (ELBE SRF-Gun-II) has been installed and commissioned at HZDR since 2014. This new gun replaces the first 3.5-cell SRF gun (SRF-Gun-I) at the SC Linac ELBE. The RF performance of the niobium cavity has been evaluated, the beam parameters for low charge bunches have been measured, and the first beam has been guided into the ELBE beam line. The results agree with the simulation very well. The photocathode transfer system has been installed for the first high current beam test planned in 2015. However, the unexpected strong degradation on the cavity and also on the photocathode was found soon after the first photocathode exchange. In this contribution the results of the SRF-Gun-II commissioning and the latest experiment will be presented in detail., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

The transport and exchange technology of Cs2Te photocathode for the ELBE superconducting rf photoinjector (SRF gun) has been successfully developed and tested at HZDR. The next goal is to realize the transport of GaAs photocathode into SRF gun, which will need a new transfer system with XHV 10⁻¹¹ mbar. The key component of the system is the transfer chamber and the load-lock system that will be connected to the SRF-gun. In the carrier four small plugs will be transported, and one of them will be plug on the cathode-body and inserted into the cavity. The new transport chamber allows the transfer and exchange of plugs between HZDR, HZB and other cooperating institutes. In HZDR this transfer system will also provide a direct connection between the SRFGUN and the GaAs preparation chamber in the Elbe-accelerator hall., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

An improved SRF gun has been installed and commissioned at HZDR since 2014. This new gun replaces the first SRF gun at the superconducting linear accelerator ELBE which had been in operation since 2007. The new SRF gun II has an improved 3.5-cell niobium cavity and a superconducting solenoid inside the gun cryostat. The gun has been tested first with a Cu photocathode. Using the standard high repetition rate laser system, this allows low bunch charge beams of a few pC only. The beam parameters for these low charges have been measured, and the first beam has been guided into the ELBE accelerator. Since 2015 the photocathode transfer system is installed for future use of Cs2Te photocathodes with high quantum efficiency photocathodes., Proceedings of the 56th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, ERL2015, Stony Brook, NY, USA

In May 2014 the first SRF photo injector at HZDR has been replaced by a new gun, featuring a new resonator and cryostat. The intention for this upgrade has been to reach for higher beam energies, bunch charges and therefore an increased average beam current, which is to be injected into the superconducting, CW ELBE accelerator, where it can be used for multiple purposes, such as THz generation or Compton backscattering. Because of the increased bunch charge of this injector compared to its predecessor, it demands upgrades of the existing and/or novel approaches to alleviate the transverse emittance growth. One of these methods is the integration of a superconducting solenoid into the cryostat. Another method, the so called RF focusing, is realized by displacing the photo cathode's tip and retracting it from the last cell of the resonator. In this case, part of the accelerating field is sacrificed for a better focus of the electron bunch right at the start of its generation. Besides particle tracking simulations, a recent study, investigating on the exact position of the cathode tip with respect to the cell's back plane after tuning and cool down, has been performed., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

As in 2007 the first 3.5 cell superconducting radio frequency (SRF) gun was taken into operation at Helmholtz-Zentrum Dresden-Rossendorf, it turned out that the specified performance to realize an electron energy of 9.4 MeV has not been achieved. Instead, the resonator of the gun was limited by field emission to about one third of this value and the measured beam parameters remained significantly below its expectations. However, to demonstrate the full potential of this electron source for the ELBE linear accelerator, a second and slightly modified SRF gun was developed and built in collaboration with Thomas Jefferson National Accelerator Facility. We will report on commissioning of this new SRF gun and present a full set of RF performance results. Additionally, investigations are shown that try to explain a particle contamination that happened recently during our first cathode transfer., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

The SRF gun has been successfully operated for the radiation source ELBE at HZDR. To achieve higher current and lower beam emittance, a new niobium cavity with superconducting solenoid and a new 13 MHz laser have been recently developed. For this reason, better photocathodes with high quantum efficiency are urgently in demand. In this work we improve the present Cs2Te preparation system for cleaner environment and more precise stoichiometric control than before. A new mask is designed to prevent cesium pollution of the cathode body. Instead of Kapton only alumina ceramics are used for isolation, and the cathode plugs are degassed at higher temperature. New evaporators are installed and tested to obtain an accurate deposition rate. Furthermore, the cathode transfer system is thoroughly cleaned for a better vacuum condition., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

At HZDR a preparation chamber for NEA-GaAs (Cs, O) has been built and commissioned. GaAs is the next photocathode material for the ELBE SRF gun, which has been successfully operated with Cs2Te layer in last years. GaAs At HZDR a preparation chamber for NEA-GaAs (Cs, O) has been built and tested. GaAs is the next photocathode material for the ELBE SRF gun, which has been successfully operated with Cs2Te photocathode in last years. GaAs photocathodes are advantageous because of their high quantum efficiency (QE) with visible light and the extensive experiences of their use in DC guns. Furthermore, GaAs photocathodes provide the possibility to realize a polarized SRF gun in the future. In this presentation we will introduce the new preparation system and the first results of the GaAs tests. The new transfer system under construction will be also presented., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

By combining the code of ASTRA and elegant in a user-friendly interface, a simulation tool is developed for the ELBE SRF Gun II. The photoelectric emission and first acceleration to several MeV in the gun cavity are simulated by ASTRA with a 1D Model, where the space charge effect is considered. The dependence of the beam quality on key parameters is studied, and a compromised optimization for a 77 pC beam is used for further elegant simulation of the beam transport through a dogleg and ELBE Linacs. Proper settings of the magnets and RF phases are the main targets of improving the beam quality. Up to now the best simulation result is an electron bunch with the energy of 47 MeV, energy spread of 66 keV, bunch length of 0.35 ps and transverse emittance of 1.9 μm and 2.7 μm in the two perpendicular directions., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Superconducting RF particle sources combine the advantages of normal conducting RF sources and high duty cycle non-RF sources. The Rossendorf SRF gun was the first to demonstrate this injecting electrons into the ELBE accelerator at 13 MHz. Recently, a new 3-1/2-gun cavity has been prepared at Jefferson Lab for its use in an updated injector which is expected to increase the electron energy from 2.4 to 7.5 MeV. Along with this new cavity, a new gun cryostat has been introduced. It combines several minor updates to the setup with the installation of a superconducting solenoid right at the exit of the gun, compensating the emittance growth of the electron bunch at an early stage. The poster is going to conclude the results of the commissioning of the new cryostat including the solenoid and compare it to the prior concept using a normal conducting solenoid outside the cryostat. As it is of great importance to this subject, studies of the magnetic shielding are going to be presented as well., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany