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ELBE is a compact, accelerator-driven photon and particle source. The variety of secondary radiation being offered extends from high-energy gamma rays to infrared and THz radiation as well as from neutrons to positrons and electrons. Since 2001 ELBE is operated as a user facility, providing more than 5500 hours of beamtime with an efficiency of more than 90% each year. The electron accelerator is based on four superconducting 9-cell TESLA cavities that are driven in CW operation to accelerate an average current of 1 mA up to beam energies of 40 MeV. In addition an upgraded 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, so that now more than 1800h of user beam are generated by this unique CW electron source every year. The talk will summarize our experiences of operating all our SRF cavities over two decades in CW. In detail, this includes the cavity performance and attempts to improve it, as well as investigations on their limitations. Additionally, we will discuss several issues that are related to the high average RF as well as beam power and we will present appropriate measures to protect the machine. In this regard we will also introduce a resonant ring for RF component tests at CW power levels up to 100 kW. Regarding the SRF gun, the main emphasis lies in seamlessly integrating a normal-conducting photocathode into the SRF cavity, alongside addressing associated intricacies like dark current, multipacting, and contamination of the resonator.

In the high gradient rf photoinjectors, dark current is the “unwanted beam” not produced by the cathode drive laser. It is a part of field emission from the cavity and photocathode, which is accelerated through the gun. Dark current can cause beam loss, increase the risk of damage to accelerator components, and create additional background for beam users. Furthermore, during operation of the ELBE srf gun, the dark current has been found to correlate with the photocathode QE and life time. Therefore, understanding the sources as well as the dynamics of dark current is crucial to machine safety and gun quality. In this paper we present our experimental investigations of the dark current at the ELBE SRF gun-II. The beam dynamics of the dark current is studied with the ASTRA code, which helps us to track the field electrons starting from the cathode area and from other sources, so that we can understand their different contributions to the dark current.

As an excellent demonstration of SRF gun technology, HZDR's SRF Gun-II has been rountinely operated as an electron gun for the ELBE THz radiation and the ELBE neutron source since 2018, delivering CW beams with bunch charges of up to 300 pC at 50-100 kHz. As is well known, the quality of the photocathodes is crucial for the stability and reliability of the gun operation. Different cathodes have been used in the HZDR SRF gun and the cathodes have been replaced three to four times per year. In the last five years, the quality of the superconducting cavity has remained almost stable, but some characteristics of the gun have been observed related to the cathodes. In this contribution, we will introduce the operational aspect of the photocathodes in HZDR SRF gun, look back at the achievements and discuss open issues and possible improvements for future application.

Superconducting radio frequency photo injectors (SRF gun) offer advantages for operating in continuous wave (CW) mode and generating high-brightness and high-current beams. A new SRF gun is designed as a low emittance photo injector for LCLS-II-HE and a prototype gun is currently being developed under collaboration between SLAC, FRIB, HZDR and ANL. The aim is to demonstrate stable CW operation at a cathode gradient of 30 MV/m. One of the crucial components for successful SRF gun operation is the photocathode system. The new SRF gun will adopt the HZDR-type cathode, which includes a cathode holder fixture (cathode stalk) developed by FRIB and a cathode exchange system designed by HZDR. This innovative cathode insertion system ensures accurate, particle-free and warm cathode exchanges. A novel alignment process targets the cathode to the stalk axis without touching cathode plug itself. To commission the prototype gun, metallic cathodes will be used. A specifically designed vacuum system ensures vacuum pressure of 10-9 mbar for transport of a single cathode from the cleanroom to the gun.

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, so that now more than 1800h of user beam are generated every year. Since the commission, a total of 24 cathodes (2 Cu, 12 Mg, 10 Cs2Te) have been used, without observing serious cavity degradation. The contribution summarized commissioning and operational experience of the last years, with special emphasis on SRF properties but also on specialties such as dark current and multipacting that are directly linked to the integration of a normal conducting cathode into the SRF cavity.

A dependable and consistent electron source is a crucial requirement for the achievement of high-power free electron lasers (FELs). Over the past two decades, it has been demonstrated that photoinjectors based on SRF technology (SRF guns) are suitable for continuous wave (CW) beam generation. SRF guns possess both the high accelerating field gradients of normal conducting RF photoinjectors and the low power dissipation thanks to mature SRF cavity technology, and therefore have the potential to provide the high-brightness, high-current beams required for CW-XFELs. After the demonstration of the first SRF gun in Dresden-Rossendorf, several SRF gun programs based on different approaches have achieved promising progress and even succeeded in routine operation. SRF guns are expected to play an important role in XFEL facilities in the near future. In this paper, we give an overview of design concepts, important parameters and development status of the worldwide SRF gun projects.

Various continuous-wave (CW) electron gun technologies are reviewed, including DC, superconducting radio frequency RF (SRF), hybrid DC-SRF and normalconducting RF. Also, the SLAC Linac Coherent Light Source II (LCLS-II) normal-conducting RF gun and injector are described, and the performance to date, including the bunch emittance achieved and the dark current observed, is presented.

The quality of the photocathodes is critical for the stability and reliability of the photoinjector’s operation. Thanks to the robust magnesium and Cs2Te photocathodes, SRFgun-II at HZDR has been proven to be a successful example in CW mode for high current user operation. In this contribution, we will present our observation of the QE evolution of Cs2Te photocathodes during SRF gun operation. The variables including substrate surface, film thickness, Cs/Te stoichiometric, multipacting, RF loading and charge extract are considered in the analysis.

At the electron accelerator for beams with high bril-liance and low emittance (ELBE), the second version of a superconducting radio-frequency (SRF) pho-toinjector 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 now more than 3400h of user beam have already been gen-erated since 2019. During this time, a total of 20 cath-odes (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 main RF properties of the gun cavity.

As well known, the quality of the photocathodes is essential for the stability and the reliability of photo injector operation. Especially for the superconducting ratio frequency photo injectors (SRF guns), the photocathode represents one of the most critical parts. Benefit from the fast de-veloping photocathode technology in last years, several SRF guns were successfully operated or tested for the beam generation at kHz - MHz repetition rate. In this paper, we will review the achievements as well as the open questions in the applications of the photocathodes for SRF gun operation. Furthermore, we will discuss the possible improvement from cathodes side for the future CW electron sources.

Recent progress on advanced photocathodes for superconducting RF guns. We will together view the achievements as well as open questions for the reliable gun operation.

The contribution highlights the developments of superconducting radio frequency photo electron source (SRF gun) in Germany. The success at HZDR, as well as the progress at HZB and DESY are discussed. The presentation completes with detailed beam dynamics simulations for the typical bunch charges and an gives perspectives of high charge (1 nC) operation.

The SRF gun-II at HZDR has been stably applied as the electron source for high power THz radiation since 2018, generating CW beams with bunch charges up to 300 pC at 100 kHz. It is an excellent demonstration that SRF guns can work reliably in a high power user facility. As well known, that the quality of the photocathodes is critical for the stability and reliability of the gun operation. In last years, thank to the successful ps UV laser cleaning, Mg photocathodes were successfully used for gun operation at kHz repitition rate. In order to satisfy user request of MHz operation, Cs2Te is still in demand. However, the Mo plugs were overheated in superconducting rf cavity due to thermal contact problem, and SRF Gun-II faced twice serious contamination in 2017. The Cs2Te could not be used in gun untill Cu is adopted in stead of Mo as new substratum. Up to now three Cs2Te cathodes on Cu plugs have been applied in the Gun-II. In this contribution, we will present the operational aspect of the photocathodes for SRF gun, and discuss the possible improvement in the future application.

The success of proposed high power free-electron lasers (FELs) and energy recovery linac (ERL) largely depends on the development of the electron source, which requires the best beam quality and CW operation. An elegant way to realize both high brilliance and high current is to com-bine the high beam quality of the normal conducting radio frequency photoinjector with the quick developing superconducting radio frequency technology, to build superconducting rf photoinjectors (SRF guns). In last decade, several SRF gun programs based on dif-ferent approaches have achieved promising progress, even succeeded in routine operation at BNL and HZDR. In the near future SRF guns are expected to play an im-portant role for hard X-ray FEL facilities. In this contribu-tion, we will review the design concepts, parameters, and status of the major SRF gun projects.

SRF injectors need materials which can promise high quantum efficiency, long lifetime and good vacuum stability, fast response time and low thermal emittance. It is assumed that GaN, like GaAs, as a novel electron source for particle accelerators shows an enormous potential. P-type GaN on different substrate material is activated by a thin layer of caesium and illuminated at the same time by ultra-violet (UV) light. As a consequence of a NEA surface and photoeffect, the generated photoelectrons enter into vacuum and are collected by an anode. The resulting photocurrent is detected during the whole activation process and stopped when a maximum photocurrent is reached. Quantum efficiency (QE) can be calculated from the photocurrent. Its decay is tracked in the following days after activation. It is also studied to re-activate the sample again by using thermal heat treatment and caesium Deposition once more. High 11.5% QE can be provided from GaN:Cs on sapphire substrate at the moment. After 600 h of lifetime this photocathode shows still 1.4% QE. Besides the outstanding experimental results so far further investigations are still required.

A short report on experiences with UV photocathode lasers and laser beam line for ELBE SRF Gun in 2020-2021

Negative electron affinity (NEA) GaAs- and GaN-based photocathodes are used in modern night vison detectors and light emitting diodes1. GaAs semiconductors are already used as electron sources in particle accelerators and well- studied2. Like GaAs, GaN belongs to the III-V semiconductor group with similar properties. It is assumed that GaN, like GaAs, shows enormous potential as a novel electron source for particle accelerators. P-type GaN on different substrate material (sapphire, silicon, copper or SiC) is activated by a thin layer of caesium and illuminated by ultra-violet (UV) light at the same time. As a consequence of negative electron affinity (NEA) and photoeffect, the generated photoelectrons enter into vacuum and are collected by a copper ring anode. The resulting photocurrent is detected during the whole activation process and stopped when a maximum photocurrent is reached. The GaN is chemical cleaned and transferred into a UHV chamber where it undergoes a thermal heat treatment at 250°C for 20 min using a halogen lamp. The aim of the thermal treatment is to remove residual adsorbed gas molecules from the sample surface. Afterwards when the sample is back at room temperature, the thermal-cleaned GaN is activated with a thin layer of caesium. The photocurrent and the QE is observed in the following days until the QE vanishes. Then it is tested to re-activate the cathode again, meaning to thermal clean it again and to activate it with caesium once more. By a comparison of differences in substrate material, chemical pre-cleaning, thermal heat treatment and activation parameters (e.g. caesium-flux), the photocurrent, quantum efficiency and the re-activation of the photocathode is studied. Additionally the GaN samples are examined by AFM, SEM and EDX. From the experimental results obtained so far, it appears that GaN:Cs could be used as a photocathode in particle accelerators, but further investigations are still required and needed.

The SRF gun-II at HZDR has been stably applied as the electron source for high power THz radiation since 2018, generating CW beams with bunch charges up to 300 pC at 100 kHz. It is an excellent demonstration that SRF guns can work reliably in a high power user facility. In order to generate higher current beam with MHz repetition rate, Cs2Te photocathodes are required. However, in last two experiments with Cs2Te, the Mo substrate plugs were overheated in superconducting rf cavity. The reason is that different thermal expansion coefficient of Mo and Cu led to a bad thermal contact between the Mo plug and Cu holder. Thus we decided to use Cu as new substratum of Cs2Te cathodes. In last year we prepared several Cs2Te photocathodes on Cu plugs and improved the vacuum of cathode transfer system in order to achieve satisfied lifetime. In this contribution, we will present the study result of QE and life time of Cs2Te photocathodes with different thickness.

Magnesium bulk cathodes and caesium telluride cathodes work routinely in SRF Gun II at ELBE. However, the particle accelerator community is always looking for new materials which can promise higher quantum efficiency, longer lifetime and good vacuum stability, fast response time and low thermal emittance. P-type GaN on different substrate material is activated by a thin layer of caesium and illuminated at the same time by ultra-violet light. As a consequence of a negative electron affinity and photoeffect, the generated photoelectrons enter into vacuum and are collected by a copper ring anode. The resulting photocurrent is detected during the activation process and stopped when a maximum photocurrent is reached. Quantum efficiency can be calculated from the photocurrent and its decay is tracked in the following days after activation. By a comparison of differences in substrate material, chemical pre-cleaning, thermal heat treatment and activation parameters (e.g. caesium-flux), the photocurrent, quantum efficiency and its decay is studied.

At the electron accelerator for beams with high bril-liance and low emittance (ELBE), the second version of a superconducting radio-frequency (SRF) pho-toinjector 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 now more than 3400h of user beam have already been gen-erated since 2019. During this time, a total of 20 cath-odes (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 main RF properties of the gun cavity.

The success of proposed high power free-electron lasers (FELs) and energy recovery linac (ERL) largely depends on the development of the electron source, which requires the best beam quality and CW operation. An elegant way to realize both high brilliance and high current is to com-bine the high beam quality of the normal conducting radio frequency photoinjector with the quick developing superconducting radio frequency technology, to build superconducting rf photoinjectors (SRF guns). In last decade, several SRF gun programs based on dif-ferent approaches have achieved promising progress, even succeeded in routine operation at BNL and HZDR. In the near future SRF guns are expected to play an im-portant role for hard X-ray FEL facilities. In this contribu-tion, we will review the design concepts, parameters, and status of the major SRF gun projects.

The photocathodes determine the beam quality in linear accelerators and represent a key component for many accelerator projects. Free-electron lasers (FEL), synchrotron- and THz radiation sources require injector systems with high brightness electron beams. High quantum efficiency, a long lifetime and good vacuum stability, fast response time and low thermal emittance are desirable parameters for a perfect photocathode used in accelerators. Semiconductors such as GaN and GaAs as novel materials for photocathodes are showing an enormous potential. GaAs is a well-known material for photocathodes. After activation with caesium and oxygen, it has a high QE for visible light (red or green). An advantage of GaAs is the opportunity of the layers to emit spin-polarized electrons. GaN is a semi-conductive material and well known for its high QE when lighted with UV light. For improving the QE only caesium for activation is required. At the moment GaN is used for photocathode-based detectors such as photomultipliers or phototubes and for LEDs. They have characteristics of low dark current, high-speed response and high sensitivity. It is very new for application in SRF Guns. It seems to be more robust and achieves higher QE than other photocathodes [1]. Crystallinity and surface parameters define the photoemission properties. Modern analytical methods are used for identification of impurities, dislocations and characterization of the crystallinity of the semiconductors and the right cleaning treatment as well as the right caesium rating. [1] Uchiyama, Shoichi et al. 2011. “GaN-Based Photocathodes with Extremely High Quantum Efficiency” 103511(2005):1–4.

In 2014, the second-generation of the ELBE SRF gun replaced its predecessor, which had been in operation since 2007. In the first two years, copper and magnesium cathodes were initially used without any discernible problems. However, after switching to Cs2Te in 2017, it was found that the layers of two cathodes evaporated within a few days during RF operation in the Gun. Since this was never observed in Gun I, an extensive root cause search was conducted using a dedicated cathode test setup. The findings pointed to loose thermal contact between the cathode plug and the cathode body and ultimately resulted in a change of the used cathode substrate from molybdenum to copper. Although this was accompanied by a lower quantum efficiency of about 5% after preparation, it stabilized to 1-2% during beam operation in the SRF gun. As of May 2020, three of these cathodes have now been successfully used for THz user beam time and a total charge of 26 C has been extracted. Together with the electrons still produced by Mg cathodes in 2019/2020, a total of 217 user shifts could be served and 2600h hours of beam time were delivered. This demonstrates the reliability of CW SRF in combination with normal conducting cathode and is so far unique in the world. During the talk, the reason for overheating, the preparation on Cu substrate as well as experiences from the past two years of user beam operation will be presented in detail.

Recent progress on advanced photocathodes for superconducting RF guns. We will together view the achievements as well as open questions for the reliable gun operation.

The SRF Gun has been running stabile using a magnesium cathode in the last year. Over 200 hours beam time have been provided in CW operation in 2019. The magnesium bulk cathodes work routinely in ELBE and are polished and chemical cleaned before inserting them into the SRF Gun II, where they are again cleaned with an UV drive laser. Magnesium cathodes derives usually quantum effeciencies (QE) between 0.3 to 0.5% in SRF Gun II and offer a low risk of contaminations and an extreme long lifetime. The UV drive laser cleaning can be repeated several times to guarantee an high quality working cathode. However, the particle accelerator community is always looking for new materials which can promise high quantum efficiency, a long lifetime and good vacuum stability, fast response time and low thermal emittance. Semiconductors such as GaN as novel materials for photocathodes are showing an enormous potential. GaN is a semi-conductive material and well known for its high QE when illuminated with UV light. For the activation only caesium is required. At the beginning of 2020 the first activation of GaN on sapphire substrate was successfully done. At first the GaN is heat treated at 610°C for 15 min and then activated with caesium to form a negative electron affinity surface. With 0.5 % quantum efficiency the first activation is all in all a successfully step for further promising GaN photocathodes.

As a new electron source with higher brilliance, the second superconducting RF photoinjector (SRF Gun II) has been built at the ELBE radiation center for high power radiation sources. One of the main goals of SRF gun II is to achieve a higher bunch charge (>200 pC) and lower emittance (3 mm mrad) than the present ELBE thermionic DC gun. SRF Gun II features a 3.5-cell niobium cavity as well as a superconducting solenoid in the same cryomodule. With Mg photocathodes the gun is able to provide medium current beam with bunch charge of more than 200 pC and sub-ps bunch length at 100 kHz repetition rate. With this contribution we present convincing results from long-term user operation of SRF gun II in combination with the bunching concept of the ELBE accelerator in order to produce THz radiation with much higher stability and power than available using the existing thermionic gun.

At present, ELBE radiation source at HZDR is optimizing the SRF cavity for the next generation ELBE SRF GUN. This paper presents a preliminary study on the geometry optimization of a 3.5-cell SRF gun cavity based on beam dynamics. By changing the lengths of the half cell and the first TESLA like cell, two new cavity models with higher electric field in the half cell are built and their RF fields are compared with SRF GUN I and SRF GUN II. Through the scanning of the RF phases and the electric fields, the simulation results indicate that new models have smaller transverse emittance at relatively lower electric field gradients and better performance on longitudinal emittance than SRF GUN I and SRF GUN II.

Quality of photocathode in a photoinjector is one of the critical issues for the stability and reliability of the whole accelerator facility. In April 2013, the IR FEL lasing was demonstrated for the first time with the electron beam from the SRF gun with Cs2Te at HZDR. Cs2Te photocathode worked in SRF gun-I for more than one year without degradation. Currently, Mg photocathodes with QE up to 0.5% are applied in SRF Gun-II, generating e- beam with bunch charge up to 300 pC in CW mode with sub-ps bunch length for the high power THz radiation. It is an excellent demonstration that SRF guns can work reliably in a high power user facility.

In order to optimize the next generation SRF gun at HZDR ELBE radiation source, the impact on beam dynamics from the SRF cavity geometry needs to be investigated. This paper presents an analysis on the electromagnetic fields and output electron beam qualities, by changing the geometry parameters of a 3.5-cell SRF gun cavity. The simulation results show the higher electric field ratio in the first half cell to the TESLA like cell, the better beam parameters we can obtain, which, however, will also lead to a higher Emax/E0 and Bmax/E0.

As a new electron source with higher brilliance, the second superconducting RF photoinjector (SRF Gun II) has been built at the ELBE radiation center for high power radiation sources. One of the main goals of SRF gun II is to achieve a higher bunch charge (>200 pC) and lower emittance (3 mm mrad) than the present ELBE thermionic DC gun. SRF Gun II features a 3.5-cell niobium cavity as well as a superconducting solenoid in the same cryomodule. With Mg photocathodes the gun is able to provide medium current beam with bunch charge of more than 200 pC and sub-ps bunch length at 100 kHz repetition rate. With this contribution we present convincing results from long-term user operation of SRF gun II in combination with the bunching concept of the ELBE accelerator in order to produce THz radiation with much higher stability and power than available using the existing thermionic gun.

we present here the status of ELBE SRF gun II and the application of Mg photocathodes in SRF gun. In order to improve the QE of Mg cathodes, we appy the ps- UV laser cleaning for bulk Mg photocathodes. Furthermore, several alternative preparation methods of Mg are also under studying.