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To improve the quality of photocathodes is one of the critical issues in enhancing the stability and reliability of photo-injector systems. Magnesium has a low work function (3.6 eV) and shows high quantum efficiency (QE) after proper surface cleaning. This paper presents the investigation of alternative surface cleaning procedures, such as ps laser cleaning, thermal cleaning and ion beam cleaning. The QE is able to be improved two magnitudes after the treatment.

Superconducting radio frequency (SRF) photoinjectors offer advantages for continuous wave (CW) operation and high brightness, high current beam generation. One of the critical components for successful operation of SRF photoinjectors is the photocathode system. HZDR is building a sophisticated cathode exchange system to ensure accurate, particle-free and warm cathode exchange. A novel alignment process aligns the cathode to the gun axis without touching the cathode plug itself. Less than 10 particles as small as 0.3 micrometer are detected during the cathode load-lock tests.

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.

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.

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.

As well known, the quality of the photocathodes is critical for the stability and reliability of photo-injector operation. Especially for the superconducting rf guns, the photocathode is one of the most important parts. In last years, thanks to the developed photocathode technology, several SRF guns were successfully operated or tested for the beam generation at kHz-MHz repetition rate. In this review, the achievements as well as open questions for the cathode requirements of the reliable SRF gun operation will be reviewed, and the possible improvement from photocathodes point of view for the future application will be discussed.

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.

A superconducting radio-frequency (SRF) photo injector is in operation at the electron linac for beams with high brilliance and low emittance (ELBE) radiation center and generates continuous wave (CW) electron beams with high average current and high brightness for user operation since 2018. The speed of emittance measurement at the SRF gun beamline can be increased by improving the slit-scan system, thus the measurement time for one phase space mapping can be shortened from about 15 min to 90 s. The convolution neural networks are applied to improve the efficiency and accuracy of beamlet images processing. In order to estimate the uncertainty in the calculation of normalized emittance, we analyze the main error contributions.

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.

The coherent infrared and THz sources driven by the superconducting electron accelerator ELBE are described. The present status of the facility is summarized and a few scientific highlights are mentioned. Finally plans for a successor facility (Dresden Advanced Light Infrastructure, DALI) are outlined along with the most important scientific and technological challenges.

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.

This work discusses the influence of different cleaning procedures on p-GaN grown on sap-phire by metal-organic chemical vapor deposition. The cleaned p-GaN surface was trans-ferred into an ultra-high vacuum chamber and studied by an X-ray photoelectron spectrome-ter, revealing that a cleaning with a so-called „piranha“ procedure results in low carbon and oxygen concentrations on the p-GaN surface. Contrary, a cleaning that solely uses ethanol represents a simple cleaning, but leads to an increase of carbon and oxygen contaminations on the surface. Afterward, the cleaned p-GaN samples underwent a subsequential vacuum thermal cleaning at various temperatures to achieve an atomically clean surface. XPS meas-urements revealed residual oxygen and carbon on the p-GaN surface. Thus, a thermal treat-ment under a vacuum did not entirely remove these organic contaminations, although the thermal cleaning reduced their peak intensities. The complete removal of carbon and oxygen contaminants was only achieved by argon ion sputtering, which is accompanied by a strong depletion of the nitrogen on the p-GaN surface. The treatments cause a large number of sur-face defects preventing the formation of a negative electron surface when the p-GaN is acti-vated with a thin layer of cesium.

A superconducting radio-frequency (SRF) photo injector is in operation at the electron linac for beams with high brilliance and low emittance (ELBE) radiation center and generates continuous wave (CW) electron beams with high average current and high brightness for user operation since 2018. The speed of emittance measurement at the SRF gun beamline can be increased by improving the slit-scan system, thus the measurement time for one phase space mapping can be shortened from about 15 min to 90 s. The convolution neural networks are applied to improve the efficiency and accuracy of beamlet images processing. In order to estimate the uncertainty in the calculation of normalized emittance, we analyze the main error contributions.

This paper compares different photocathodes that are applicable for electron injector systems and summarizes the development in cathode technology in the last years. The photocathode is one of the key components of the facilities that provides electrons for many research experiments. Typically, a high efficiency and a long operation time are desired, thus the photocathode needs to be robust against any rest gases occasionally available during operation. Low thermal emittance and fast response time are special requirements for the accelerator community. These parameters are commonly used to compare the various cathode materials. Metals and plasmon-enhanced materials emit electrons from the near surface, whereas semiconductors emit photoelectrons mostly from the bulk region. We compare metal photocathodes such as magnesium, copper and lead, with semiconductor photocathodes such as cesium telluride, antimonide photocathodes and III-V semiconductor photocathodes. GaAs and its typical application for the generation of spin-polarized electrons is discussed and special attention has been paid to the emerging GaN as a potential novel photocathode. The above mentioned state-of-the-art cathodes are compared regarding their preparation approaches, quantum efficiency, lifetime, response time and their status of application. This work is aimed to provide a guideline for particle accelerator researchers in their choice of the cathode material. Thermionic cathodes and field emission cathodes are not discussed in this review.

This study shows residual surface-C’s influence on photocathodes’ quantum efficiency based on p-GaN grown on sapphire by metal organic chemical vapor deposition. An X-ray photoelectron spectrometer (XPS) built in an ultrahigh vacuum system allowed the in-situ monitoring of the photocathode surface beginning immediately after their cleaning and throughout the activation and degradation processes. An atomically clean surface is necessary to achieve a negative electron affinity, which is the main prerequisite for high quantum efficiency. The p-GaN samples were cleaned with ethanol and underwent a sub-sequential thermal vacuum cleaning. Although carbon and oxygen contaminations, undesired impurities from the metal organic chemical vapor deposition, remained on the surface, p-GaN could still form a negative electron affinity surface when exclusively activated with cesium. After the activation with cesium, a shift to a higher binding energy of the photoemission peaks was observed, and a new species, a so-called cesium carbide, was formed, growing over time. The XPS data allowed elucidating the critical role of these cesium carbide species in photocathode degradation. The X-ray damage to the p-GaN:Cs photocathodes, especially the influence on the cesium, was additionally discussed.

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.

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 summarizes commissioning and operational experience of the last 8 years of gun operation, 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.

As well known, the quality of the photocathodes is critical for the stability and reliability of photo-injector operation. Especially for the superconducting rf guns, the photocathode is one of the most important parts. In last years, thanks to the developed photocathode technology, several SRF guns were successfully operated or tested for the beam generation at kHz-MHz repetition rate. In this review, the achievements as well as open questions for the cathode requirements of the reliable SRF gun operation will be reviewed, and the possible improvement from photocathodes point of view for the future application will be discussed.

This folder "XPS data" contains original and evaluated XPS data (.vms) on a p-GaN sample which was treated at various temperatures and underwent Ar+ irradiation. Furthermore, the folder "REM Images" contains REM images (.tif) and EDX data (.xlsx) on the used excessively treated sample. All images that are published in the main manuscript are collected as .tif files in the folder "images".

The LCLS-II-HE Project at SLAC seeks to increase the photon energy reach of the LCLS-II FEL to at least 20 keV. In addition to upgrading the undulator system, and increasing the electron beam energy to 8 GeV, the project will also construct a low-emittance injector (LEI) in a new tunnel. To achieve the LEI emittance goals, a low-MTE photocathode will be required, as will on-cathode electric fields up to 50% higher than those achievable in the current LCLSII photoinjector. The beam source for the LEI will be based around a superconducting quarter-wave cavity resonant at 185.7 MHz. A prototype gun is currently being designed and fabricated at the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU). This paper presents performance goals for the new gun design, an overview of the prototype development effort, status, and future plans including fabrication of a “production” gun for the LEI.

Achieving an atomically clean surface is an important step to improving the quality of semiconductor photocathodes, but it is a challenging requirement for surface treatment [1]. In order to understand the surface during the cleaning, the cesium deposition, and the storage of the photocathode, the use of an x-ray photoelectron spectrometer (XPS) is needed. The XPS probes the electronic structure of the p-doped gallium nitride (GaN) photocathode after each step of the preparation process. Using energies between 1200-0 eV the core levels of Ga, N, O, C and Cs are monitored.

The quality of the photocathodes is critical for the stable operation of the electron sources of the accelerator-based light sources (for example X-ray FEL at DESY [1], THz at ELBE [2], etc). In this field, high quantum efficiency (QE) and long operation lifetime are the most important parameters for the ideal photocathode materials. Cs2Te photocathodes have been successfully applied in the e- guns at DESY and in the superconducting RF gun at HZDR. Recently GaN as a new photocathode candidate has been paid more and more attention because of its high QE and robustness. However, even under the extremly high vacuum environment, photocathodes lose QE slowly and change the QE distribution during the operation. In this contribution, we will present the surface study on the Cs2Te and GaN photocathodes, including the QE evolution during operation (see Fig. 1) and the XPS analysis of the 'used' photocathodes (see Fig. 2). Several possible reasons leading degradation of those photocathodes might be: 1. rest gases during operation react with the sensitive alkali-rich surface; 2. the low energy ions, ionized rest gas molecules by the electrons, reach the cathode surface and react with the surface.

The LCLS-II-HE Project at SLAC seeks to increase the photon energy reach of the LCLS-II FEL to at least 20 keV. In addition to upgrading the undulator system, and increasing the electron beam energy to 8 GeV, the project will also construct a low-emittance injector (LEI) in a new tunnel. To achieve the LEI emittance goals, a low-MTE photocathode will be required, as will on-cathode electric fields up to 50% higher than those achievable in the current LCLSII photoinjector. The beam source for the LEI will be based around a superconducting quarter-wave cavity resonant at 185.7 MHz. A prototype gun is currently being designed and fabricated at the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU). This paper presents performance goals for the new gun design, an overview of the prototype development effort, status, and future plans including fabrication of a “production” gun for the LEI.

Superconducting radio-frequency (SRF) electron guns are attractive for delivery of beams at a high bunch repetition rate with a high accelerating field. An SRF gun is the most suitable injector for the high-energy upgrade of the Linac Coherent Light Source (LCLS-II-HE), which will produce high-energy Xrays at high repetition rate. An SRF gun is being developed for LCLS-II-HE as a collaborative effort by FRIB, HZDR, ANL, and SLAC. The cavity operating frequency is 185.7 MHz, and the target accelerating field at the photocathode is 30 MV/m. The photocathode is replaceable. The cathode is held by a fixture ('cathode stalk') that is designed for thermal isolation and particle-free cathode exchange. The stalk must allow for precise alignment of the cathode position, cryogenic or room-temperature cathode operating temperature, and DC bias to inhibit multipacting. We are planning a test of the stalk to confirm that the design meets the requirements for RF power dissipation and biasing. In this presentation, we will describe the cathode stalk design and RF/DC stalk test plan.

Superconducting radio-frequency (SRF) electron guns are attractive for delivery of beams at a high bunch repetition rate with a high accelerating field. An SRF gun is the most suitable injector for the high-energy upgrade of the Linac Coherent Light Source (LCLS-II-HE), which will produce high-energy Xrays at high repetition rate. An SRF gun is being developed for LCLS-II-HE as a collaborative effort by FRIB, HZDR, ANL, and SLAC. The cavity operating frequency is 185.7 MHz, and the target accelerating field at the photocathode is 30 MV/m. The photocathode is replaceable. The cathode is held by a fixture ('cathode stalk') that is designed for thermal isolation and particle-free cathode exchange. The stalk must allow for precise alignment of the cathode position, cryogenic or room-temperature cathode operating temperature, and DC bias to inhibit multipacting. We are planning a test of the stalk to confirm that the design meets the requirements for RF power dissipation and biasing. In this presentation, we will describe the cathode stalk design and RF/DC stalk test plan.

The LCLS-II-HE Project at SLAC seeks to increase the photon energy reach of the LCLS-II FEL to at least 20 keV. In addition to upgrading the undulator system, and increasing the electron beam energy to 8 GeV, the project will also construct a low-emittance injector (LEI) in a new tunnel. To achieve the LEI emittance goals, a low-MTE photocathode will be required, as will on-cathode electric fields up to 50% higher than those achievable in the current LCLSII photoinjector. The beam source for the LEI will be based around a superconducting quarter-wave cavity resonant at 185.7 MHz. A prototype gun is currently being designed and fabricated at the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU). This paper presents performance goals for the new gun design, an overview of the prototype development effort, status, and future plans including fabrication of a “production” gun for the LEI.

The LCLS-II-HE Project at SLAC seeks to increase the photon energy reach of the LCLS-II FEL to at least 20 keV. In addition to upgrading the undulator system, and increasing the electron beam energy to 8 GeV, the project will also construct a low-emittance injector (LEI) in a new tunnel. To achieve the LEI emittance goals, a low-MTE photocathode will be required, as will on-cathode electric fields up to 50% higher than those achievable in the current LCLSII photoinjector. The beam source for the LEI will be based around a superconducting quarter-wave cavity resonant at 185.7 MHz. A prototype gun is currently being designed and fabricated at the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU). This paper presents performance goals for the new gun design, an overview of the prototype development effort, status, and future plans including fabrication of a “production” gun for the LEI.

An x-ray photoelectron spectrometer (XPS) is used in the HZDR photocathode lab to understand the surface states of GaN photocathodes during its cleaning, cesium activation and degradation. The XPS probes the electronic structure of the p-doped GaN photocathode after each step of the preparation process. Using energies between 1200-0 eV the core levels and auger photoemission peaks of Ga, N, O, C and Cs are monitored. In our experiments, p-GaN on sapphire samples were cleaned with 99 % ethanol in an ultrasonic bath, followed by a thermal cleaning in a vacuum with the intention to remove carbon and oxygen contaminations on the p-GaN surface. Although still some carbon remained on the surface, the p-GaN was successfully activated by the deposition of a thin layer of cesium. Quantum efficiencies (QE) of 3 - 9 % were achieved. XPS photoemission spectra show a shift towards higher binding energies for the photoemission peaks, which is caused by a new component, so-called organic – cesium. During the storage under ultra – high vacuum, the GaN:Cs photocathodes were measured from time to time in the photocurrent and by XPS. We found a shift of 0.35 eV towards lower binding energies, which is related to the formation of the organic – cesium islands. This island growth is assumed to be in close correlation to the photocathode degradation. The p-GaN:Cs photocathodes showed a big QE loss after XPS analysis and therefore we investigated the potential damage from x-ray irradiation. The long-time irradiation experiments show that the x-ray damage has a high influence on the cesium component and the degradation of the p-GaN:Cs photocathode.

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.

The quality of the photocathodes is critical for the stable operation of the photoinjector. Thanks to the robust Cs2Te photocathodes, SRF gun at HZDR has been proven to be a type of successful CW e- source. In this contribution, we will present the operation experience of Cs2Te photocathodes in SRF gun, especially the QE evolution of Cs2Te photocathode during user operation. The possible reason for QE degradation will be discussed.

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.

Achieving an atomically clean surface is an important step to improving the quality of semiconductor photocathodes, but it is a challenging requirement for surface treatment [1]. In order to understand the surface during the cleaning, the cesium deposition, and the storage of the photocathode, the use of an x-ray photoelectron spectrometer (XPS) is needed. The XPS probes the electronic structure of the p-doped gallium nitride (GaN) photocathode after each step of the preparation process. Using energies between 1200-0 eV the core levels of Ga, N, O, C and Cs are monitored.

An x-ray photoelectron spectrometer (XPS) is used in the HZDR photocathode lab to understand the surface states of GaN photocathodes during its cleaning, cesium activation and degradation. The XPS probes the electronic structure of the p-doped GaN photocathode after each step of the preparation process. Using energies between 1200-0 eV the core levels and auger photoemission peaks of Ga, N, O, C and Cs are monitored. In our experiments, p-GaN on sapphire samples were cleaned with 99 % ethanol in an ultrasonic bath, followed by a thermal cleaning in a vacuum with the intention to remove carbon and oxygen contaminations on the p-GaN surface. Although still some carbon remained on the surface, the p-GaN was successfully activated by the deposition of a thin layer of cesium. Quantum efficiencies (QE) of 3 - 9 % were achieved. XPS photoemission spectra show a shift towards higher binding energies for the photoemission peaks, which is caused by a new component, so-called organic – cesium. During the storage under ultra – high vacuum, the GaN:Cs photocathodes were measured from time to time in the photocurrent and by XPS. We found a shift of 0.35 eV towards lower binding energies, which is related to the formation of the organic – cesium islands. This island growth is assumed to be in close correlation to the photocathode degradation. The p-GaN:Cs photocathodes showed a big QE loss after XPS analysis and therefore we investigated the potential damage from x-ray irradiation. The long-time irradiation experiments show that the x-ray damage has a high influence on the cesium component and the degradation of the p-GaN:Cs photocathode.

Accelerator scientists have high demands on photocathodes possessing high quantum efficiency (QE) and long operational lifetime. p-GaN, as a new photocathode type, gained recently more and more interest because of its ability to form a negative electron affinity (NEA) surface. Being activated with a thin layer of cesium, p-GaN:Cs photocathodes promise higher QE and better stability than the known photocathodes. In our study, p-GaN samples grown on sapphire or silicon were wet chemically cleaned and transferred into an ultra-high vacuum (UHV) chamber, where they underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with cesium to obtain p-GaN:Cs photocathodes and their performance was monitored in respect to their quality, especially their QE and storage lifetime. The surface topography and morphology were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) in combination with energy dispersive x-ray (EDX) spectroscopy. We have shown that p-GaN could be efficiently reactivated with cesium for several times. This paper compares systematically the influence of wet chemical cleaning as well as thermal cleaning at various temperatures on the QE, storage lifetime and surface morphology of p-GaN. As expected, the cleaning influences strongly the cathodes’ quality. We show that high QE and long storage lifetime are achievable at lower cleaning temperatures in our UHV chamber.

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.

Superconducting radio-frequency (SRF) electron guns are attractive for delivery of beams at a high bunch repetition rate with a high accelerating field. An SRF gun is the most suitable injector for the high-energy upgrade of the Linac Coherent Light Source (LCLS-II-HE), which will produce high-energy Xrays at high repetition rate. An SRF gun is being developed for LCLS-II-HE as a collaborative effort by FRIB, HZDR, ANL, and SLAC. The cavity operating frequency is 185.7 MHz, and the target accelerating field at the photocathode is 30 MV/m. The photocathode is replaceable. The cathode is held by a fixture ('cathode stalk') that is designed for thermal isolation and particle-free cathode exchange. The stalk must allow for precise alignment of the cathode position, cryogenic or room-temperature cathode operating temperature, and DC bias to inhibit multipacting. We are planning a test of the stalk to confirm that the design meets the requirements for RF power dissipation and biasing. In this presentation, we will describe the cathode stalk design and RF/DC stalk test plan.

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.

Superconducting radio-frequency (SRF) electron guns are attractive for delivery of beams at a high bunch repetition rate with a high accelerating field. An SRF gun is the most suitable injector for the high-energy upgrade of the Linac Coherent Light Source (LCLS-II-HE), which will produce high-energy Xrays at high repetition rate. An SRF gun is being developed for LCLS-II-HE as a collaborative effort by FRIB, HZDR, ANL, and SLAC. The cavity operating frequency is 185.7 MHz, and the target accelerating field at the photocathode is 30 MV/m. The photocathode is replaceable. The cathode is held by a fixture ('cathode stalk') that is designed for thermal isolation and particle-free cathode exchange. The stalk must allow for precise alignment of the cathode position, cryogenic or room-temperature cathode operating temperature, and DC bias to inhibit multipacting. We are planning a test of the stalk to confirm that the design meets the requirements for RF power dissipation and biasing. In this presentation, we will describe the cathode stalk design and RF/DC stalk test plan.

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.

The quality of the photocathodes is critical for the stable operation of the photoinjector. Thanks to the robust Cs2Te photocathodes, SRF gun at HZDR has been proven to be a type of successful CW e- source. In this contribution, we will present the operation experience of Cs2Te photocathodes in SRF gun, especially the QE evolution of Cs2Te photocathode during user operation. The possible reason for QE degradation will be discussed.

The heart of the ELBE Center for High Power Radiation Sources is a superconducting RF (SRF) linac, which accelerates electrons up to 35 MeV for driving a diverse set of secondary radiation sources. The ELBE linac is particularly unique in the capability of accelerating a continuous beam of ultrashort bunches of electrons at very high repetition rates (up to 26 MHz). This extremely high-power electron beam is selectively directed into specially designed beamlines to drive secondary radiation sources for THz/IR photons (FELBE and TELBE), positrons (pELBE), neutrons (nELBE), and gamma radiation (ELBE), each with dedicated laboratories and instrumentation. Users from all over the world utilize the advanced radiation sources at ELBE for a wide array of both fundamental and applied studies of matter, health, energy, and technology. The ELBE accelerator was first commissioned in 2001 in the form of a grid-pulsed 250 kV thermionic gun followed by two stages of RF bunching to inject beam into a linac comprised of two SRF cryomodules, each containing two 9-cell DESY TTF-type niobium accelerating cavities. Through continuous development and research, the ELBE facility has achieved many major advancements in accelerator technology, the most important being the ELBE SRF Gun program, which has designed, built, and commissioned several prototype SRF electron guns for high bunch charge, high average current, and low emittance. The ELBE SRF Gun-II is the first and only electron source of its type to deliver electron beam to a user experiment, and is now in routine operation for the TELBE and pELBE beamlines. An overview of the performance parameters of the ELBE accelerator and secondary sources will be presented in this talk along with a summary of the experimental capabilities available to users. Highlights of recent user results will also be presented to help illustrate the great potential ELBE provides for a diverse scientific community. Beamtime proposals are accepted and revie

We share the status of long term operation of Cs2Te in SRF-gun for CW mode facility, which is intested for the ERL society.

we share the experience of Cs2Te operation in SRF gun for ELBE user facility.

Status report of Cs2Te photocathodes for SRF gun-II at ELBE

An x-ray photoelectron spectrometer (XPS) is used in the HZDR photocathode lab to understand the surface states of GaN photocathodes during its cleaning, cesium activation and degradation. The XPS probes the electronic structure of the p-doped GaN photocathode after each step of the preparation process. Using energies between 1200-0 eV the core levels and auger photoemission peaks of Ga, N, O, C and Cs are monitored.

A Quarter Wave Resonator based SRF Gun for the LCLS II High Energy project