Photocathodes for High Brightness Photo Injectors

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ScienceDirect Physics Procedia 77 (2015) 58 – 65

International Conference on Laser Applications at Accelerators, LA3NET 2015

Photocathodes for High Brightness Photo Injectors Rong Xiang*, Jochen Teichert Helmholtz Zentrum Dresden Rossendorf (HZDR), 01328 Dresden, Germany

Abstract The development of the photo injector has become a key technology for accelerator-based light sources and for the electron collider. There are a lot of opportunities to improve the injector quality, as well as photocathode function. A better photocathode can reduce the specification requirements of the drive laser system. The identification of better photocathodes is always a principal technical challenge, especially for the high brightness injectors. In this contribution, we will briefly review the status of photocathode research and the developing trends that are foreseen for the future. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder under responsibility of University the University of Liverpool. Peer-review responsibility of the of Liverpool Keywords:photocathodes, high brightness, photoinjector

1. Introduction For each accelerator facility, e.g. the light source and the future electron-ion collider, the development of the photo injector is a key technology. There are various solutions of photo injectors for different application requirements, but the common challenge is to obtain a high quantum efficiency (QE), long lifetime and cold photocathode. Especially in the high luminosity collider, x-ray Free Electron Laser and Compton backscattering facilities, a high brightness source is a crucial part. The definition of the normalized beam brightness shows that higher brightness results from higher beam current and/or lower emittance:

* Corresponding author. Tel.: +49-351-2603548; fax:+49-351-260 3690. E-mail address: [email protected]

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the University of Liverpool doi:10.1016/j.phpro.2015.11.010

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Bn

2I

S H n , xH n , y 2

(1)

Here Bn is the normalized brightness, I is the electron beam current, εn,x and εn,y are the normalized transverse emittances. Thus, for the high brightness electron guns, there are four important aspects for the photocathodes: high QE for high current, low thermal emittance and prompt time response for low emittance, long life time for sufficient user beam time. The QE needs to be more reliable, and the cathode material must be more robust. Thus there is strong motivation to push cathode R&D: on one hand to modify the present cathodes; on the other hand to search for new materials. There are two main types of photocathodes - metallic and semiconductor photocathodes. In the metallic cathodes, the “conventional” normal conducting (NC) metallic photocathodes, such as copper and magnesium, are most robust for radio frequency (RF) guns, but their QEs are unfortunately very low, mostly at the level of 10-5 to 10-6. The superconducting photocathodes consisting of niobium and lead have been well investigated for superconducting RF (SRF) guns, but the deep UV drive laser is still challenging. Recently the new plasma-enhanced metallic cathodes and alkali-activated metal cathodes have been developed and are very promising for future application. The semiconductor photocathodes, alkali antimonites, Cs2Te and GaAs(Cs), have the best QE of 1~10% but a critical working environment is mostly required. Among them, Cs2Te is relatively robust and can be used in most RF/SRF guns. And Cs2KSb achieves the highest current record 65 mA in the Cornell DC gun [1]. Although GaAs(Cs) relies on very high vacuum technology, it is attractive because it is up to now the unique candidate for a polarized electron source and also has high QE for non-polarisation application. 2. Metallic photocathodes Copper has been used for a long time as the preferred material for the RF gun cavity and for a robust photocathode. However, its work function at 4.5 eV is so high that the QE is rather poor for high current application [2]. In spite of the low work function Φ (less than 3 eV), the pure alkali metals are practically difficult to be operated as photocathodes. Magnesium is a metal with low work function of about 3.66 eV [2] and can be applied in the RF photoinjector [3]. The best QE record for Mg photocathodes reaches 0.2 % [4]. For SRF guns niobium and lead are considered to serve in the niobium cavity at BNL and DESY, because of the high transition temperature and acceptable work function (4.25 eV for Pb and 4.3 eV for Nb) [5]. Although the deep UV drive laser is challenging, Pb has been well investigated to achieve a high QE up to 10 -3 by Smedley et al. [6]. The test in the SRF gun showed however a relatively low QE of 9×10-5 at 258 nm by Barday [7], showing that it is possibly not suitable for high brightness electron sources. If the metal cathode is activated with a low work function alkali metal, for example, cesium (Φ =1.95 eV), the QE will be greatly increased. The QE for a commercial dispenser cathode was found to be as high as 0.22% at 266 nm by Jensen et al. [8]. The activation can be realized with vacuum deposition or ion implantation [9]. Recently a new plasma-enhanced metal cathode has been developed by Mustonen et al. [10] and shows very promising characteristics (see section 5.1). 3. Semiconductor photocathodes D. H. Dowell et al. [11] has listed most of photocathode materials. For semiconductor photocathodes, the band gap and the electron affinity define their response spectra and experimental QEs for a given wavelength. The band gap, i.e. the difference between the top of the valence band and the bottom of the conduction band, is the intrinsic character of a semiconductor. However, the electron affinity, the difference between the surface energy level and the vacuum energy level, is affected by the surface conditions. Thermionic electrons in the conduction band have more chance to overcome lower electron affinity, leading to a higher QE. Some materials have even negative electron affinity (NEA), which means all electrons with thermal energy in the conduction band are able to escape the solid surface.

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3.1. Positive Electron Affinity (PEA) At CERN, DESY, HZDR and LBNL, Cs2Te shows its high quality as the high brightness photocathode [12-16]. Several nC bunch charge has been routinely produced in DESY and CERN RF guns. It is relatively robust and can endure high peak E-field up to 60MV/m [17]. For application in the RF/SRF gun, Cs2Te has a fast response time in sub-ps or ps range and low thermal emittance. Its preparation process is reliable and very well reproducible. At INFN Milano, the Cs2Te photocathodes has maintained a high QE (>10%) for more than one year in the transport system. And the life time in the DESY RF gun and the HZDR SRF can surpass one year. The DESY PITZ gun can produce high bunch charge with slice brightness of 135 A/(mm mrad)2 [18]. The only disadvantage is its band gap plus the electron affinity of 3.5 eV, requiring a UV laser, which means a slow efficient frequency conversion and difficult laser pulse shaping. The high current of 100 mA required by the ERL machine motivates the bi-alkali antimonite material study. Up to now, the highest current record of photoinjectors is from the antimonite photocathode in the Cornell DC gun [1]. At BNL, the in-situ x-ray diffraction (XRD) and in-situ grazing incidence small angle x-ray scattering (GISAXS), as well as after growth x-ray reflectivity (XRR) measurements were performed at synchrotron radiation sources for the alkali antimonite photocathode research, so that the photocathode researchers can analyses the relationship between the preparation process, the crystal structure and the practical QE [19]. 3.2. Negative electron affinity NEA NEA photocathodes have the surface energy level lower than the vacuum level, so that all of the thermal electrons in the conduction band are able to escape from the cathode to vacuum, which leads to a high QE. The most famous NEA photocathode is GaAs(Cs), which achieves a QE of more than 10% in visible light.

Figure 1. The spin polarization (data points) and cathode quantum efficiency (solid curve) at room temperature as a function of excitation photon energy for strained GaAs sample [20].

Besides its high QE as a non-polarized cathode, GaAs(Cs) is still the best candidate for polarized electron sources. For some high energy physics experiments, polarized electron sources can deliver better resolution than from non-polarized beams. From the curves in Fig. 1, the photoemission changes with the photon energy [20]: with longer wavelength one gets higher polarization but a lower QE. Another advantage of this material is its low Mean Transverse Energy (MTE) less than 10 meV, which means ultra-cold electrons emission [21]. Bazarov and his colleagues have found that the rms normalized emittance from GaAs when excited with 860 nm light is as small as 0.121± 0.004 μm/mm radius [22]. Based on the state-of-art preparation technology, GaAs(Cs) has been successfully applied in worldwide DC guns: JLAB, Cornell, Daresbury, Mainz, KEK and so on [23-28]. At Cornell an average current of 52 mA has been

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achieved with GaAs in the DC gun [1]. And at JLab the development of an advanced GaAs photoemission sources has enabled a high polarized beam with polarizations up to 85 % at beam currents up to 180μA [26]. However, it is believed that NEA photocathodes cannot be applied in RF guns, because GaAs(Cs) has very demanding vacuum requirements, which cannot be achieved in RF guns. For the SRF guns at BNL and HZDR, GaAs(Cs) is one of the photocathode candidates [27,28]. The response time is another focus topic [29] for GaAs application in RF or SRF guns. An interesting simulation with Monte Carlo electron transport model shows that the temporal response from GaAs is at the level of ps, agreeing well with the experimental result [30]. 4. Main challenge 4.1. Sufficient life time The first challenge for cathode application is to produce a satisfactory beam over reasonable life time. A long life time reduces the cathode exchange frequency and enables continuing user beam time. The life time of photocathodes can be described as real time or as charge amount. Vacuum in the whole cathode environment is of course important, while good chamber surface treatment and powerful pumps can greatly help to reduce the active rest gas and avoid the ion back bombardment during the gun operation. L. Cultrera studied a used K2CsSb photocathode under a scanning electron microscope (SEM) in reference by Cultrera et al. [31]. Also the reported work on GaAs by Grames et al. [32] showed that the cathodes during the operation could be damaged, at least in part, by the back-bombardment of residual gas ionized by the beam. So the inert gas in gun environment must also be removed. SRF guns have a cryogenic environment and very good vacuum, which will benefit the cathode life time. On the other hand, a robust coating may provide another solution to protect the sensitive photocathode layer, for example a thin layer Cs2Te on GaAs, since Cs2Te is more robust in the high RF field [17] and the reported life time for Cs2Te in the high bunch charge rf gun is more than 300 hours [33]. 4.2. High Quantum Efficiency The second challenge for photocathode application, also the main mission of the photocathode research community, is how to further improve the QE. The usual way is to reduce the surface energy barrier for the chosen material, or modify the material structure or content to reduce its work function. For metallic photocathodes, the proper surface treatment to remove the contamination or the coating with low work function material can help to improve the QE [34,35]. For semiconductor photocathodes, good growth recipes and accurate deposition technology are the crucial issues. Here the photoemission theory and the fundamental understanding of materials can guide the research direction. For example, S. Schubert performed XRD measurements at synchrotron radiation sources to study developing of the crystal structure during the deposition of alkali antimonite photocathode [19]. Another successful example is the research with Auger Electron Spectroscopy (AES) to show the formation of different stoichiometric compounds during Cs deposition and to obtain the correct Cs/Te ratio corresponding to the QE maximum [36]. The present state-of-the-art Cs2Te photocathode preparation technology follows this discovery. 4.3. Low Thermal emittance The intrinsic thermal emittance of photocathodes decides the minimum transverse emittance from a photo injector. For the high brightness electron source, the low thermal emittance of photocathode plays a big role in reducing the total phase space area and thus increasing the beam brightness. The mean transverse energy (MTE) is the parameter one can precisely determine in the lab, and the exact transverse electron momentum distribution can be mapped at the meV level [37, 38]. MTE determines the normalized thermal emittance of photocathode:

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H nx ,th V x

MTE 3m0c 2

(

2

)

Here εnx.th is the normalized thermal emittance, and σx is the bunch size in the x direction. MTE is the kinetic energy of photoemitted electrons, and theoretically it is equal to the energy difference of the photo energy and work function. Besides the material band structure, the surface roughness is believed to be another factor affecting the transverse emittance [39]. L. Cultrera and his colleagues have found that by cooling the Cs3Sb photocathode to cryogenic temperature a significant reduction in the mean transverse energy or thermal emittance near the photoemission threshold appears, opening new frontiers in generating ultra-bright beams [40]. 5. Developing trend and new ideas 5.1. Modify present cathodes with novel methods To further improve the photoemission properties based on the present photocathode materials is the first idea. For example, one can improve the life time through a protecting layer [41], enhance the QE of metallic photocathodes with alkali coating [35] or design a needle shape photocathode with strong Schottkey enhancement [41]. A very new method is to produce nano-structure on metallic cathode surface to realize the plasmatic enhancement. Pacific Northwest National Laboratory published an interesting experiment with plasmonic nanoholes in gold thin films to enhance the photoemission of gold [42]. The photoemission is enhanced in the region of propagating surface plasmons launched from light coupled into the nanohole array. The intensity ratio of photoemission from the nanostructured gold to the flat metal surface can be as high as 108 at a laser intensity of 1 GW/cm2. This construct can be a promising candidate for high-brightness photoemission sources. 5.2. Search for new materials The enthusiasm for research on identifying new photo materials has never cooled down. In recent years, the diamond amplifier [43], the new dispenser cathodes [8] and multilayer semiconductors have generated fruitful inspiration. GaN(Cs) is another III-V semiconductor, which can be activated to NEA like GaAs, but the activation process is much easier and more reliable. Designed GaN(Cs) has the highest QE up to 68.7 % at 240 nm [44]. Although GaN(Cs) has a wide band gap and must be driven by UV light, it has extremely high QE and is more robust than GaAs(Cs). Its thermal emittance and other crucial parameters have been characterized by Bazarov et al.[45]. If the band gap can be manipulated with some narrow band gap material, GaN is a very promising candidate for the next high QE photocathode for the high brightness gun. 5.3. Modern theoretical work All of the theory to understand photoemission phenomena is based on the quantum photoelectric effect and Spicer’s three-steps theory [46]. The Spicer model has been successful in explaining critical parameters of high brightness semiconductor photocathodes, such as thermal emittance and response time. Together with the band structure theory, the diffusion model and electron quantum tunneling theory [47], nearly all physical questions about photocathodes can be answered (shown in Fig.2). Simulation tools have also developed greatly in recent decades. For instance, S. Karkare and co-authors have performed detailed Monte Carlo electron transport simulations to study the exact mechanism of electron emission from GaAs. Simulations show a quantitative agreement with the experimental results for QE, energy distributions of emitted electrons, and response time. This agreement between simulation and experiment sheds light on the

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mechanism of electron emission and provides an opportunity to design novel semiconductor photocathodes with optimized performance [30].

Figure 2. Brief description of the photoemission process [12]. The photoemission process can be divided into three steps: 1) photoexcitation, 2) transport to the surface and 3) escape to vacuum. For example, in semiconductors, photoexcitation is overcoming the band gap Eg; during the transport free electrons will lose part of energy by scattering with phonons; photoelectrons are the part of electrons with enough energy overcoming the electron affinity into the vacuum.

6. Summary A dream photocathode for high brightness beam has the following characteristics: high QE, long life time, low emittance and fast response time. All research efforts have the goal to improve one or more of these parameters, either to modify present photocathodes or to search for new materials. From the technical work in the labs, one must optimize the preparation process to ensure the cathode quality and reproducibility, use new XHV technology for the preparation chamber, the transfer system and the gun environment, and develop precise methods to accurately characterize the photocathode. Besides experimental improvement and advances in theoretical work, the research community is encouraging the development of cooperation networks between the labs for sharing of facilities and experiences and to get together for exchanging problems, solutions and new ideas [48, 49]. Acknowledgements The work is supported by the European Community under the FP7 programme (EuCARD-2, contract number 312453, and LA3NET, contract number 289191) and by the German Federal Ministry of Education and Research (BMBF) grant 05K12CR1. Reference [1] Dunham, B., Barley, J., Bartnik, A., Bazarov, I., Cultrera, L., Dobbins, J., Hoffstaetter, G., Johnson, B., Kaplan, R., Karkare, S., and others. Record high-average current from a high-brightness photo injector. Applied Physics Letters 2013; 102: 034105. [2] Lide, D. R.. Properties of Solids, in: CRC Handbook of Chemistry and Physic, Internet Version 2005. Boca Raton, FL: CRC Press; 2005, P. 124.

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