Cesium telluride cathodes for the next generation of high-average current high-brightness photoinjectors

Transcription

1 Cesium telluride cathodes for the next generation of high-average current high-brightness photoinjectors D. Filippetto, 1, a) H. Qian, 1 and F. Sannibale 1 Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California,94720, USA (Dated: 21 July 2015) We report on the performances of a Cs 2 T e photocathode under extreme conditions of high peak timedependent accelerating fields, continuos wave operations and MHz pulse extraction with up to 0.3 ma average current. The measurements, performed in a normal conducting cavity, show extended lifetime and robustness, elucidate the main mechanisms for cathode degradation, and set the required system vacuum performance for compatibility with the operations of a high average power X-ray Free Electron Laser user facility, opening the doors to the next generation of MHz-scale ultrafast scientific instruments. PACS numbers: Bx, Hd, i The next generation of scientific instruments holds promise to widen the reach of human eye both in time and space, by combining fully coherent femtosecond pulses with Angstrom wavelengths and high average X-ray flux in high repetition rate Free Electron Lasers 1 (FELs). In order to fulfill such requirements, a tremendous leap forward is requested to the electron sources driving FELs in terms of performance 2 : producing beams of extraordinary peak brightness, and at the same time providing an average particle flux four orders of magnitude over the state-of-the-art FEL drivers 3,4. The pressing demand for higher electron flux has pushed the R&D towards innovative electron source technologies addressing the need of simultaneous high peak field and high repetition rate, from DC sources 5,6 to Superconducting Radiofrequency Guns 7,8 (SRF). Our group at Lawrence Berkeley National Laboratory has focused the effort on the design of a normal conducting radiofrequency (RF) gun resonating in the VHF frequency range (APEX 9 ). Such design has demonstrated high performances, being the only one able to run reliably in continuos wave mode (CW) with accelerating fields (in excess of 20 MV/m) required to produce low emittance-high charge electron beams with high peak current needed to drive the next generation of Free Electron Lasers 2. High brightness, high-yield photocathode materials are essential for high repetition rate electron sources. A large variety of compounds have been studied in recent years (see for example 10 for a review), with photoemission properties well suited for photo-injector sources. Unfortunately such materials are usually very reactive semiconductors and their performances tend to degrade very fast with time and extracted charge. Cathode degradation can be a serious limitation to the operation of a future facility, and has been subject of intense studies in recent years. Different types of Alkali cathodes have been tested in photoemission guns 11 15, but the extreme a) dfilippetto@lbl.gov conditions produced by the APEX gun have not been tested yet. Unlike DC and SRF guns, the vacuum level in normal conducting RF cavities is affected by operations. The residual gas pressure in the accelerating cavity volume increases due to desorption from the cavity walls induced by RF heating. At the same time, the high fields reached in the cavity exponentially increase the amount of generated field-emitted electrons from surface imperfections. Such particles can be accelerated back to the cathode surface, together with ions generated by inelastic scattering of electrons from the primary beam with residual gas molecules in the cavity. Electronic and/or ionic sputtering can therefore take place continuously on the cathode surface, rapidly removing the deposited Cs 2 T e film, leading to irreversible degradation of the QE. Therefore, while thin films of Cs 2 T e semi-conductive material ( nm) are routinely grown with quantum efficiencies above 10% 11 (enough for MHz/mA operation), as of today it is not known whether such cathodes would resist to the above-described environment. In this article we report on the test and demonstration of Cs 2 T e performances in a normal conducting RF gun under simultaneous presence of continuos high electric fields ( 20MV/m) and up to 0.3 ma average current, in line with the demands of next generation of FELs. We have studied cathode degradation and the quantum efficiency evolution as function of time and extracted charge. A mechanism of cathode self-cleaning has been found, and post-mortem analysis has revealed the main mechanism of non symmetric cathode degradation. The experimental setup, with a detailed description of the electron gun and beamline, has been presented elsewhere 9. Table I reports a summary of the parameters relevant to the following discussion. An in-vacuum cathode exchange system, the load lock, hosts up to five cathode plugs and allows swapping without venting the system. A thin film of Cs 2 T e was deposited on a 5 mm diameter round area on top of the plug 16 by INFN-LASA 17,18. Polycrystalline molybdenum was used as substrate, to minimize the diffusion of cathode film material in it 19. A map of the quantum efficiency of the fresh cathode is reported

2 2 TABLE I. Gun parameters of relevance for the discussion. Eacc in CW mode 20 MV/m frf MHz Ek 780 kev Base Pres. RF off Torr Base Pres. RF on Torr on the left side of fig. 1, together with a picture of the deposited area on top of the plug. of the same cathode plug for a period of a week, changing the average current extracted. Figure 2 shows two examples of continuos runs of the APEX gun at 0.1 ma (top) and 0.3 ma (bottom). The measurements were done on the same cathode plug at different times, characterized by different QE values. Figure 3 summarizes the results Cs2T Cs2Te, 0.3 ma, ~ 5 C FIG. 2. Examples continuos QE measurements during 1MHz runs at 100 pc (top) and 300 pc (bottom). The total extracted charge for each run is specified. FIG. 1. Left: image and QE map of the Cs2 T e cathode before operations (Courtesy of D.Sertore, INFN/LASA). Right: residual gas analysis in the APEX gun without RF power (blue) and during 0.1 ma operations (red). As already mentioned, low gas pressure in the gun is of paramount importance for cathode lifetime: the total pressure determines the rate of ion production, causing cathode bombardment, while the presence of specific elements and compounds leads to chemical contamination. In the case of Cs2 T e, it has been demonstrated its receptivity to O and CO2, while it has shown to be fairly insensitive to CO, N2 and CH4 18. The right plot of fig.1 reports residual gas analysis traces in the VHF gun cathode-anode volume in absence of electric field and during operations, with nominal CW power and 0.1 ma average current. In both cases the total pressure is dominated by the partial pressure of hydrogen, with the other chemical species down by at least two decades. The gas pressure is fairly insensitive to the value of extracted average current, while directly dependent on the average RF power feeding the cavity. The cathode thermal emittance was characterized, by measuring the electron beam size at the first viewscreen along the beamline as function of the solenoidal lens strength. The emittance of 500 fc beam was measured for different laser spot sizes at the cathode, and a linear regression of the results lead to a value of respectively 0.72±0.07 and 0.79±0.05 µm/mm RMS for horizontal and vertical emittances, in line with previous results on the same type of cathode20. The slight difference between the two planes is symptomatic of skew quadrupole errors in the solenoid field breaking the cylindrical symmetry. In order to characterize and understand the dependencies of the cathode performance on the specific operating conditions, we performed continuous measurements of the measurement campaign, providing answers to the open questions regarding cathode degradation. Cathode sputtering by ion and electrons bombardment is a major concern when running at high average currents. Such effect can be very large in DC guns,12, while it is mitigated in RF structures due to the limited ion capture efficiency of time dependent fields21. The damage rate for ion backbombardment is proportional to the total pressure in the gun and to the number of electrons extracted and accelerated, i.e. the average beam current. The top-right graph of Fig.3 reports the relative change in lifetime (data is normalized respect to the lifetime at 0.02 ma) as function of the average current extracted, for a set of four 6-hour runs. Such acquisition time is much shorter than the cathode lifetime itself, increasing the measurement error bars. No correlation between QE lifetime and average current was detected within the experimental accuracy upon variation of average current by more than one order of magnitude, implying negligible contribution to the degradation by the back-bombardment at such currents and vacuum levels. In particular there was no evidence of locally enhanced degradation due to either bombardment or laser heating at the cathode center. The right plot of Fig.4 shows a QE map taken at the end of the measurement campaign. According to previous findings18, the main cause of contamination is caused by reaction of Cesium with oxygen and carbon dioxide, with the poisoning effect of oxygen being about 100 times faster. Therefore, to a good approximation, only oxygen pressure can be considered in assessing the degradation by contamination. Residual gas analysis of the APEX gun volume (Fig.1) shows an increase of the oxygen line by about 2 orders of magnitude when the RF power is feeding the cavity. Oxygen is desorbed by the cavity walls during operations, due to heating and X-rays absorption by the cavity surface. Most of the contamination will therefore happen

3 3 during operations. The bottom plot of Fig. 3 reports measurements done on the same cathode during operations. The horizontal axis reports the hours of operation of the electron gun, with RF power feeding the cavity, both with and without electron beam extraction. The top-axis shows the total exposure to oxygen, expressed in Langmuirs (10 6 torr s). A fit of the measurements reveals a 1/e lifetime of about 14.5 Langmuirs which, in our case, corresponds to about 402 hours of operations, consistent with previous measurements at low fields and low currents 18. It has already been shown 18 that cathode hour runs Average current (ma) FIG. 3. Top-left: Fractional amount of time spent on different operations for the cathode under study. About 39 C of charge were extracted. Top right: lifetime ratio (t/t 20µA ) versus extracted current, showing no clear correlation. Bottom: exponential fit of QE as function of operational time and exposure to oxygen. surface passivation can be partially recovered. Such rejuvenation requires heating and UV illumination for breaking the strong ionic bonds between oxygen and cesium. Consistently with such previous findings we observed a slight increase of QE at the beginning of each run. Quantum efficiency degradation was observed also after long machine shutdowns. Without RF power in the cavity oxygen levels are too low for explaining the measured rate of degradation with cathode oxidation. One possible alternative is the formation of weak bonds (Van der Walls-like) between the cathode surface elements and the residual gas molecules in the cavity (expecially water). Such bonds are very weak, with characteristic distances in the range of nm and energies in the mev range, and an externally applied field of 20MV/m is sufficient to break them apart. In order to verify such hypothesis we run the electron gun at the maximum voltage of 21.5 MV/m for 15 consecutive hours, without photocurrent extraction. Figure 4 shows QE maps before and after the run. The QE at the cathode center before the 3-week shutdown was 11%, while it dropped down to t/t 20µA (arb. units) about 5.5% at the restart (left map of Fig. 4). After 15 hours of RF-only operations we had fully recovered the old QE all over the cathode surface (right plot of Fig. 4). FIG. 4. Maps of quantum efficiency during operations, before and after 15 hours of RF-only operation. We have shown negligible correlation of cathode lifetime with average current, and indicated the surface oxidation during operations as the main mechanism of QE degradation. In fact, we experienced another deterioration mechanism: cathode sputtering from field-emitted electrons. The high fields in the cavity cause field emission from particulates and imperfections in the cavity walls. The geometry at the boundary between the cathode plug and the gun cavity causes local distortions of the electric field lines. Consequently field emitted particles from specific locations around that area can be accelerated at high angles 16. Some electrons can be accelerated toward the cathode plug causing erosion and ejection of atoms from the active film. Others hit the anode walls producing secondary electrons/ions that can be accelerated back at the opposite phase of the field, hitting the cathode. Figure 5-A shows the evolution of iso-qe lines (12%) of a fresh cathode at constant time intervals ( 5.5 hours) during operations. After an initial quick degradation of the right side of the cathode ( 10 hours) the QE stabilizes. The inset of fig.5-a shows an image of the cathode on a downstream screen revealing the fieldemitters distribution. The field emitters are distributed along the edges of the circular gap between cathode plug and cavity wall, and are concentrated on the right side of the cathode. Post-mortem analysis of the cathode confirms the cause of asymmetric degradation. Figure 5-B shows interferometric measurements done on the plug. A peak and a valley are clearly visible inside the (white) inner circle delimiting the active region of the plug (deposition area). Such regions correspond to high and low QE areas measured on the cathode, and their height difference is on the order of the initial Cs 2 T e film thickness, suggesting reduction of deposited material. Such hypothesis is confirmed by TEM measurements in Fig. 5- C.1(C.2). Two 5µm-wide samples, chosen from the peak and the valley as shown in the figure, were prepared by Focused Ion Beam lift-out technique and exposed to 200 kv electron beam for imaging 22. Platinum strips were deposited on the region of interest to protect from ion implantation and damage of the surface of interest. The large thickness difference of active deposition measured between the two areas explains the difference in quantum efficiency 19. Such cathodes are grown by deposition

4 4 (A) (B) doors to the next generation of scientific instruments, as high repetition rate FELs, where peak brightness meets high flux. The authors would like to thank M. Zolotorev for useful discussions, Russell Wells for the coordination of all mechanical activities that allowed to perform the presented measurements, and to the personnel of INFN/LASA for providing the cathodes. The authors acknowledge support from DOE grants No. DE-AC02-05CH J. FIG. 5. (A) Iso-QE lines during operations. The inset shows the asymmetric distribution of field-emitted particles, imaged at the beamline viewscreen. (B) Post-mortem interferometry on the cathode surface showing asymmetric erosion. The difference between peaks and valleys is at the level of film deposition thickness. (C.1-2) Post-mortem TEM measurements of the Cs2 T e layer in the peak and valley as shown in figure. of 10 nm of Tellurium and consequent deposition of Cesium by evaporation. About 70 nm of Cesium have been experimentally found to be the optimum for QE, with an atomic ratio between Cs and Te of about We have performed Energy dispersive X-ray spectroscopy on the film to measure the element composition. Nine different point spectra at different locations around the film where measured with 10 and 20 kv beam energies, and an overall mapping using 8 kv beam was performed. Traces of molybdenum (substrate) have been found on all the measurements, as well as small amounts of oxygen and carbon. The atomic percent of Te and Cs varies along the cathode and, while in the high-qe region (lower-right side of the film) the atomic percent ratio Cs/Te is close to 1.5, it rapidly decreases well below 1 moving toward the low-qe region (upper-left side). Such decrease of Cesium on the surface explains the difference in QE between different areas18, while the asymmetric distribution of field emitted electrons matching the cathode QE map suggests the cause of the degradation speed between different areas of the film to be electron/ion sputtering. In conclusion, we have tested Cs2 T e cathodes for continuos operations in high average current electron injectors for the next generation of Free Electron Lasers. The cathode has been characterized in a normal conducting CW-RF system providing simultaneously very high accelerating fields and ma-scale current. The results demonstrate that such cathodes are capable of withstanding continuos operations in the APEX-like operating conditions. The blending between the normal conducting APEX VHF gun and semiconductor cathodes opens the Galayda, Proceedings of LINAC2014, Geneva, Switzerland, (2014). 2 J. Schmerge, A. Brachmann, D. Dowell, A. Fry, R. Li, Z. Li, T. Raubenheimer, T. Vecchione, F. 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6 Cs 2 T Cs 2 Te, 0.3 ma, ~ 5 C

7 0.0 Average current (ma) hour runs t/t 20µA (arb. units)

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9 (A) (B)