Secondary Electron Yield Measurements Research Articles

Secondary electron emission characteristics of alumina coating on metallic substrate prepared by atomic layer deposition

The main objective of this study is to research the secondary electron emission characteristic of different thickness alumina coatings which are prepared by atomic layer deposition (ALD) technique on several metallic secondary-emitting materials. The experimental results reveal that secondary electron yield (SEY) property concerned with the metal work function and the surface topography of alumina coatings when coating thickness is in the nanometer level. In addition, after utilizing SEY measurement setup and pulsed electron gun, the SEY curves of metallic substrates with different thickness alumina coatings and primary energy (PE) of incident electron could be measured. From an analysis of the SEY curves, some key parameters related to secondary electron emission characteristics such as the maximum SEY value and its corresponding PE of incident electron, and the penetration depth of incident electron can be obtained. Furthermore, basing on the ALD technique and on the SEY results of stainless steel substrate coated with 80-cycle alumina thin film, a novel 18-stage discrete dynode electron multiplier (DDEM) was fabricated and tested. The gain of DDEM 1.6× 106 was obtained.

Origin and mitigation of the beam-induced surface modifications of the LHC beam screens

All over Run 2, the LHC beam-induced heat load on the cryogenic system exhibited a wide scattering along the ring. Studies ascribed the heat source to electron cloud build-up, indicating an unexpected high Secondary Electron Yield (SEY) of the beam screen surface in some LHC regions. The inner copper surface of high and low heat load beam screens, extracted during the Long Shutdown 2, was analysed. On the low heat load ones, the surface was covered with the native Cu2O oxide, while on the high heat load ones CuO dominated at surface, and it exhibited a very low carbon coverage. Such chemical modifications increase the SEY and inhibit a proper conditioning of the affected surfaces. Following this characterisation, the mechanisms for CuO build-up in the LHC beam pipe were investigated on a newly commissioned cryogenic system allowing electron irradiation, surface chemical characterisation by X-ray Photoelectron Spectroscopy and SEY measurements on samples held below 15 K. In parallel, curative solutions against the presence of CuO in the LHC beam screens were explored, which could be implemented in-situ to recover a proper conditioning and lower the beam-induced heat load.

Surface characterization and secondary electron yield measurement of the Ti-Zr-V films deposited on laser-etched copper

The electron multipacting effect and distributed vacuum profile, induced by the interaction of the particles in vacuum chamber with the surrounding walls, are the significant issues in modern accelerators. Laser-etching the inner surface of the vacuum chambers can effectively eliminate the electron multipacting effect, and depositing the non-evaporable getter (NEG) film on the vacuum chamber walls can provide distributed pumping properties. In the present study, Ti-Zr-V NEG films were prepared by magnetron sputtering technique on laser-etched oxygen-free copper (OFC) with different deposition times. The secondary electron yields (SEY), activation kinetics, and resistance of the samples were evaluated. The results show that although the SEYs of the coated laser-etched samples are higher than those of the uncoated laser-etched sample, the maximum SEY values are still below the threshold of the operation requirements. The Ti-Zr-V film deposited on laser-etched surface exhibits obvious nucleation phenomenon with columnar microstructure and can be activated at 180^∘C. Moreover, the coated laser-etched samples present lower resistance compared with the uncoated laser-etched sample. The combination of the laser-etching technique and the NEG thin film is aim to provide a practical treatment for the vacuum chambers in modern accelerators.

Effect of the Surface Morphology of Porous Coatings on Secondary Electron Yield of Metal Surface.

Surface roughening is an important material surface treatment technique, and it is particularly useful for use in secondary electron yield (SEY) suppression on metal surfaces. Porous structures produced via roughening on coatings have been confirmed to reduce SEY, but the regulation strategy and the influence of process parameters both remain unclear in the practical fabrication of effective porous structures. In this paper, the effect of the surface morphology of porous coatings on the SEY of aluminum alloy substrates was studied. Surface characterization and SEY measurements were carried out for samples with a specific process technique on their surfaces. An exponential fitting model of the correlation between surface roughness and the peak values of SEY curves, , was summarized. Furthermore, an implementation strategy to enable low surface SEY was achieved from the analysis of the effect of process parameters on surface morphology formation. This work will aid our understanding of the effect of the irregular surface morphology of porous coatings on SEY, thereby revealing low-cost access to the realization of an easy-to-scale process that enables low SEY.

Cryogenic secondary electron yield measurements on structural materials applied in particle accelerators

Multipacting phenomenon induced by large secondary electron yield (SEY) in cryogenic vacuum chambers and superconducting cavities is one of the significant issues in operating particle accelerators. Studying the characteristic and the factors influencing the secondary electron emission on cryogenic surfaces has become an inevitable task in the construction of accelerators. Here, custom cryogenic SEY measurement system with cold helium circulation facility has been established. Cryogenic SEY of structural materials (copper, aluminum, stainless steel, niobium and technical treated niobium) as a function of primary electron energy were evaluated. The experimental results show that SEY of materials at cryogenic temperature is lower than that at room temperature due to the adsorption of residual gases on the surfaces. Secondary electron emission can be effectively restrained by buffered chemical polishing treatment due to the removal of the contaminations and the build-up of graphitelike C–C bonds. After annealing treatments, the surface morphology of technical treated niobium has been modified, causing an increased roughness. Secondary electrons trapped by rough surface lower the SEY of technical treated niobium further. This paper is aimed to explore a rationale of cryogenic SEY characteristic of structural materials and evaluate the potential technical treatments for design and construction of Hefei advanced light facility (HALF).

Low and Stable Secondary Electron Yields of Alodine-Treated Aluminum Alloys Under Continuous Electron Bombardment

High secondary electron yields of aluminum (Al) alloys limit the performances of high-power microwave devices and even extremely degrade their lifetimes. In this work, a strategy of alodine treatment for Al alloys was proposed to achieve a goal of the secondary electron emission suppression under continuous electron bombardment. It was found that a porous chromate conversion film including the mixture of basic chromium chromate and alumina with a mean pore size of 69.1 nm was formed on the surface of the Al alloy treated by alodine solution. On the basis of the secondary electron yield (SEY) measurement results, this alodine treatment brings about a reduction of maximal SEY of the Al alloy from 1.67 to 1.33 with a ratio of 20.4% and has just a remarkably tiny SEY change within 0.01~0.03 under 240-min continuous electron bombardment.

The impact of H2 and N2 on the material properties and secondary electron yield of sputtered amorphous carbon films for anti-multipacting applications

Amorphous carbon thin films were prepared by direct current hollow cathode sputter deposition in Ar discharge with the injection of small amounts of H2 and/or N2. The influence of these additives on the film properties with particular focus on the application as a coating for electron cloud mitigation in particle accelerators is characterized by optical spectroscopy, X-ray photoelectron spectroscopy, and secondary electron yield (SEY) measurements. The SEY maximum increased from initially 0.98 with pure Ar in the gas discharge up to 1.38 at 0.5% H2 while the addition of 1% of pure N2 enabled to reduce it to 0.88. The simultaneous addition of N2 to the H2 containing discharge allowed an average SEY maximum reduction of 20%. The optical bandgap revealed a correlation between the increase/decrease of the bandgap and the SEY increment/reduction for H2/N2 addition. The surface composition changes and the resulting modification of the sp2/sp3 ratio correlate with the changes in SEY and optical properties. The obtained results highlight the potential of intentionally injected N2 to counteract the detrimental effect of the inevitable H2 partial pressure in the coating systems during the production of amorphous carbon thin films for anti-multipacting applications in particle accelerators.

Measurement of the transmission secondary electron yield of nanometer-thick films in a prototype Timed Photon Counter

We measure the transmission secondary electron yield of nanometer-thick Al2O3/TiN/Al2O3 films using a prototype version of a Timed Photon Counter (TiPC) . We discuss the method to measure the yield extensively. The yield is then measured as a function of landing energy between 1.2 and 1.8 keV and found to be in the range of 0.1 (1.2 keV) to 0.9 (1.8 keV). These results are in agreement to data obtained by a different, independent method. We therefore conclude that the prototype TiPC is able to characterise the thin films in terms of transmission secondary electron yield. Additionally, observed features which are unrelated to the yield determination are interpreted.

Secondary electron emission from reticulated cellular copper surfaces

An experimental and computational study of the secondary electron yield (SEY) of copper foam is presented. Ray-tracing Monte Carlo (MC) simulations, based on primary electron transport and interactions with the electron system in solid and foam-type copper, are also included. The 3D reticulated foam geometry is explicitly represented in the MC model. This allows the influence of the complex copper surface on the energy and angular dependence of the SEY to be determined. Experimental measurements of SEY were performed in a high-vacuum electron gun chamber. Solid copper and copper foam with 100 pores per inch and a 4.6% volume fraction were tested for energy and angular dependence of SEY. Incident angles were varied from 0° to 75° and electron beam energy ranged from 20 eV to 570 eV. The agreement between MC simulations and experiments suggests that a general reduction of the SEY by around 20% is due to the interaction and subsequent adsorption of a fraction of emitted secondary electrons on the internal surfaces of the reticulated foam. Moreover, it is found that the SEY becomes nearly independent of the incident electron energy above 200 eV for steep angles of incidence (θ>45°).

Angular dependence of secondary electron yield from microporous gold surfaces

We report exhaustive measurements of the secondary electron yield (SEY) from a gold film containing an array of micropores as a function of the angle of incidence of the primary electrons. The SEY measurements are in good agreement with Monte-Carlo (MC) simulations. A highly accurate empirical fit to the SEY data as a function of the incident electron impact angle is also proposed. In this study, the micropores have aspect ratios (ratio of pore height over pore diameter) ranging from about 1.5 to 3.5. The effect of the pore array density (porosity) and pore aspect ratio is analyzed in greater detail. It is found that increasing the pore aspect ratio and porosity leads to a sharp reduction in the total SEY in agreement with MC simulations.

The Effect of Ultrasonic Cleaning on the Secondary Electron Yield, Surface Topography, and Surface Chemistry of Laser Treated Aluminum Alloy.

Laser ablation technique is a novel method for obtaining a surface with a low secondary electron yield (SEY) that can mitigate electron cloud in high-energy accelerators. Before the installation of laser processed aluminum alloy, surface cleaning is of the essence to reduce the contaminations of ultra-high vacuum systems for providing appropriate pressure for beam operation consequently. Laser processed aluminum alloy is one of the crucial candidates for the vacuum system construction of future accelerators. Moreover, ultrasonic cleaning is an essential procedure for most materials applied in vacuum systems. Therefore, in order to verify the stability of the laser created structures by ultrasonic cleaning and evaluate the impact of the cleaning on the SEYs, the surface topographies, and the surface chemistries of laser treated aluminum alloy, SEY measurements and related tests were performed. After ultrasonic cleaning, the SEYs of laser treated aluminum alloy increased from 0.99, 1.05, and 1.16 to 1.43, 1.74, and 1.38, respectively. Compared to the surface roughness of uncleaned laser treated aluminum samples, the cleaned laser treated ones decreased from 10.7, 7.5, and 14.5 to 9.4, 6.9, and 12.9, respectively. The results indicate that ultrasonic cleaning can induce the SEY increase of laser processed aluminum alloy. The correlative mechanism between the surface morphology, the surface chemistry, and SEY increase were analyzed for the first time.

Surface roughness evolution induced low secondary electron yield in carbon coated Ag/Al substrates for space microwave devices

High power microwave devices have been suffering from secondary electron yield (SEY), which occurs at the surface and leads to increases in energy dissipate and noise level in microwave cavities. The suppression of SEY in microwave materials is significant for space applications. In this work, we report a surface roughness evolution mechanism induced by carbon coating on a widely-used microwave cavity material Ag/Al. The competition between the preferred growth and the island growth modes leads to the roughness enhancement in the 60 nm carbon coated Ag/Al substrates. By extracting the smooth factor Ks in Vaughan model from the angular dependent SEY measurement, we attribute the suppression of SEY to the intrinsic low SEY of the carbon film and the high surface roughness. Our work demonstrates a low-cost, easy-to-scale coating process to effectively prevent the formation of secondary electron avalanche in space borne microwave devices.

Extreme UV secondary electron yield measurements of Ru, Sn, and Hf oxide thin films

Background: The secondary electron yield (SEY) of materials is important for topics as nanoparticle photoresists and extreme ultraviolet (EUV) optics contamination. Aim: Experimentally measure SEY and secondary electron energy distributions for Ru, Sn, and Hf oxide. Approach: The SEY and energy distribution resulting from 65 to 112 eV EUV radiation are measured for thin-film oxides or films with native oxide. Results: The total SEY can be explained by EUV absorption in the topmost nanometer of (native) oxide of the investigated materials. Conclusions: Although the relative SEY of Ru and Sn is well-explained by the difference in EUV absorption properties, the SEY of HfO2 is almost a factor 2 higher than expected. Based on the energy distribution of secondary electrons, this may be related to a lower barrier for secondary electron emission.

Secondary Electron Yield Measurements of Carbon Nanotube Forests: Dependence on Morphology and Substrate

Total, secondary, and backscatter electron yield data were taken with beam energies between 15 eV and 30 keV, in conjunction with energy emission data, to determine the extent of suppression of yield caused by carbon nanotube (CNT) forest coatings on substrates. CNT forests can potentially lower substrate yield due to both its inherently low-yield, low-atomic number (Z) carbon composition, and its bundled, high-aspect-ratio structure. Rough surfaces, and in particular, surfaces with deep high-aspect-ratio voids, can suppress yields, as the electrons emitted from lower lying surfaces are recaptured by surface protrusions rather than escaping the near-surface region. Yields of multilayered materials can be modeled essentially serially as a combination of the constituents. However, it is shown that suppression of yields due to CNT forest morphology is more significant than simple predicted contributions of homogeneous layered components. This effect is found to be most pronounced at low energies, where the incident electrons interact preferentially with the CNTs. CNT forests between 20 and $50~\mu \text{m}$ tall were grown on a thick silicon substrate capped with a 3-nm diffusion barrier of evaporated aluminum using a wet injection chemical vapor deposition (CVD) method. Yields of an annealed substrate and constituent bulk materials were also investigated. At incident electron energies above ~1200 eV, the substrate secondary yield dominated those of the CNT forests, as incident electrons penetrated through the low-density, low-Z CNT forests, and backscattered from the higher-Z substrate. At lower energies <1200 eV, the CNT forests substantially reduced the overall yields of the substrate, and for <500-eV, CNT forest yields were <1, well below the already low yields of bulk graphite. This suppressed yield at low energies is attributed to the porosity and preferred vertical alignment of the CNT forest. The yield’s dependence on the height and density of the CNT forest is also discussed.

Design of a mechatronic system for accurate measurement of secondary electron emission

Accurate measurement of Secondary Electron Emission (SEE) in scientific experiments provides better insight and understanding of field emission and multipacting properties in accelerator cavities. SEE properties of Niobium (Nb) are of significant importance to the performance and operation of accelerator cavities. Additionally, accurate Secondary Electron Yield (SEY) measurements from primary electrons generated from samples measured under different incident angles represent a technical challenge. For that purpose, the sample positioning system presented in this article is a design that allows for precise sample orientation and handling during single pump-down of the vacuum system in the experimental setup. Measurement techniques supported by a mechatronic system provide the means for acquisition of large quantities of data and at the same time reduce the experimental errors related to sample positioning. The authors describe a sample manipulation system and method for controlling the position of samples used for the measurement of SEY.

Measurement of Secondary Electron Energy Spectra of Polymethyl Methacrylate**Supported by the National Natural Science Foundation of China under Grant Nos U1537210 and 11375139, and the National Key Laboratory of Space Microwave Technology China under Grant No 9140C530101130C53013.

We report on a novel and convenient method of measuring secondary electron spectra for insulators in a secondary electron yield measurement system with a planar grid analyzer configuration and a metal mesh probe. In this measurement, the planar grid is negatively biased to force some emitted secondary electrons to return to the sample surface and to neutralize charges accumulated on the sample during the previous beam irradiation. The surface potential of the sample is then measured by use of a metal mesh probe. The grid bias for neutralization corresponding to the zero surface potential is determined based on the linear relationship between the surface potential and the grid bias. Once the surface potential equals zero, the secondary electron spectra of polymethyl methacrylate (PMMA) are studied experimentally by measuring the S-curve and then fitting it to Everhart’s formula. The measurement results show that the peak energy and the full width at half maximum of the spectra are 4.26 eV and 14.06 eV, respectively.

Fabrication of Porous Ag/TiO2/Au Coatings with Excellent Multipactor Suppression

Porous Ag/TiO2/Au coatings with excellent multipactor suppression were prepared by fabrication of porous Ag surface through two-step wet chemical etching, synthesis of TiO2 coatings by electroless-plating-like solution deposition and deposition of Au coatings via electroless plating. Porous structure of Ag surface, TiO2 coatings on porous Ag surface and Au coatings on porous Ag/TiO2 surface were verified by field-emission scanning electron microscopy, the composition and crystal type of TiO2 coatings was characterized by X-ray photoelectron spectroscopy and X-ray diffraction. Secondary electron yield (SEY) measurement was used to monitor the SEY coefficient of the porous Ag coatings and Ag/TiO2/Au coatings. The as-obtained porous Ag coatings were proved exhibiting low SEY below 1.2, and the process was highly reproducible. In addition, the porous Ag/TiO2/Au coatings showed excellent multipactor suppression with the SEY 1.23 and good environmental stability. It is worth mentioning that the whole preparation process is simple and feasible, which would provide a promising application in RF devices.

Positive bias and vacuum chamber wall effect on total electron yield measurement: A re-consideration of the sample current method

The measurement of the total secondary electron yield (TEY, δ) is of fundamental importance in areas such as accelerator, spacecraft, detector, and plasma system. Most of the running TEY facilities in the world are based on the kind of bias strategy. The applied bias can assist in the collection of the secondary/primary electrons. In the prevailing sample current method, the TEY is obtained by the measurement of the current from the sample to ground with a negative/positive bias applied to the sample. One of the basic assumptions in this method is that the positive bias can retain most of the electrons emitted by the sample. This assumption is generally recognized based on the seeming fact that the low energy secondary electrons dominate the emitted electrons. In this work, by considering the full electron energy spectrum including both the true secondary and backscattered electrons, we give a new insight in this TEY measurement method. Through the analytical derivation as well as the Particle-in-Cell numerical simulation, we show that it is due to the following two factors, other than the assumption mentioned above, which make the sample current method works satisfactorily: (a) the TEY relative error is related to the TEY itself in the form of |1−δ|/δ, which indicates a smallest error when measuring samples with TEY closest to 1; and (b) the compensation effect of the vacuum chamber wall. Analytical results agree well with numerical simulations and furthermore, we present a correction method for reducing the TEY relative error when measuring samples with TEY below 1. By sweeping the positive bias from 50 to 500 V, a flat silver sample in the as-received state with maximum TEY larger than 2 and a laser etched sample with maximum TEY close to 1 were measured for further verification. The obtained experimental results agree well with the theoretical analysis.

Activation treatment effects on characteristics of BeO layer and secondary electron emission properties of an activated Cu–Be alloy

Dependence of characteristics of surface BeO layer and secondary electron emission properties of a Cu–Be alloy on activation conditions were studied by measurements of X-ray photoelectron spectroscopy and secondary electron yield of the alloy based on varying conditions of an optimized activation treatment. The results showed that the thickness and composition distribution of the surface BeO layer of the alloy were closely related to the pre-heat treatment and the affecting factors (oxidation duration, oxidation temperature, and oxygen pressure) of the oxidation process in the activation treatment. The improper treatment leaded to a too thick or too thin surface BeO layer, the presence of metallic Be, and poor distribution of metallic and oxidized Cu in the layer, hence significantly reduced the secondary electron emission yield of the activated Cu–Be alloy.

In-situ measurements of the secondary electron yield in an accelerator environment: Instrumentation and methods

The performance of a particle accelerator can be limited by the build-up of an electron cloud (EC) in the vacuum chamber. Secondary electron emission from the chamber walls can contribute to EC growth. An apparatus for in-situ measurements of the secondary electron yield (SEY) in the Cornell Electron Storage Ring (CESR) was developed in connection with EC studies for the CESR Test Accelerator program. The CESR in-situ system, in operation since 2010, allows for SEY measurements as a function of incident electron energy and angle on samples that are exposed to the accelerator environment, typically 5.3GeV counter-rotating beams of electrons and positrons. The system was designed for periodic measurements to observe beam conditioning of the SEY with discrimination between exposure to direct photons from synchrotron radiation versus scattered photons and cloud electrons. The samples can be exchanged without venting the CESR vacuum chamber. Measurements have been done on metal surfaces and EC-mitigation coatings. The in-situ SEY apparatus and improvements to the measurement tools and techniques are described.