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.

Laser melting the surface of 304 stainless steel allows controlled grain growth in the direction of the laser scan [1]. We demonstrate the application of laser surface melting with a Yb fibre laser as a technique for single crystal grain refinement and security marking in polycrystalline metals via localized grain nucleation. Single crystals were achieved after three consecutive passes with constant laser parameters throughout the length of the laser raster at 19.17 kJ/cm(2) average energy at 300K, 0.1% O-2 environment. The depth of localized grain nucleation was measured to be approximately 20 mm for a single pass, making hidden messages in the bulk of the material possible after mechanically removing the immediate surface melt. The patterns are undetectable by conventional optical microscopy but can be viewed with interferometric microscopy due to fine height differences between untreated and laser treated surface regions, this way establishing a metal security marking technique.

We present a general empirical model of secondary electron yield (SEY), which successfully fits the experimentally measured SEY of a flat gold surface for both normal and oblique incidence of primary electrons. This empirical model is applied in a two-dimensional Monte Carlo (MC) simulation to estimate the effective SEY reduction of a microporous surface. The simulation results are in very good agreement with the experimental data.

This work investigates secondary electron yield (SEY) mitigation from a metal surface with a microporous array fabricated using the laser drilling technique. We propose a general empirical model to fit the experimentally measured SEY of a flat gold surface for normal and oblique incidences of primary electrons. Using this empirical model, we develop a two-dimensional Monte Carlo (MC) simulation scheme to determine the effective SEY of a microporous array. It is found that the SEY from a porous surface is significantly reduced compared to that of the flat surface. By taking into account all the generations of secondary electrons inside a well, our MC results are found to be in very good agreement with the experimental data. The dependence of the SEY on the aspect ratio of the micropores and porosity of the surface is examined. A simple empirical formula has been proposed to evaluate the effective SEY of the gold microporous array for pores of arbitrary aspect ratios.

The authors regret to inform that the Acknowledgement section of the paper has incorrect grant numbers. Here is how it is now in the paper: Acknowledgments Work supported by US Air Force contract FA8650-11-D-5401 at the Materials & Manufacturing Directorate (AFRL/RXAP). The authors thank Lt Col Victor Putz of AFOSR/EOARD and Jason Marshall at AFOSR.D.G and M.S. wish to thank the EPSRC (EP/K503241/1). Here is how it needs to read Acknowledgments This material is based upon work supported by the Air Force Office of Scientific Research under award numbers FA9550-17RXCOR428 and FA9550-17-1-0317. The authors also wish to thank the EPSRC (EP/K503241/1). Authors would like to apologize for the inconvenience caused.

Multipactor and vacuum breakdown are serious problems for RF systems that utilize vacuum electronic devices. These devices contain metallic surfaces that serve as the source of unwanted particles released into vacuum. The primary culprits are desorbed gas molecules and secondary electrons. Outgassing results from stimulated or thermal desorption of molecular species in metals and can lead to undesired plasma formation in the vacuum gap. Secondary electrons (SEs) can lead to the multipactor effect when an avalanche of SEs obtain resonance with the internal RF field. Here we discuss surface treatments for mitigating these problems. Laser surface melting (LSM) entails irradiating a metallic sample with the output of a high energy, continuous laser beam, thereby causing melting, flow and re-solidification of the material. LSM processing reduces H outgassing by decreasing the number of grain boundaries through which H can diffuse. We have demonstrated a 50x reduction over an untreated stainless steel surface1. For SE reduction, both surface roughening and patterning have been investigated. Surface roughening was accomplished by using specific grades of metallographic polishing pads imparting controlled levels of roughness and surface features. Electron beam bombardment experiments showed that the secondary electron yield (SEY) increased with surface roughening2. Surface patterning with micro-pores has also been shown to reduce SEY. Modeling has shown that the amount of reduction depends on the aspect ratio of the pore (the ratio of the pore height to the pore diameter), with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes, and the models were validated with experimental results3, These results will be discussed.

High Pulsed Power Electromagnetic (HPEM) devices are used in numerous applications including vacuum electronics, particle accelerators, and microwave generators. Stable, long term HPEM device operation is presently constrained by pulse shortening due to plasma formation in the anode-cathode gap region. Plasma is formed through collisions between secondary electrons and gas molecules, both of which are produced by high energy electron impact at the anode; hydrogen is the most abundant species desorbed from metal surfaces by high energy electron impact. The purpose of the work described here was to determine the feasibility of using anodes of 304 stainless steel (SS), processed by laser surface melting (LSM), to reduce hydrogen outgassing. The LSM technique entails irradiating a sample with the output of a high energy, continuous laser beam, thereby causing melting, flow and re-solidification of the material as the laser beam is scanned across the anode surface. When compared to more conventional processing techniques, such as electropolishing, LSM processing introduces significantly fewer contaminants (especially hydrogen) into the anode surface and has the potential to reduce outgassing by decreasing the number of grain boundaries through which hydrogen can diffuse.

Metal anodes in high power source (HPS) devices erode during operation due to hydrogen outgassing and plasma formation, both of which are thermally driven phenomena generated by the electron beam impacting the anode s surface. This limits the lowest achievable pressure in an HPS device, which reduces its efficiency. Laser surface melting the 304 stainless steel anodes by a continuous wave fiber laser showed a reduction in hydrogen outgassing by a factor of ~4 under 50 keV electron bombardment, compared to that from untreated stainless steel. This is attributed to an increase in the grain size (from 40 - 3516 micrometer2), which effectively reduces the number of characterized grain boundaries that serve as hydrogen trapping sites, making such laser treated metals excellent candidates for use in vacuum electronics.

Dept of Physics and Astronomy, Ohio University Athens, OH USA One of the problems associated with long term operation of high pulsed power, vacuum electronic devices is pulse shortening, which is caused by hydrogen outgassing and by secondary electron emission from the anode. We recently showed the feasibility of Laser Surface Melting (LSM) of stainless steel (SS) anodes to reduce hydrogen outgassing from SS samples subjected to 50 keV electron bombardment. The results showed a reduction in outgassing from LSM-treated SS. This was attributed to a reduction in the number of grain boundaries, which serve as trapping sites for hydrogen. We have since measured the hydrogen depth profiles of treated and untreated samples by Elastic Recoil Detection in order to more completely understand the mechanism for reduced outgassing. The results indicate a significant reduction in residual hydrogen within the melt depth $( \sim 15 \mu \mathrm {m})$ of LSM-treated samples due to the small solubility of hydrogen in molten steel. We describe here a more complete model of the mechanism for reduced hydrogen outgassing that includes both a reduction in the number of trapping sites as well a reduction in the residual hydrogen concentration within the melt depth of LSMtreated samples. We conclude by describing the use of vacuum arc re-melted steel as an anode material and describe the effect of laser patterning of such samples to reduce secondary electron yield.