Secondary electron yield engineering of copper surfaces by 532 nm ultrashort laser pulses

Nanostructured surfaces exhibit outstanding properties and enable manifold industrial applications. In this study the laser surface processing of polycrystalline, flat copper surfaces by 532 nm picosecond laser irradiation for secondary electron yield (SEY) reduction is reported. The laser beam was scanned in parallel lines across the sample surface in order to modify large surface areas. Morphology and SEY are characterized in dependence of the process parameters to derive correlations and mechanisms of the laser-based SEY engineering process. The nano- and microstructure morphology of the laser-modified surface was characterized by scanning electron microscopy and the secondary electron yield was measured. In general, an SEY reduction with increasing accumulated laser fluence was found. In particular, at low scanning speed (1 mm/s - 10 mm/s) and “high” laser power (~ 1 W) compact nanostructures with a very low SEY maximum of 0.7 are formed

Influence of wavelength and accumulated fluence at picosecond laser-induced surface roughening of copper on secondary electron yield

Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu 2 O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (,1000 J/cm2 ), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10 -4 C/mm2 . Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between pro- cessing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders

Picosecond pulsed 532 nm laser system for roughening and secondary electron yield reduction of inner surfaces of up to 15 m long tubes

Laser-induced surface structuring is a promising method to suppress electron mulitpacting in the vacuum pipes of particle accelerators. Electrons are scattered inside the rough surface structure, resulting in a low Secondary Electron Yield (SEY) of the material. However, laser processing of internal pipe surfaces with a large aspect ratio is technologically challenging in terms of laser beam guidance and focusing. We present a 532 nm ultrashort-pulse laser setup to process the inner parts of 15 m long beam vacuum tubes of the Large Hadron Collider (LHC). Picosecond pulses at a repetition rate of 200 kHz are guided through an optical fiber toward an inchworm robot traveling inside the beam pipe. The system was installed, characterized, and tested for reliability. First surface treatments achieved the required scan precision. Cu2O-dominated nano-features were observed when processing at high average laser power (5 W) and slow scanning speed (5 mm s−1) in nitrogen flow, and the maximum SEY of copper was decreased from 2.1 to 0.7

A Reliable Monitoring and Control System for Vacuum Surface Treatments

Secondary electron yield (SEY) of beam-screens in the LHC puts limits on the performance of the accelerator. To ramp up the luminosity for the HiLumi LHC project, the vacuum surface coatings team are coming up with ways to treat the surfaces to control the electron cloud and bring the SEY down to acceptable levels. These treatments can take days to weeks and need to work reliably to be sure the surfaces are not damaged. An embedded control and monitoring system based on a CompactRIO is being developed to run these processes in a reliable way. This paper describes the techniques used to create a LabVIEW-based real-time embedded system that is reliable as well as easy to read and modify. We will show how simpler approaches can in some situations yield better solutions

Pulse Duration Dependence of Infrared Laser-Induced Secondary Electron Yield Reduction of Copper Surfaces

The irradiation of metals with ultrashort laser pulses enables the rapid and cost-effective production of nanostructured surfaces with a wide range of industrial applications. The laser-induced surface roughening modifies the interaction processes upon electron impact, leading to a modification of the secondary electron emission. In this study, the nanostructuring as well as the secondary electron yield (SEY) variation of polycrystalline copper surfaces was investigated by irradiation with 1030 nm infrared ultrashort laser pulses at a constant repetition rate of 100 kHz. The influence of varying the pulse duration between 238 fs and 10 ps, the laser power and the number of laser pulses per unit area (induced by varying the scanning speed) on the surface topography and the SEY was investigated. Irrespective of the pulse duration, irradiation with low scan speed (v ≤ 20 mm/s) and high laser power (P ≥ 2.6 W) results in the formation of a surface with compact nanostructures and a very low maximum SEY δmax < 0.7. The δmax increased slightly with increasing pulse duration at similar laser parameters. Increasing the pulse duration also resulted in a slight decrease in the ablation threshold and volume. The observed SEY dependence is probably explained by the pulse duration dependence of the ablation. The results suggest that nanostructured copper surfaces with very low SEY can be produced with ultrashort laser pulses over a wide range of pulse duration

Detection of Surface Creases in Range Data

We propose a fully automatic and view-independent computational procedure for detecting salient curvature extrema in range data. Our method consists of two major steps: (1) smoothing given range data by applying a nonlinear diffusion of normals with automatic thresholding; (2) using a Canny-like non-maximum suppression and hysteresis thresholding operations for detecting crease pixels. A delicate analysis of curvature extrema properties allows us to make those Canny-like image processing operations orientation-independent. The detected patterns of creases can be considered as ``shape fingerprints''. The proposed method can be potentially used for shape recognition, quality evaluation, and matching purposes

Laser-induced surface structuring for electron cloud mitigation in particle accelerators

Pulsed laser processing of vacuum component surfaces is a promising method for electron cloud mitigation in particle accelerators. By generating a hierarchically structured surface, the escape probability of secondary electrons is reduced. The choice of laser treatment parameters – such as laser power, scanning speed and line distance – has an influence on the resulting surface morphology as well as on its performance. The impact of processing parameters on the surface properties of copper is investigated by Secondary Electron Yield (SEY) measurements, Scanning Electron Microscopy (SEM), ablation depth measurements in an optical microscope and particle release analysis. Independent of the laser wavelength (532nm and 1064nm), it was found that the surface morphology changes when varying the processing parameters. The ablation depth increases and the SEY reduces with increasing laser fluence. The final application requires the capability to treat tens of meters of vacuum pipes. The limiting factors of this type of surface treatment for the applicability in particle accelerators are discussed

Influence of wavelength and accumulated fluence at picosecond laser-induced surface roughening of copper on secondary electron yield

Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu2O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (<1000 J/cm2), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10-4 C/mm2. Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between processing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders

Laser-induced periodic surface structuring for secondary electron yield reduction of copper: dependence on ambient gas and wavelength

One of the main limitations for future high-performance accelerators operating with positively charged particles is the formation of an electron-cloud inside the beam vacuum chamber, giving rise to instabilities. The Secondary Electron Yield (SEY) of the beam-facing surfaces gives a measure of the mechanism which drives this phenomenon. The laser-induced periodic structure formation on Cu surfaces has been demonstrated as a promising process to reduce SEY. In view of applications in beam chambers, we studied the laser process influence on SEY for 515 and 1030 nm wavelength femtosecond pulses on copper in different ambiences (air, nitrogen, vacuum). Depending on used process conditions, the surface composition differs, structures with varying aspect ratio are formed, i.e., periodic ripples and large-scale channels. Treatment in air at 515 nm is the most efficient for the formation of deeper structures allowing SEY maximum reduction first down to 1.6–1.7 and then below unity upon electron irradiation, thereby totally suppressing electron-cloud. Increasing the laser fluence, SEY will further reduce due to surface roughness enhancement via nanoparticle redeposition. This study reveals the fundamental role of LIPSS treatments to enable surface treatment in large-scale accelerator installations, where particle-free components are desired, and paves the way to potential future applications