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.

Generation of silver nanoparticles with controlled size and spatial distribution by pulsed laser irradiation of silver ion-doped glass.

Silver ions were driven into glass by a direct current electric field-assisted ion exchange technique. The silver ion exchanged glass was then irradiated by laser pulses of 10 ns and 10 ps in length at 355 nm for comparison purposes. In both cases, laser irradiation led to the formation of a metallic-like film at the surface of the ion exchange glass. Scanning electron microscopy showed that the films consist of a very dense single layer of silver nanoparticles with similar particle sizes and separation. Irradiation with different laser parameters shows no significant difference in transmission spectra and modification width between ps- and ns-pulsed lasers. Particle sizes and separation at the surface are increasing with increasing laser power, and are larger for picosecond pulsed laser irradiation. It is also shown that the film formation is a thermal process.

Optical analyses of the formation of a silver nanoparticle-containing layer in glass.

We present results of our observations on the formation of a silver nanoparticle-containing layer in glass over time. First, silver ions are driven into the glass by field-assisted ion exchange at 300 °C. A following annealing step at 550 °C resulted in the formation of silver nanoparticles (< 4 nm in diameter). This annealing was performed for five different durations (1h, 2h, 4h, 8h, 48h), and thin slices of the cross sections of the glasses have been prepared. The sequence of slices showed the growth of the nanoparticle-containing layer over time. Transmission spectra of the slices have been measured with a spatial resolution of 1.5 µm. Simulating spectra using the Maxwell-Garnett theory allowed us to determine the volume filling factor distribution of the nanoparticles across the layers. A first attempt to simulate the diffusion of silver is performed.

Diffractive optical element embedded in silver-doped nanocomposite glass.

A diffractive optical element is fabricated with relative ease in a glass containing spherical silver nanoparticles 30 to 40 nm in diameter and embedded in a surface layer of thickness ~10 µm. The nanocomposite was sandwiched between a mesh metallic electrode with a lattice constant 2 µm, facing the nanoparticle containing layer and acting as an anode, and a flat metal electrode as cathode. Applying moderate direct current electric potentials of 0.4 kV and 0.6 kV at an elevated temperature of 200 °C for 30 minutes across the nanocomposites led to the formation of a periodic array of embedded structures of metallic nanoparticles. The current-time dynamics of the structuring processes, optical analyses of the structured nanocomposites and diffraction pattern of one such fabricated element are presented.