Linear particle accelerators (Linacs) are primarily composed of radio frequency cavities (cavities). Compared to traditional manufacturing, Laser Powder Bed Fusion (L-PBF) holds the potential to fabricate cavities in a single piece, enhancing Linac performance and significantly reducing investment costs. However, the question of whether red or green laser PBF yields superior results for pure copper remains a subject of ongoing debate. Eight 4.2 GHz single-cell cavities (SCs) were manufactured from pure copper using both red and green PBF (SCs R and SCs G). Subsequently, the surface roughness of the SCs was reduced through a chemical post-processing method (Hirtisation) and annealed at 460 °C to maximize their quality factor ($Q_0$). The geometric accuracy of the printed SCs was evaluated using optical methods and resonant frequency ($f_R$) measurements. Surface conductivity was determined by measuring the quality factor ($Q_0$) of the SCs. Laser scanning microscopy was utilized for surface roughness characterization. The impact of annealing was quantified using Energy-Dispersive X-ray Spectroscopy and Electron Backscatter Diffraction to evaluate chemical surface properties and grain size. Both the SCs R and SCs G achieved the necessary geometric accuracy and thus $f_R$ precision. The SCs R achieved a 95% $Q_0$ after a material removal of 40 µm. The SCs G achieved an approximately 80% $Q_0$ after maximum material removal of 160 µm. Annealing increased the $Q_0$ by an average of about 5%. The additive manufacturing process is at least equivalent to conventional manufacturing for producing cavities in the low-gradient range. The presented cavities justify the first high-gradient tests.

Positive nitrogen‐vacancy centers (NV⁺) in diamond are predicted to exist in conjunction with neutral (NV⁰) and negative (NV⁻) centers. However, the existence of NV⁺ has only been indirectly inferred through a shift of the Fermi level. Evidence of NV⁺ coexisting with NV⁰ and NV^– in diamond has not yet been observed. In this paper, positron annihilation spectroscopy in combination with in situ light illumination is applied, in order to investigate the presence of NV⁺ centers in nitrogen implanted and subsequently annealed diamond. Switching of NV⁺ to NV⁰ centers is observed with a threshold photon energy of 1.234(8) eV. In complete darkness, a decay of NV⁰ centers with a decay time of 1.73(22) h can be detected. In conclusion, previously converted NV⁰ centers are metastable and partially decay in darkness, leading to the reformation of NV⁺ centers.

We have performed first four-dimensional age-momentum correlation (4D-AMOC) measurements at a pulsed high intensity positron micro beam and determined the absolute value of the three-dimensional momentum of the electrons annihilating with the positrons in coincidence with the positron age in the sample material. We operated two position sensitive detectors in coincidence to measure the annihilation radiation: a pixelated HPGe-detector and a microchannel plate image intensifier with a CeBr₃ scintillator pixel array. The transversal momentum resolution of the 4D-AMOC setup was measured to be about 17 × 10⁻³ (FWHM) and was circa 3.5 times larger than the longitudinal momentum resolution. The total time resolution was 540 ps (FWHM). We measured two samples: a gold foil and a carbon tape at a positron implantation energy of 2 keV. For each sample discrete electron momentum states and their respective positron lifetimes were extracted.