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.

Compared to conventional manufacturing, additive manufacturing (AM) of radio frequency (RF) cavities has the potential to reduce manufacturing costs and complexity and to enable higher performance. This work evaluates whether normal conducting side-coupled linac structures (SCCL), used worldwide for a wide range of applications, can benefit from AM. A unit cell geometry (SC) optimized for 75 MeV protons was developed. Downskins with small downskin angles $\alpha$ were avoided to enable manufacturing by laser powder bed fusion without support structures. SCs with different $\alpha$ were printed and post-processed by Hirtisation (R) (an electrochemical process) to minimize surface roughness. The required accuracy for 3 GHz SCCL (medical linacs) is achieved only for $\alpha >{45}^{\circ }$. After a material removal of 140 µm due to Hirtisation (R), a quality factor $Q_0$ of 6650 was achieved. This corresponds to 75% of the $Q_0$ simulated by CST^®. A 3 GHz SCCL concept consisting of 31 SCs was designed. The effective shunt impedance $Z{T}^2$ simulated by CST corresponds to $60.13\frac{\mathrm{M}\mathrm{\Omega }}{\mathrm{m}}$ and is comparable to the $Z{T}^2$ of SCCL in use. The reduction in $Z{T}^2$ expected after Hirtisation (R) can be justified in practice by up to 70% lower manufacturing costs. However, future studies will be conducted to further increase $Q_0$.

Linear particle accelerators (Linacs) are essential for numerous applications in industry, medicine, and research. However, their use is often constrained by high investment costs, which are largely driven by conventional manufacturing methods. Studies have shown that additive manufacturing (AM) has the potential to significantly reduce these costs while simultaneously enhancing Linac performance. This study investigates this potential for the production of radio-frequency quadrupoles (RFQs), one of the most important Linacs for ion acceleration. For the first time, a multi-material RFQ prototype was fabricated using multi-material (MM) laser powder bed fusion (PBF-LB/M) AM, integrating the copper alloy CuCr1Zr for the internal cavity and tool steel (1.2709) for the outer shell in a single manufacturing step. The prototype’s design features six ConFlat® (CF) flanges for sealing without O-rings, highlighting the advantages of MM AM in simplifying assembly and enhancing functionality. Experimental evaluations included assessments of geometric precision, surface roughness, microstructural integrity, vacuum performance, resonance frequency (f_R), and quality factor (Q₀). The results demonstrated the successful application of MM PBF-LB/M for RFQ production, achieving a vacuum pressure below 8.3 ∙ 10^–8 mbar in the prototype. However, challenges in geometric precision and material transition zones necessitate further process optimization. This work underscores the potential of MM PBF-LB/M to address limitations in mono-material AM, particularly for Linac applications. Future developments, such as enabling pure copper processing and improving thermal and mechanical performance, could establish MM PBF-LB/M as a transformative technology for advanced Linac designs, paving the way for enhanced Linac applications.

This work investigates the impact of an internal electric field on the annihilation characteristics of positrons implanted in a $180\left(10\right)\mathrm{nm}$ SiO₂ layer of a Metal-Oxide-Silicon (MOS) capacitor, using Positron Annihilation Lifetime Spectroscopy (PALS). By varying the gate voltage, electric fields up to $1.72\mathrm{MV}/\mathrm{cm}$ were applied. The measurements reveal a field-dependent suppression of positronium (Ps) formation by up to $64\%$, leading to an enhancement of free positron annihilation. The increase in free positrons suggests that vacancy clusters are the dominant defect type in the oxide layer. Additionally, drift towards the SiO₂/Si interface reveals not only larger void-like defects but also a distinct population of smaller traps that are less prominent when drifting to the Al/SiO₂ interface. In total, by combining positron drift with PALS, more detailed insights into the nature and spatial distribution of defects within the SiO₂ network and in particular near the SiO₂/Si interface are obtained.