## 2 Experimental Setup and Methods  
Two nozzles with different geometries were measured regarding oxygen concentration distribution. These are a three-jet nozzle, which is an in-house product of _Oerlikon Metco_ and not commercially available, and the _Coax 40S_ coaxial nozzle from _Fraunhofer ILT_. The _Metco_ powder nozzle has a powder channel diameter of \(d_{c}\) = 2 mm and a standoff of 12 mm. Standoff is defined as the distance between the nozzle tip and the powder focus. The ILT coaxial nozzle has an angular cavity of 200 \(\upmu\)m between the outer and inner cone and a standoff of 7 mm. The experimental setup (see Figure 1) was developed for the measurement. A hole plate was fabricated so that the diameter of the hole matches the outside diameter (diameter: 1/16'') of the sampling tube of the oxygen sensor. The _Dansensor ISM-3_ was used to measure the oxygen concentration with an accuracy better than \(\pm\)1% of the displayed measured value. To obtain a first insight into the spatial extent of the zone with low oxygen concentration (<10,000 ppm), both nozzles were approached from the outside in the direction of the gas sampling hose at a carrier gas flow of 4 L/min and a shield gas flow of 15 L/min at the working distance of the respective nozzle (standoff three-jet nozzle: 12 mm, standoff coaxial nozzle: 7 mm) until a change to the ambient oxygen concentration (20.9 vol.%) occurred. Below a certain distance, a strong oxygen gradient was present. To better resolve this area, the point grid was refined in this region. These experiments were used to create an oxygen concentration distribution at the respective working distance of each nozzle with turned off laser and powder flow. The laser must be turned off because it would destroy the gas sampling hose and the powder flow must be turned off because it would destroy the ceramics which are inside the oxygen sensor. This has no impact on the measured oxygen concentration because the influence of thermally induced flows is negligible (which is shown in the Numerical Modeling section) and because of the low volume percentage of the powder particles inside the carrier gas  
Figure 1: Measuring principle for determining the spatial oxygen concentration. A scanning strategy or a stationary measurement can be utilized. flow. Further experiments investigating the oxygen concentration change directly above the gas sampling hose (at working distance of the respective nozzle) at different carrier and shield gas flows were performed. Those allowed us to investigate the change in oxygen concentration depending on the carrier and shield gas flows. In addition to the nozzle characterization in ambient environment, the oxygen distribution measurements were performed inside a shield gas chamber to measure the impact of the chamber on the resulting oxygen concentration. Therefore, the oxygen point grid experiments were repeated, as well as the carrier and shield gas variations. The influence of the shield gas chamber on the spatial oxygen concentration can thus be demonstrated. Before the measurements could be carried out in the shielding gas chamber, the shortcomings of the shielding gas chamber regarding ventilation still had to be eliminated. Due to the lack of pressure equalization holes as well as the one-sided ventilation, a homogeneous inflow in the shielding gas chamber is not possible. This results in too high oxygen concentrations (2231 ppm) inside the chamber, as can be seen on the oxygen sensor display (see Figure 2). To achieve a more homogeneous inflow, pressure equalization holes were fabricated on the top of the chamber and a diffusor ring was 3D printed, which was flow optimized using computational fluid dynamics simulations (see Figure 3). The diffusor ring with the slit (bottom picture of Figure 3) allows a uniform, homogeneous inflow of argon from all directions at simultaneous low inflow velocities. This avoids turbulence within the shielding gas chamber, which has a favorable effect on the present oxygen concentration.