## 3 Experimental Set-Up
### Final Experimental Set-Up  
The results of the pre-test numerical analysis formed the basis for the final set-up of the HELOKA loop. The validity of the cooling scheme was confirmed by the RELAP5-3D analysis, and the final layout of the pipes connecting the manifolds with the FWMU was decided following a dedicated stress analysis (Figure 13). The complex arrangement of these pipes is due to the fact that the pipes had to fit within the lower half of the HELOKA vacuum chamber (3 m in diameter) while maintaining the stress levels below the allowable limits defined by DIN-EN 13480. With the electron beam gun mounted on the top side of the vacuum tank, the upper half of the vessel was kept free of any helium piping to avoid the risk of accidentally heating them up while applying the heat load on the mock-up surface. To keep the pressure losses in the connecting pipes at a reasonable level, the inlet and outlet pipes vary in diameter along their path from and to the manifolds. Thus, the pipe segments connected to the manifolds have a 21-millimeter inner diameter (1-inch precision pipes). Similar to the mock-up, the size of the pipes had to be reduced to 9 mm inner diameter (1/2 in pipes) to keep within the allowable limits the stress levels in the mock-up nozzles. Figure 12: Average helium temperature in the FWMU channels. Figure 13: Inlet and outlet pipes layout. At the connection between the 1 in and 1/2 in pipes several sensors were installed as follows:  
* Six coolant temperature sensors (TS-T-002, TS-T-003, TS-T-004, TS-T-007, TS-T-008, TS-T-009) were installed at the outlet pipes of channels 2, 3, 4, 7, 8, and 9. The sensors are 3 mm in diameter type K, tolerance class 1. To increase the response time, the sensors were directly immersed in the helium stream. * The pressure drop (TS-P-006, TS-P-005, TS-P-004, TS-P-001, TS-P-002, TS-P-003) and the outlet pressure (TS-P-016, TS-P-015, TS-P-014, TS-P-011, TS-P-012, TS-P-013) of the six channels of interest (2, 3, 4, 7, 8, and 9) were also monitored. The absolute pressure sensors are of type DMP 320 from BD Sensors GmbH (<0.1% accuracy at a measuring range of 10 MPa) while the differential pressure sensors are Siemens SITrans P DSIII with a measuring range of 0 to 5 bar (accuracy 0.065% \(\times\) 0.5 MPa). * Two sensors (TS-T-013 and TS-T-018) were clamped on the outside of the channels 3 and 8 inlet pipes, measuring the coolant inlet temperature (1-millimeterthick, type K, tolerance class 1 sensors). * No dedicated measurements were made for channels 1, 5, 6, and 10. A complete list of the sensors used for the present analysis is presented in Table 3. The table also includes the accuracy of the sensors as given by the manufacturer. The only exception is the mass flow rate, which was calculated according to DIN-EN-ISO 5167-1 and 2:2004-01 standards from the measurements of the pressure loss over the orifice (TS-P-F01 and TS-P-P02) and the helium density at the pressure and temperature measured by the absolute pressure (TS-P-F11, TS-P-F12) and temperature sensors (TS-T-F01, TS-T-F02), respectively. The heat load on the mock-up surface was applied using an electron beam gun. To have a sharp transition at the edges of the heated zone and limit the power deposition