## 2 Experimental Apparatus  
The examined eight-turn FPPHP (Figure 1) [28] is constituted by two main heat transfer sections, named the evaporator, where power is externally provided, and the condenser, where power is dissipated to the external environment, respectively. The remaining portion, i.e., the section in between the two mentioned heat transfer zones, is named the adiabatic section: here, no significant heat transfer phenomena generally occur between the device and the external environment. The FPPHP has a total of 16 channels with 3 \(\times\) 3 mm\({}^{2}\) section in the adiabatic zone, resulting from the machining process of a copper plate (width: 80 mm, length: 200 mm, thickness: 3.5 mm). The first machined plate is brazed to a second copper plate, having same dimensions and thickness equal to 0.5 mm. Power is provided to the 80 mm long evaporator zone by a wire electrical heater (Thermocoa(r) Type NcAc15, 1 mm external diameter), Figure 1b, and connected to an electrical power supply (ELC(r) AL8220, \(\pm\)10 mV). The condenser section is instead cooled by a single-phase loop connected to a copper cold plate (width: 100 mm, length: 80 mm), directly brazed on the rear side of the FPPHP (Figure 1b). The loop temperature is controlled by a cryostat (Huber(r) CC240 wl). Two pipes are brazed on the top and bottom side of the device, allowing the filling and vacuuming of the system, respectively. The evaporator temperature was monitored by means of five T-type thermocouples (Figure 1a) directly attached on the FPPHP surface, while the condenser temperature was acquired through three T-type thermocouples on the rear surface of the cold plate. The thermocouples' location at the evaporator was chosen with the aim of achieving a satisfactorily uniform assessment of temperature referred to each heating section over its whole extension, without unnecessarily increasing the complexity of the manufacturing process. Given the high thermal conductivity of the adopted manufacturing material, a limited number of thermocouples at the specified locations was believed to provide reliable evaporator temperature measurements, especially for the sake of the device thermal characterization, which is based on an average value of evaporator temperature (computed over a long time-span) rather than on punctual temperature values. The ambient temperature was measured by one T-type thermocouple. The thermocouples signals were sampled at 1 Hz. In addition, the front side surface of the device was coated with a highly emissive paint (\(\varepsilon\) = 0.92 \(\pm\) 0.01) to allow measurements of the outer wall temperature within the adiabatic section by a medium wave infrared (MWIR) camera (FLIR(r) SC5500, sampling frequency: 50 Hz, pixel resolution: 320 \(\times\) 256). The opaque paint emissivity, along with its uncertainty, was empirically estimated by performing dedicated IR acquisitions on a coated isothermal surface (same opaque paint, at controlled temperature and monitored through thermocouples). The emissivity parameter set in the IR camera software (Altair(r), version 5.91.010, FLIR, Wilsonville, OR, USA) was adjusted until a match between the temperature measured by thermocouples and the one by the infrared camera was reached in the range of uncertainty of the camera (\(\pm\)1 \({}^{\circ}\)C), resulting in an emissivity of 0.92 \(\pm\) 0.01. The estimated value is in accordance with the emissivity of longer waves of most opaque, commercial paints [29]. Since the emissive layer had a thickness lower than 100 \(\upmu\)m, its thermal resistance was considered as negligible, thus reasonably assuming with that the temperature measured on top of the coating corresponded to the temperature of the device surface. In particular, the adiabatic section was framed over a 40 mm wide portion (green window in Figure 2a). For visualization purposes, no insulation was adopted along the adiabatic section.