# Experimental Characterization of the Capacitively Coupled RF-Plasma Thruster
## 2 Materials and Methods
### Diagnostics
#### 2.3.1 Emissive Probes  
The emissive probe design is shown in Figure 5. The probe tip is made of a tungsten wire with a diameter of 0.025 mm. The tungsten wire was fused into the glass capillary tubes. The other end of these tubes was connected onto the copper contact wires. The whole assembly was placed into a long Al2O3 tube holder. The length of the wire exposed to the plasma is approximately 2 mm. The typical value of the electron current collected by the cold emissive probe varies between 1 and 50 microamperes. The probe has two electrical contacts for connecting the heating power supply. The needed values of heating current lie in the region of 200-250 mA at voltages between 1 and 2 volts, which corresponds to a heating power of 0.2-0.5 Watts. Figure 4: (**a**) General view of 2D moving platform with mounted emissive probes and (**b**) the definitions of measurement domain. Figure 5: (**b**) Photo of the set of two emissive probes and (**a**) structure of the single emissive probe: 1—Al\({}_{2}\)O\({}_{3}\) tube with outer diameter (OD) 1.6 mm, inner diameter (ID) 1.0 mm; 2—capillary glass tubes OD 0.5 mm, ID 0.3 mm, melted tips of glass capillary are indicated by white ellipses; 3—tungsten wire, diameter 25 \(\upmu\)m; 4—enameled copper wire, diameter 0.3 mm; 5—fixing glue for copper wires. An electrical scheme of the emissive probe measurements is depicted in Figure 6. A DC power supply Voltcrraft VSP 1410 was used to deliver the heating power to the emissive probes. A Keithhey 2410 source measure unit (SMU) connected to a PC was used for the measurements. The voltage-current characteristics of the probe were measured. It can be shown [10] that the plasma potential lies within 1.5 T\({}_{\text{e}}\) from the value of the floating potential of the emitting probe. The electron saturation current was used for the estimation of the electron density. Two kinds of measurements were performed. One is the so called \"full\" measurement, where the complete VCC of the probe was collected in each step. During the \"quick\" measurements only the floating potential of the hot probe was measured. The first type of the measurement allowed for the estimation of both the plasma potential and the electron density, the second type delivered only plasma potential information, but was performed with higher resolution. To illustrate the working principle of the emitting probe the voltage-current characteristics are shown in Figure 7a. The measurements at two different spatial positions with two different emission currents are presented. Figure 6: Electrical scheme of emissive probes. Figure 7: (**a**) Example of the emissive probe voltage current characteristics at two heating powers. The curves taken at two distances and two heating currents are shown for hot probe. (**b**) The comparison of the hot and cold regimes of the probe. The VCC of the cold probe (solid line) is compared to that of the hot probe (broken line). Distances from the engine cut-off are given. Arrow indicates the plasma potential. The electrons emitted by a hot probe are able to leave the probe only if the probe is biased negatively relative to the plasma potential. As long as the probe is more positive than the plasma potential the thermionic electrons do not contribute to the probe current. Thus in this range the probe is working like a Langmuir probe in collecting mode and the VCC of the hot probe should coincide with that of the cold probe. When scanning the probe voltage the transition through the plasma potential value is marked by the sudden change of the probe current by the value of the thermionic current. Therefore the emissive probes are well suited for the determination of plasma potential. The transition from the collecting mode (probe is positive) to the emitting mode (probe is negative) is marked by the sign reversal of the probe current. It can be shown, that in the limit of the large emission current the floating potential of the emissive probe approaches the value of the plasma potential [11].