## 4 Experimental Setup and HPT Test Plan
At higher neutral pressures, an MEICP reactor can transition into wave-heated mode (W-mode) even at relatively low RF power levels, specifically below 500 W [36]. To promote the development of the blue core within the plasma column at relatively low RF power and prevent thermal damage to both the antenna and the discharge chamber, a PWM was proposed. A software routine was developed to automatically control the RF power and propellant flow, following the specific pattern shown in Figure 10, to generate pulsed-power-modulated high-density argon plasma within the HPT. The duty cycle consists of three phases: an Off Timer (toff), during which both RF power and gas flow are either turned off or set to a low value; an On Timer (ton), where both RF power and gas flow are activated or set to a high value simultaneously, and a delay (tdelay) between the activation of the gas flow and the RF power. In the proposed testing program, two key parameters were modulated: the RF input power and the argon mass flow rate. Starting from the initial parameters listed in Table 3, the optimal ignition sequence was experimentally determined by adjusting the initial values. The initial and optimized PWM ignition sequences are shown in Table 3.  
Figure 9: A schematic sketch of the communication connection overseen for the HPT experimental testing campaign. During the initial phase of the on-time, the electron temperature rapidly rises above steady-state levels. This rapid increase, driven by the application of higher power relative to continuous wave (CW) operation, enhances the confinement of charged particles within the thruster's plasma production stage. Conversely, during the early off-time phase, the electron temperature quickly decreases, reducing Bohm velocity and, consequently, lowering particle losses. This sequence leads to a higher time-averaged power output. The innovation of the proposed testing program lies in its ability to control the reaction within the MEICP reactor by adjusting the modulation period. This approach enables multi-mode operation, allowing for transitions between inductive (H) mode and wave (W) mode. The pulse mode starts with an ultimate vacuum of \(2.4\cdot 10^{-5}\) mbar. In this approach, gas insertion is synchronized with the RF power, with both being activated and deactivated according to the pulse scheme. This method allows the gas to enter the thruster tube, creating a momentary increase in pressure which, when combined with the RF pulse, ignites the plasma more efficiently and effectively than in continuous mode. The argon propellant flow rates remain constant at 12 sccm for the final testing campaign. The pulse parameters, including the duration of the RF pulse, the gas flow duration, the delay between gas flow and RF pulses, and the interval between pulses, RF power levels, and gas flow rates, were all adjustable to optimize reflected power and plasma discharge characteristics. The impedance-matching network's capacitance was adjusted by altering the settings on the two capacitors, achieving optimal impedance matching for the plasma discharge. This match was validated by monitoring the RF power supplied to the plasma, as indicated on the RF power supply display. Additionally, the effectiveness of the match was confirmed through visual assessment of the plasma's brightness and stability, observed through a vacuum window port. Table 4 summarizes the operating parameters for the HPT test program. Figure 10: RF envelope-auto ignition of the MECP reactor under a pulsed wave modulation (PWM) regime. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**State** & **Plow (W)** & **Phigh (W)** & **Flow (sccm)** & **Fhigh (sccm)** & **Toff (s)** & **Tdelay (s)** & **Ton (s)** \\ \hline Initial & 17 & 150 & 5 & 20 & 6 & 1 & 0.2 \\ Optimized & 8 & 500 & 0 & 12 & 0.4 & 0 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Initial and optimized PWM ignition sequences.