## 5 Discussions  
Visual observation of the plasma reveals that increased RF input power has a significant impact on both the color and intensity of the emitted radiation. The plasma \begin{table}
\begin{tabular}{c c} \hline \hline
**Source Type** & **MEICP—Helicon Waves Plasma** \\ \hline RF antenna type & Half-wavelength right helical \\ RF antenna length & 78 mm \\ Ionization chamber length & 150 mm \\ Inner radius & 15 mm \\ RF power & \textless{}600 W \\ Pulse length & 1 s between 200 and 500 W \\ Radio frequency & 13.56 MHz \\ Propellant gas & Argon \\ Nominal propellant flow rate & 12 sccm \\ Background magnetic field & \textless{}800 G \\ Core magnetic field strength & 400 G \\ The ultimate vacuum & \(2.4\cdot 10^{-5}\) mbar \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Representative operating parameters envisioned by the HPT test program. plume, containing high-energy electron-excited ions and low-energy electron-excited neutral particles, was analyzed using OES within the 400-850 nm wavelength range for argon discharge. For both modes, the argon propellant was fixed at 12 standard cubic centimeters per minute (sccm). Figures 11-13 illustrate the Helicon Plasma Thruster in operation, with Figure 11 representing the thruster under 200 W RF power in the Inductively Coupled Mode (H-mode) while and Figure 12 highlighting the emission spectrum characteristic of the ICP mode. Figure 13 point out showing the thruster operating at 500 W RF power in the Wave Coupled Mode (W-mode). In H-mode, the argon plasma displays a red-pink color, with its luminosity primarily concentrated near the HWRH antenna. In contrast, during W-mode, the plasma develops a distinct bright core throughout the entire discharge chamber, attributed to intense emission from excited argon ions (ArII). This bright core is well documented in the literature as a hallmark of the transition from H-mode to W-mode [19; 64]. Two ignition methods have been identified in the proposed HPT: the E-H mode transition for power levels below 300 W and the E-W mode transition for RF input energy of 400 W or higher. Figures 12 and 14 show two distinct particle groups that can be observed in the HPT: high-energy electrons excited ionic lines (ArII) and low-energy electrons excited neutral lines (ArI). Figure 14 illustrates a more pronounced rise in the intensity of the high-energy electron-excited ionic population compared to the low-energy electron-excited neutral population as the input power increases from 200 W to 500 W in a pulsed modulated power sequence. The decrease in neutral emission within the MEICP reactor of the HPT can be attributed to an increased ionization fraction of the argon, driven by more efficient electron heating through wave-particle interactions. During H-mode operation, the emission from argon neutral lines becomes more pronounced. A comparison between the ICP and HW modes of operation (Figure 15) in a Helicon Plasma Thruster was established using twenty emission lines with the highest signal-to-noise ratio (SNR) from a single emission spectrum. The following selected high-energy electron-excited ionic lines should be noted: 415.9 nm, 419.7 nm, 420 nm, 425.9 nm, 430 nm, 434.8 nm, 451.3 nm, 461 nm, 480.6 nm, and 488 nm. In the current RF-powered propulsion technology, the W-mode of operation has been experimentally demonstrated at 500 W, a lower power threshold compared to other experiments that generate BC modes inside a helicon reactor. [PERSON] et al. [64] reported BC generation at around 1000 W using argon gas and a double-loop antenna. [PERSON]. [PERSON] et al. [74] achieved BC at 1100 W with argon as the working gas and a double saddle antenna. Additionally, [PERSON] [75] demonstrated BC using argon and 1500 W RF energy with a Nagoya Type III antenna, while [PERSON] et al.