## 3 Development Strategy of a HPT Based on MEIC Plasma Reactor  
The most significant figures of merit in EP systems are specific impulse, thrust efficiency, and thrust-to-weight ratio. The first two figures are closely tied to the characteristics of the plasma discharge. Specific impulse \(\left(I_{sp}\right)\) is directly proportional to the square root of the plasma's internal energy, meaning that achieving a high specific impulse requires a high plasma temperature. A higher internal energy translates to higher exhaust velocities, increasing specific impulse and improving propellant efficiency. Achieving high thrust efficiency involves several key factors. First, efficient wave-plasma energy conversion from the antenna is critical to ensuring that the maximum amount of input energy is transferred to the plasma. Second, efficient plasma heating is essential to raising the plasma's internal energy, which contributes to higher specific impulse and thrust. Finally, the system must effectively convert the internal energy of the plasma into directed axial energy, which propels the spacecraft. Minimizing plume divergence is crucial in this process, as it ensures that most of the energy is directed in the desired direction, maximizing thrust and overall efficiency. Table 2 outlines the HPT requirements. The proposed HPT is engineered with a multi-stage architecture, as illustrated in Figure 3. This architecture is strategically divided into two interdependent stages that work in synergisty: the plasma ionization stage and the plasma acceleration stage. \begin{table}
\begin{tabular}{c c} \hline Power Class & 500 W \\ \hline Thrust & 10 mN \\ Specific Impulse & \textgreater{}1000 s \\ Plasma Density & \(10^{12}\)\(\div 10^{13}\) cm\({}^{-3}\) \\ Propellant Flow Rate & \(8-20\) sccm \\ Operating Frequency & \(13.56\) MHz \\ Electron Temperature & \(3\div 5\) eV \\ \hline \end{tabular}
\end{table} 
Table 2: Helicon Plasma Thruster requirements. Figure 2: Representation of the electric field pattern of the \(m=+1\) azimuthal mode in an MEICP reactor. The first stage is represented by a compact MEICP reactor with multi-dipole cusp confinement based on permanent magnets. In this configuration, RF power, operating in the megahertz range, primarily couples with the electrons in the plasma. The energy transferred to the electrons heats them, and this energy is subsequently converted into ion beam energy through electrostatic acceleration. This process occurs via an ambipolar electric field, specifically within a current-free double layer [58; 59], where electrons that overcome the potential drop effectively neutralize the accelerated ions [60; 61]. The second stage of the HPT is defined by the MN. In this stage, an internal azimuthal electric field current is spontaneously generated within the plasma. This current facilitates electromagnetic plasma acceleration through the Lorentz force, which efficiently directs and accelerates the plasma, contributing to the overall thrust of the system [19; 62; 63]. The thrust in a HPT is generated progressively within the magnetic nozzle, where the internal plasma energy is converted into axial kinetic energy. This process involves efficient plasma detachment and momentum transferring from the plasma to the spacecraft. The MEICP reactor is composed of several key elements:  
* A cylindrical ionization chamber made from a low thermal expansion material, with a diameter of 30 mm and a length of 150 mm, featuring a convergent nozzle at the outlet with a 10 mm diameter;
* A cooper half-wavelength right helical (HWRH) antenna (Figure 4), measuring 78 mm in length and 1.25 mm in thickness, surrounded the ionization chamber.