# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 6 Radio-Frequency Ion Thruster (RIT)  
### Operating Principle  
Radio frequency ion thrusters belong to a subset of gridded ion thrusters that generate thrust by accelerating the ionized propellant (plasma) through an electrostatic grid. Electron bombardment and microwave thrusters are some of the other gridded ion thrusters [63]. In RITs, the stored propellant is let into the discharge chamber where it is ionized (and becomes plasma) by means of Radio Frequency (RF) power (from RF coils). The ionized propellant is then extracted (from the discharge chamber) and accelerated by a series of grids (ion optics) called screen and accelerator grids. The screen grid extracts propellant cations (for instance, Xe\({}^{+}\), Kr\({}^{+}\) ions) from the ionized plasma and directs them downstream towards the accelerating grid [63]. Bigger RF ion engines that are used on larger spacecraft also have a third grid called the decelerator grid, however the it is typically not used in CubeSat propulsion systems. A neutralizer cathode, present on the exterior of the thruster in all ion engines, provides electrons to neutralize the ionized propellant that is emitted from the thruster [63]. The specific impulse of a gridded thruster can be varied by changing the voltage that is applied to the accelerating grids [64]. Electron bombardment and microwave thrusters are some of the other types of gridded ion thrusters where the ionization occurs due to electron bombardment with the neutral propellant and microwave power respectively [63]. The schematic of an RF Ion propulsion system is shown in Figure 5 that includes the propellant storage, RF coil, discharge chamber, grids (screen and accelerator) and a neutralizing (external) cathode. Ion thrusters are characterized by high thruster efficiency (60% to \(>\)80%) resulting in high specific impulse (from 2000 s to over 10,000 s) [63]; however, they have been plagued with issues that are caused by cathode wear [65] and contamination over prolonged usage [66]. Various types and composition of contamination are explained in the following sub-section.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Company/Institution with Location** & **Engine** & **Thrust (mN)** & **I\({}_{\text{pr}}\) (s)** & **Power (W)** & **Propellant** & **Remarks** & **Ref.** \\ \hline SSTL, Guildford, UK & LPR & 18 & 48 & 30 & Xe & flown on NovaSAR-S (100 kg) & [59; 60] \\ \hline CU Aerospace, Champaign, IL, USA and VACCO Industries Inc., Huntsville, AL, USA & & 5.4 & 65 & 15 & SO\({}_{2}\) & - & [30; 61] \\ \hline CU Aerospace, Champaign, IL, USA and VACCO Industries Inc., Huntsville, AL, USA & CHIPS & 30 & 82 & 30 & R134a, R236 fa & - & [58; 62] \\ \hline Busek, Natick, MA, USA & AMR & 10 & 150 & 15 & R134a, R236 fa & - & [9; 30; 49] \\ \hline University of Southern California, Los Angeles, CA, USA & FMMR & 0.129 & 79.2 & - & Water & - & [54] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of Resistojet Propulsion Systems.  
The ion exhaust velocity (shown in Equation (12)) and thrust (shown in Equation (13)) are both functions of the charge of propellant ion (\(q\)), mass of propellant ion (\(n_{ion}\)) and ion accelerating voltage (\(V_{i}\)). Ion engines use heavier elements (elements with higher atomic mass) as propellants because the thrust generated is proportional to mass of the ion (propellant). Thrust, however, also depends on the ion beam current (\(I_{i}\)) [63]. Specific impulse (shown in (14)) is a function of ion accelerating voltage and mass of ion [63]. The performance factors of ion engines are explained below with their mathematical equations:  
\[v_{e_{i}}=\sqrt{\frac{2 qV_{i}}{m_{ion}}} \tag{12}\]  
\[\tau=\sqrt{\frac{2m_{ion}V_{i}}{q}}I_{i} \tag{13}\]  
\[I_{sp}=1.417\times 10^{3}\gamma_{c}\eta_{m}\sqrt{\frac{V_{i}}{m_{ion}}}, \tag{14}\]  
where \(\eta_{m}\) is the thruster mass utilization efficiency and \(\gamma_{c}\) is the total thrust correction factor.