# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 9 Pulse Plasma Thruster (PPT)
### Design Considerations and Technologies  
Analyses indicate that \(45\pm 11\%\) of ablated mass per pulse in a PPT is expelled at negligible speeds relative to the effective plume exhaust velocity (1500 m/s) and on a time scale longer than the 0.5 s primary discharge [117]. Due to delayed ablation and particulate emissions, PPTs have a very low efficiency (10-20% [116]) [114]. Particulates, sometimes referred to as Macro-Particles (MP), are emitted during a pulse and may interact with the surrounding plasma. In addition, particulate emissions consume about 40% of the total propellant mass, while contributing only 1% towards the total thrust generated [114]. In the case that late ablation is diminished or totally prevented, the specific impulse and efficiency of the thruster can be substantially raised [115]. Teflon is a widely used inert and non-toxic [118] solid propellant in PPTs, and as a result, it is of great importance to study the phenomena that occur with its use. One such phenomenon is the charring of electrodes. It was experimentally observed that current density and Teflon surface temperature have their maxima near the electrodes due to their preferential ablation. Microscopic analysis of the charred  
Figure 8: Schematic of a Pulse Plasma Thruster. area has shown the presence of carbon due to the carbon flux returned from the plasma. In some cases, metal layers originating from electrode erosion were also found under the carbon char [113]. Spark (igniter) plugs are normally used in a PPT to provide a large supply of free electrons to initiate an electric discharge between the electrodes across the exposed surface of the propellant (Teflon). They are extensively used because of the unavailability of any other suitable alternatives. Various studies are being conducted to prove the feasibility of shining an Infra-Red (IR) laser pulse on the thruster's backplate or a self-triggering design where the voltages in the electrodes can exceed the breakdown voltage of the discharge gap. Although the proposed studies are effective theoretically, they seem to pose design challenges for spacecraft usage [119]. Development of an efficient pulsed plasma accelerator is a challenging proposition due to the problems with propellant conversion efficiency hindering the breakdown of propellant gas. Additional challenges result due to electrode erosion and the difficulty in transferring all energy input to the ionized propellant (gas). Planar Pulsed Inductive Thruster (PIT) is a modified PPT technology first developed in the early 1960s to overcome the existing issues (efficiency and electrode wear) with PPTs. PIT is an electrode-less system that could accommodate a wider variety of propellants like CO\({}_{2}\), ammonia, hydrazine and H\({}_{2}\)O. PITs have also demonstrated their operation with efficiency being relatively constant over a wide range of I\({}_{sp}\). They potentially process high levels of power that result in relatively high thrust using a single thruster [112]. Vacuum arc discharges that ablate and consume cathode material (solid propellant) in vacuum produce fully ionized plasma jets having high velocity. Such devices are called Vacuum Arc Thrusters (VATs) and have been studied for propulsion applications since 1960s [120]. Vacuum arcs may also be used as plasma sources in electrostatic accelerators such as ion or Hall thrusters [121]. VATs offer several potential advantages like simplified thruster design and lower mass (due to the absence of gas feed system), higher efficiency (due to the highly-ionized plasma generation) and discrete pulse operation without sacrificing plasma production efficiency (this control allows for fine-tuning of spacecraft maneuvers) [120; 121].