###### Abstract  
This paper presents an investigation into the plume characteristics of composite propellants fabricated by polytetrafluoroethylene (PTFE) filled with different carbon additives (nano-carbon powder, graphite, and graphene) under laser irradiation in a vacuum environment. The dynamic plumes generated by the laser ablation of different modified propellant samples were captured using a high-speed camera, and the feature parameters of the plumes were extracted by image processing. The results indicated that doping carbon particles in PTFE enhanced the quality of the plasma plumes. The plume area increased up to a certain value and then stabilized, while end of plume clusters remained for a short time. Further analysis revealed that the propellant sample doped with graphene exhibited the maximum plume length and expansion rate, whereas the propellant sample doped with nano-carbon demonstrated the largest plume area. Moreover, a higher graphene doping ratio promoted greater plume length, expansion speed, and plume area. However, when the doping ratio exceeded 3%, the gain of the plume parameters gradually became saturated, and the optimal doping ratio appeared to be 5%. laser-assisted pulsed plasma thruster; composite propellant; carbon dopant; plume +
Footnote †: journal: Applied  
According to the electrode layout, conventional PPTs can be divided into parallel and coaxial types. Compared with parallel PPTs, coaxial PPTs can more effectively restrain plume expansion and improve energy utilization, which is expected to improve performance [15]. PPTs are compatible with solid, liquid, and gas propellants. In contrast to the solid propellants, the liquid- and gas-fed PPTs exhibit a wide-range of control, reliable ignition, and have various material candidates such as water, ethanol, and liquid metal for liquid propellants [16] and helium, xenon, argon for gas propellants [17], but these are limited by their complex supply systems, leakage potential, and valve lifespan. Solid propellants (normally PTFEs) still receive more attention [18]. Figure 1a presents a schematic diagram of a PPT in its coaxial form where a solid propellant bar between two electrodes is connected to a charged capacitor in a vacuum environment. The operation of the PPT starts with the generation of small amounts of charged particles from the spark plug that initiate a surface flashover of the propellant, leading to the formation of an electric arc. The intense heat produced by this arc causes the propellant to sublimate and ablate, transforming it into a gas and eventually into a plasma, which initially advances in the discharge channel at a low speed (<500 m/s) due to aerodynamics [5]. As the plasma conducts the circuit, it allows a large current to flow through it. This flow of electrons induces a strong electromagnetic field, exerting a Lorentz force on the plasma and accelerating it out of the PPT at an extremely high velocity (>30 km/s) [5]. However, the propulsive efficiency is very low (-10%) compared with other forms of electric propulsion due to energy losses caused by late-time ablation and rapid conductive heat transfer from the propellant to the rest of the spacecraft. Moreover, the unstable ignition of the ablative pulsed plasma thrusters (APPTs) remains a challenge because of spark plug soot [19]. Laser-assisted pulsed plasma thrusters (LAPPTs) were originally conceived to address the problem of late-time ablation in conventional PPTs while featuring a straightforward design, precision control, and a relatively high impulse-coupling coefficient [20; 21]. As shown in Figure 1b, an LAPPT employs a laser as the ignitor instead of a spark plug. The propellant then undergoes decomposition and vaporization through laser irradiation, transforming it to a gaseous state. Subsequently, the gaseous substance is further ionized into a plasma in the discharge tunnel, which is then accelerated by a Lorentz force to produce the thrust.