Development and Investigation of a High Pulse Rate Planar Inductive Pulsed Plasma Thruster

Abstract

Planar inductive pulsed plasma thrusters promise to offer efficient, high thrust density electric spacecraft propulsion at moderate to high specific impulse. Unlike many other types of electric propulsion, they can be readily throttled while maintaining optimal performance and can operate on a wide range of propellants, including those that are reactive or molecular. A combination of theoretical work and experimental results from other pulsed thrusters has suggested that operation of planar inductive pulsed plasma thrusters at high pulse rates, which are defined here as rates in excess of 1 kHz, could simplify the operation and improve the performance of these devices. To be viable in the near to medium term, however, a high pulse rate planar inductive pulsed plasma thruster would have to operate at much lower discharge energies (≲ 10 J) than previous, lower pulse rate (≲ 10 Hz) designs. To investigate whether efficient thruster operation remained possible at these lower discharge energies, a compact planar inductive pulsed plasma thruster, along with a supporting power processing unit, was built and tested. A highly modular prototype power processing unit based on a paralleled boost converter design has been developed for recharging the capacitor bank of inductive pulsed plasma thrusters operating at pulse rates of 1 kHz to 10 kHz. Operation at these rates, which are one to two orders of magnitude higher than in previous designs, has been hypothesized to allow for efficient quasi-steady thruster operation while also improving lifetime and reliability. Benchtop testing of a prototype unit has demonstrated successful operation at pulse rates from 1 kHz to 10 kHz, voltage gain as high as 25.5, power handling of 1 kW to 9.1 kW, and electrical efficiency up to 70%. Quasi-steady operation has been achieved up to 20 pulses and the design has been shown to maintain functionality in the event of a simulated component failure. Theoretical and circuit simulator models of the prototype were developed and leveraged to identify the dominant loss mechanisms. The efficiency was determined to be primarily constrained by high switching losses, which were estimated to account for over 70% of total unit losses in the cases investigated. Scaling laws for the efficiency of the design were developed using simple linearized circuit equations and indicated thruster performance was primarily dependent on the ratio of the input and output voltages, the inductive recapture ratio of the thruster, and ratios of the relevant charging and switching timescales.

The evolution of the current sheet is fundamental to the understanding and operation of planar inductive pulsed plasma thrusters. Methods for experimentally determining the time-varying mutual inductance and plasma resistance associated with the current sheet are presented. From these, time-histories of the resistive heating in and electromagnetic acceleration of the sheet are found. Analysis of experimental data obtained from a compact, low discharge energy inductive pulsed plasma thruster shows that current sheet evolution is well described by three phases: ionization, formation, and acceleration. Evidence is provided for the plasma current and resistive heating becoming localized near the upstream edge of the current sheet as the sheet forms. The influence of initial propellant density and pre-ionization on the characteristics of the sheet are examined. Both higher propellant densities and increased pre-ionization are found to produce current sheets which exhibit greater magnetic impermeability. Higher densities cause the sheet to form closer to the coil face, improving coupling, while stronger pre-ionization leads to more rapid resistive heating, causing the sheet to form earlier in time. Formation of a well-defined, magnetically impermeable current sheet is central to the operation of planar inductive pulsed plasma thrusters. Existing scaling laws for these devices, however, are primarily derived from models that either overlook or greatly simplify the current sheet formation process. Data obtained from a compact, low discharge energy planar inductive pulsed plasma thruster were used to gain insight into the fundamental physics governing current sheet formation in these devices. It was found that maximum inductive coupling and current density in the sheet scaled with the ratio of the resistive diffusion to ionization timescales. The ratio of these timescales was shown to describe the combined role of diffusion and ionization in influencing the electron population within the current sheet. When the ionization timescale was fast relative to the diffusive timescale, the thruster discharge was found to be much more effective at concentrating current in the sheet. A connection between the strength of the current sheet and the thruster design and operational parameters was also established and used to explore the parameter space over which efficient thruster operation was possible. Evidence was found that it was difficult to form a magnetically impermeable current sheet at low discharge energies and mass bits, which strongly suggests that the process of current sheet formation cannot be neglected when investigating the performance of planar inductive pulsed plasma thrusters operating in this parameter space.