Motivation and background
With the increase in the number of manned and unmanned space exploration missions and interplanetary space missions executed by governments and private companies alike, the development of durable, reliable, powerful, and highly efficient propulsion systems with extended operational lifetime is paramount. To date, the prevailing electric propulsion technologies such as Hall effect thrusters and many other EP technologies such as high power magnetoplasmadynamic thrusters use electrodes for propellant ionisation, ion acceleration, and to neutralise the ion discharge. These electrodes erode over time, limiting their operational lifetime and degrading their performance. Also, cathode poisoning issues significantly limit the variety of propellants that can be used and can act as a potential point of failure in electric thrusters. These effects of electrode erosion become more prominent when the thruster operates at high power levels. Therefore, high power advanced EP systems with extended operational lifetimes and high efficiency are crucial to making interplanetary space travel more efficient and interstellar exploration more feasible.
Thruster overview
The Spherical Tokamak Thruster is a novel high power plasma thruster for in space propulsion, currently under development at the Imperial Plasma Propulsion Laboratory. Inspired by the operational principles of spherical tokamaks and magnetic confinement fusion, this electrodeless pulsed thruster employs the double null merging (DNM) technique for the purposes of plasma formation. This technique, developed within the fusion energy community, is used to generate two spherical plasma rings in the upper and lower regions of the plasma chamber using two sets of poloidal field coils. These plasma rings are then merged at the midplane of the chamber via a phenomenon known as magnetic reconnection which efficiently converts the stored magnetic energy into plasma thermal energy. The superheated core plasma is confined by the toroidal and poloidal magnetic fields generated by the toroidal and poloidal field coils respectively. Plasma formation via DNM can theoretically produce high temperature plasmas and high plasma currents ensuring high propellant ionisation and utilisation rates. The position, control and shaping of the plasma is facilitated by the poloidal field coils. A divertor coil is utilised to generate a poloidal magnetic null point below the core plasma whereby the plasma propagates from the core through the separatrix (last closed flux surface) and into the SOL region via cross-field transport. The SOL is a region of ‘open’ magnetic field lines that directs the plasma towards the lower region of the thruster where it leaves the system, thereby generating thrust.
The thruster consists of a cylindrical dielectric plasma chamber with 6 external toroidal field coils and 5 poloidal field coils positioned inside the chamber. The thruster is 25 cm in diameter and 33 cm in height. The thruster operates at a ~ 6 MW power level per pulse (< 3 ms). The thruster's electrodeless design not only allows for extended operational lifetime but also ensures compatibility with a broad range of propellants, spanning noble gases like xenon and krypton to molecular options such as water and nitrogen. These capabilities, combined with the exceptionally high specific impulse, make it an attractive candidate for application to both unmanned and manned interplanetary and deep space exploration missions. A lab prototype of the thruster has been manufactured. Experimental characterisation of the thruster via various test campaigns is currently in progress.