Effect of mode transition on discharge features and power coupling in the source of a helicon plasma thruster

Influence of multiple mode transitions on discharge features and power coupling in the source region of a helicon plasma thruster were investigated. Abrupt variations of plasma density and wave structure were observed in experiments to verify the different discharge modes. Plasma morphology and emission spectrum were captured to characterize the discharge modes under two operating conditions: increasing input power and increasing magnetic field. Spatial distributions of plasma density were found to be closely related to magnetic field intensity. A pressured balance model was used to calculate the neutral density in different modes. Severe neutral depletion was observed in wave modes, and near-complete ionization (approaching 100 %) was found in the highest order wave mode, corresponding to the density upper limit (∼1.4 × 10¹⁹ m⁻³ at 0.3 Pa). Furthermore, the electron energy equilibrium was considered to estimate power deposition in the plasma. A significant increase in power deposition (up to 2.8 W cm⁻¹) was observed during mode transitions. In the so-called “Blue Core” modes, hollow density profiles were evident, attributed to extreme neutral depletion. We found that higher-order modes significantly enhance power absorption by plasmas, thereby improving propellant ionization. The applied magnetic field further determines the radial distribution of absorbed power, affecting the spatial configuration of the electric field and radial gradients within the magnetic nozzle that drive the azimuthal electron current responsible for generating j × B thrust. The results help to optimize the power absorption and radial allocation in the source region to enhance the propellant ionization and thrust generation of the helicon plasma thruster.