Radiofrequency Plasma Generation and Transport in Converging-Diverging Magnetic Fields

Abstract

Low-pressure plasmas evolving through converging-diverging magnetic fields, commonly called magnetic funnels or nozzles, are encountered in space plasmas and space-propulsion devices. A new purpose-built device produces an argon rf plasma in a 1.5 m long discharge glass tube along which two movable solenoids can be positioned from being over the rf antenna to being up to 0.8 m away, giving access to a wide range of magnetic field configurations. With the solenoids typically located 30 cm to 40 cm downstream of the rf antenna, two modes of operations are observed, depending on the intensity of the magnetic field. Below a critical intensity, the plasma density is seen to have two local maxima along the tube, one under the antenna and one under the solenoids. Above the critical value, the plasma is characterised by a single-peaked density profile collocated with the solenoids. Measurements of the wall potential and of the electron energy distribution functions (EEPFs) under the antenna as the magnetic field intensity is varied showed that the mode transition is correlated with the change in ion magnetisation under the rf antenna. When ions are unmagnetised, the increased local ion flux to the wall promotes the loss of energetic electrons created in the rf skin layer under the antenna. The discharge becomes more localised to the antenna when these electrons cannot travel downstream along the magnetic streamlines to promote remote ionisation. In the single-peak mode, a blue core with densities ≥ 10¹² cm−³ for 200 W of rf power is characterised. The EEPFs show the presence of electrons with energies sufficient to ionize neutrals, suggesting that electrons act as the remote rf energy carriers as they lose their energy to ionisation along the streamlines. Strong Ar II emissions features support this finding. Measurements of the plasma potential contours show that the radial potential profiles reduce the ions’ cross-field diffusion under the solenoids. Axial potential profiles show an axial Coulomb force set to counteract the adiabatic reflection of electrons at the mirror. The force can effectively accelerate electrons and confine them to the solenoids’ region. The evolution of the electron’s internal energy is measured along the central streamline and along one of the most radial streamlines to cross the antenna. Departure from the non-locality of the EEPFs along the streamlines is the first weak direct evidence of the remote ionisation from electrons energised under the antenna and accelerated by the ambipolar field. The polytropic index along both streamlines is calculated and cautiously interpreted as giving clues of non-local ionisation along the most radial streamline with γ < 1.