Response of an ion-ion plasma to dc biased electrodes

Electronegative plasmas are plasmas containing a significant fraction of negative ions, when magnetized they are very often segregated: the core is electropositive or weakly electronegative whereas a highly electronegative plasma forms at the periphery. At strong magnetic fields this segregation can lead to the formation of ion-ion plasmas almost free of electrons close to the walls or extraction surfaces and allows access to both positive and negative ions. The PEGASES thruster aims at alternately extracting and accelerating positive and negative ions from the ion-ion plasma region to provide thrust by both types of ions. The acceleration schemes depend on the possible control of the potential in an ion-ion plasma relative to the acceleration grids. In this paper continuous extraction and acceleration of positive ions from the PEGASES thruster is investigated by a retarding field energy analyser. It is shown from the measured ion energy distribution functions that the continuous acceleration potential can be controlled by biasing bare electrodes in contact with the region of the plasma with high electron density (i.e. the weakly electronegative plasma core). A grounded grid placed in the ion-ion region allows consequently the acceleration of positive ions, where the ion velocity is controlled by the bias applied to the electrodes in the plasma core. In contrast, when the grid in the ion-ion region is biased, positive ion beams are not detected downstream of the grid. The results indicate that biasing a grid positively in the ion-ion region may result in an electronegative space-charge sheath in front of the grid, which traps the positive ions inside the thruster.