Experimental investigation of ion sonic point location in helicon plasma magnetic nozzles using a para–perp Mach probe

The axial location of the ion sonic point in helicon plasma magnetic nozzles is experimentally investigated using a parallel–perpendicular Mach probe under uniform and non-uniform axial magnetic field configurations at RF powers of 0.8–2 kW. Earlier lower-power studies reported that the sonic transition occurred in the downstream region of the expansion chamber. In the present work, the sonic transition is observed to shift upstream toward the magnetic throat with increasing RF power and to approach ideal magnetic nozzle behavior at higher RF powers and magnetic field strengths. A one-dimensional axial force-balance analysis, based entirely on experimentally measured plasma parameters, demonstrates that the ambipolar electric field associated with electron-pressure gradients exceeds the inertial contribution over most of the axial region. These results show that magnetic field topology and RF power jointly regulate the location of the ion sonic point and the efficiency of plasma acceleration in helicon magnetic nozzles, thereby extending previous lower-power experimental observations.