Modifications of wave and plasma structures by a mechanical aperture in a helicon plasma thruster

Plasma and wave structures in a helicon plasma thruster, and an imparted thrust are experimentally investigated with two configurations having a mechanical aperture (MA) near the thruster exit and no MA. A standing helicon wave is excited by locating the MA, where a large amplitude and no spatial change of the phase of the axial component of the radio frequency (rf) magnetic field are detected between the rf antenna and the MA; simultaneously, a higher plasma density by a factor of 2-2.5 is obtained in the source compared with that obtained with no MA. On the other hand, the plasma density downstream of the thruster exit with the MA is lower than that obtained without the MA. The magnetic field measurement downstream of the thruster exit shows the presence of a traveling electromagnetic wave, axial and radial wavenumbers of which are in the range of the slow wave dispersion branch. The directly measured thrust with the MA is only 4.5 mN for 1.5 kW rf power due to the lower plasma density in the magnetic nozzle, while the thrust without the MA is 12 mN.