Structure of the ion acceleration region in cylindrical Hall thruster plasmas

We investigated the structure of the ion acceleration region and the shape of the ion velocity distribution function (IVDF) in cylindrical Hall thruster plasmas, using laser-induced fluorescence spectroscopy on Xe II metastable ions. On the thruster axis, the acceleration front is located deeper than a half-length of the discharge channel length, and the acceleration region reaches up to 3 times the discharge channel length (several centimeters) away from the channel exit, regardless of the discharge condition. It is noteworthy that ion acceleration mostly (more than 70%) takes place outside the discharge channel. The IVDF is close to a single Gaussian inside the discharge channel. It however becomes substantially asymmetric when moving downstream. Double Gaussian distributions including cold and hot ion groups was in good agreement with the measured ion velocity distributions downstream with an R-squared greater than 0.995.