Time evolution of fast ions created in an expanding helicon plasma

High levels of ion production (~10¹³ cm^-3 plasma density) by helicon plasma sources and supersonic ion exit speeds (between 8 and 15 km/s) have generated considerable interest in the possibility of a helicon source electric double layer thruster. Pulsing the helicon discharge might solve important thruster issues such as plasma detachment, turbulent cross-field diffusion, and antenna heating. Therefore, for thruster applications, an understanding of the temporal evolution of the ion velocity distribution function (ivdf) downstream of an expanding helicon source is needed to choose the optimal operational parameters (duty cycle, pulse length, input power, driving frequency, etc.) to obtain the desired specific impulse along the expansion direction while minimizing the ion energy in the perpendicular direction. We report on laser induced fluorescence measurements of the temporal evolution of the argon-ion velocity distribution function in the expansion region of a pulsed helicon plasma. The measurements were taken in the expansion region, 19 cm downstream of the helicon source. Temporal resolution of 1 ms allowed investigations of different plasma pulse lengths and duty cycles. It was revealed that below a threshold pressure of ~2 mTorr, the ivdfs show a bimodal structure comprised of a slow ion population that appears simultaneously with the inception of the rf pulse and a fast ion population (~7-8 km/s axial flow speed) that appears few tens of ms later.