Plasma bullet current measurements in a free-stream helium capillary jet

A commercial current monitor has been used to measure the current associated with plasma bullets created in both the positive and negative half cycles of the sinusoidal driving voltage sustaining a plasma jet. The maximum values of the positive bullet current are typically ∼750 µA and persist for 10 µs, while the peaks in the negative current of several hundred μA are broad, persisting for about 40 µs. From the time delay of the current peaks with increasing distance from the jet nozzle, an average bullet propagation speed has been measured; the positive and negative bullets travel at 17.5 km s−1 and 3.9 km s−1 respectively. The net space charge associated with the bullet(s) has also been calculated; the positive and negative bullets contain a similar net charge of the order of 10−9 C measured at all monitor positions, with estimated charged particle densities nb of ∼1010–1011 cm−3 in the bullet. (special)