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Atmospheric pressure plasma jets offer a unique route to transport reactive chemical species generated under highly non-equilibrium conditions to a remote downstream sample; as such, they have become the device of choice in many biological and materials processing applications. 1 While our understanding of the propagation mechanism behind the plasma plume is ever increasing, 2 little is known of how, once launched, the properties of the ionisation front can be manipulated in such a way that a particular chemistry can be delivered to a particular spatial location downstream.

The propagation dynamics of an atmospheric pressure plasma jet resemble that of a cathode directed streamer and are determined, in part, by the localised electric field in the streamer head. This contribution employs an optical spectroscopy technique based on the polarisation dependant Stark splitting and shifting of visible helium lines to non-invasively measure the streamer head electric field. It is demonstrated that the streamer head comprises of a high field region with a peak magnitude of ~24 kV.cm-1 which is followed by a low field region, ~9 kV.cm-1 , identified as the streamer tail. The application of varying polarity voltage pulses to supplementary electrodes placed along the axis of streamer propagation was shown to influence the streamer head electric and afford a level of control over the propagation dynamics of the plasma jet, a finding that has considerable application potential.