Hairpin probes: time-resolution and electronegative plasma

A hairpin probe is an open, quarter-wave transmission line. When immersed in a plasma its resonant frequency measures local electron density directly and immediately through the plasma's dielectric response. At low electron density the sheath around the hairpin wires can be relatively large so it is necessary to allow for this. The use of a hairpin in photodetachment studies is explored: issues of time, electronegativity and spatial sensitivity are addressed. Temporal response is expected be limited by the time to establish resonance and the time for the sheath to equilibrate. The resonance is typically $\sim $~GHz so the former limit is $\sim $~10~ns. The latter limit applies when the sheath is wide. Experimentally, in pulsed RF plasmas (eg He), a fully-floating hairpin is seen to have $\sim $~100~ns resolution in the after-glow. In electronegative plasmas the effect of negative ions on the sheath structure is important. Test show that the hairpin functions with adequate sensitivity in the highly electronegative environment of SF$_{6}$; it is even possible to track the effects of fluorine-based etching of the hairpin wire (W). The spatial sensitivity of the hairpin peaks close to the open end. A dielectric rod has been used to mimic a photodetachment trail indicating that photodetachment signals will be a only a few {%} of full volume detachment.