Measurement of Time-Resolved Electron Density using a Microwave Hairpin Resonance Probe in a Laser Ablation Plasma Plume

Summary form only given. Laser ablation is a useful technique for applications such as nanomaterial production, micromachining and pulsed laser deposition. For laser ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. The supersonic flow in the ablation plume complicates the analysis of the current-voltage characteristic, particularly in determining the electron density. The ion current signal on negatively biased Langmuir probe situated some distance in front of the target yields measures the ion velocity by time-of-flight and thence the ion density is determined from the signal amplitude. The resonance frequency of a hairpin resonance probe depends on the plasma frequency and thus the absolute electron density. It follows that the hairpin probe can be used to measure the time at which a particular electron density occurs in an evolving plasma. In this paper we compare the temporal variation of ion density in a laser ablation plume, as measured with a planar Langmuir probe, with the electron density as measured with a floating hairpin probe. The laser plasma plume was produced by the laser ablation of silver using a 248 nm, 26 ns, KrF excimer laser. There was good agreement between the histories of plasma density as measured by the two methods.