A floating hairpin resonance probe technique for measuring time-resolved electron density in pulse discharge

We present an automated hairpin resonance probe for obtaining time-varying plasma electron density in a pulsed-magnetron discharge, operated with a 13.56 MHz radio-frequency source. When the resonator is placed in plasma, its characteristic resonance frequency in vacuum shifts to a higher value. From the frequency shifts, electron density is easily determined. By applying a fixed microwave frequency, the probe immersed in plasma resonates only at a specific time of the pulse waveform. At a different time of the pulse, the probe resonates at a different frequency. The procedure is automated using a Labview™ program, which increments the applied microwave frequency in small steps of the prescribed value and reads the corresponding resonance peak from an oscilloscope. The spatial and temporal electron density measured using this technique shows a sharp drop in density during the first few microseconds in the on-phase, followed by an increase in density as the discharge develops in the steady-state on-phase. The off-phase shows that decay in electron density at different rates is faster in the region where the magnetic field lines intersect the target. A quantitative model is described to explain different features observed in the experiment.