Microsecond resolved electron density measurements with a hairpin   resonator probe in a pulsed ICP discharge
  Time resolved electron density measurements in pulsed RF discharges are shown using a hairpin resonance probe using low cost electronics, on par with normal Langmuir probe boxcar mode operation. Time resolution of less than one microsecond has been demonstrated. A signal generator produces the applied microwave frequency; the reflected waveform is passed through a directional coupler and filtered to remove the RF component. The signal is heterodyned with a frequency mixer and read by an oscilloscope. At certain points during the pulse, the plasma density is such that the applied frequency is the same as the resonance frequency of the probe/plasma system, creating a dip in the reflected signal. The applied microwave frequency is shifted in small increments in a frequency boxcar routine to determine the density as a function of time. The system uses a grounded probe to produce low cost, high fidelity, and highly reproducible electron density measurements that can work in harsh chemical environments. Measurements are made in an inductively coupled system, driven by a single frequency pulsing generator driven at 13.56 MHz and are compared to results from literature. 
David Peterson, David Coumou, Lynda Larson, Steven Shannon
physics.plasm-ph