Response of a double probe in a low density D.C. discharge with an analog circuit model comparision

The objective of this work was to obtain experimental data in a low density d.c. discharge using a double Langmuir probe system and compare these results to an analog circuit simulation. The form of the applied voltage on the probe was sinusoidal which induced sheath capacitance effects in the acquired current-voltage data. A computer simulation was used to help resolve the electron temperature, T_e, plasma potential, V&subp;. and electron density, n_e, from the experimental data.

Double probe measurements were performed in a low density argon d.c. discharge at a pressure of 300µm, using two electrodes having a large difference in surface areas. A specially designed circuit imposed a sinusoidal voltage on the probe, while minimizing the effect of the circuit capacitance on the results. Probe current and voltage measurements were obtained for several frequencies between 1-300Hz and an applied voltage of 120V p- p. An analog circuit simulation of the probe-plasma system was constructed using an interactive circuit design program, Design Center by MicroSim. A voltage-controlled current device was used to simulate a theoretical Langmuir probe response for each of the electrodes, while linear capacitors were used to simulate the sheath capacitance. The data taken at 1Hz was used to predict several plasma parameters for the theoretical relationships used in the simulation.

The experimental current-voltage data indicated a plasma break down had occurred at approximately 28V. Although the signal noise in the data hampered the determination of actual Langmuir probe curves, the simulation agrees with experimental data for a small range of applied voltage, below ±20V. The sheath capacitance caused a distinct phase shift in the I-V curves at the higher frequencies and, although linear capacitors provided most of the necessary phase shift, non-linear capacitors may be required for a more complete fit to the data.