In the clean room we discovered that when we lower the pressure in the chamber the plasma extinguishes (current goes to zero). what happened?
1. the pressure was dropped by driving a vacuum pump harder which probably tripped a circuit breaker that also supplied current to the power supplies driving the discharge. if there is no applied voltage there can be no current.
2. the mean free path increases as the pressure drops and if there are not enough ionizing collisions the process is not self-sustaining and the number of charge carriers available to carry the current drops to zero.
3. the plasma extinguishes when the pressure is below the minimum pressure required to maintain the plasma.
4. according tot he ideal gas law, the density of atoms or molecules drops when the pressure is decreased n = n/v = p/(kb*t). plasmas require a minimum density to operate.