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There's a current/voltage graph on wikipedia for glow discharge that I don't understand.
Aren't we driving the voltage? How can the line possibly go down on the voltage axis i.e. have two values of current for a particular driving voltage? Is this some kind of hysteresis effect? What does the graph look like if you slowly increase the voltage from zero?
Note that in diagrams like these, the power supply driving the discharge is current-limited so when you get into the "line going down" zones, the current does not rise without bound and degenerate into a power arc.
The voltage required to achieve a given current flow in the glow discharge regime goes down because as more and more of the electrode area gets covered with discharge, the gap resistance goes down- until the entire electrode area is glowing. This means it takes less voltage to sustain a glow discharge than it takes to initiate it.
