DC power supply is a electronic device, which converts AC into DC. There are two knobs we could manipulate; the limit current knob and supplying voltage knob. The main function of this power supply is protecting the current overflow in a circuit which could be a significant damage to the other electronic setups.
In a general simple circuit, consisting of a DC supply and a fixed resistance, the circuit current I is determined by the voltage I supply. For example, think this circuit has 10 Ohms, and the set limit current is 1A. The real current will be determined by Ohm's law V = IR, which means I = V/R. With a green light in voltage knob, let's increase the voltage gradually. Soon we would find out the green light would be diminished and the red light next to the current knob will show up, which means the real current in circuit reached to the limit value, 1A. You can't increase your voltage more, unless you increase the limit value of the current.
Consider the situation using this DC power supply in measuring the plasma breakdown point in hollow cathode. The neutral Ar gas is ionized under high initial voltage between tip and keeper, which is the main structure of hollow cathode. Consider that I increased the voltage in supply to V_m and, breakdown has occured. I found out the voltage value is decreasing from V_m significantly, right after the breakdown occured. I think this is the very prevention of the current overflow in plasma-circuit, since the resistance has been decreased dramatically from infinity to the certain value, which means, the current is determined as I = V_m/R. Since, this current V_m/R can't be over the limit current I set, the supply drops the voltage value as V_0, which satisfies I = I_limit = V_0/R. That's why the supply voltage value is reduced largely when plasma breakdown occurs.
