A plasma system (1) has a circuit including a DC source (6), a power supply line (9), an electrodes (2, 3), and a return line (18). A perturbation signal source (8) delivers a perturbation signal into the circuit in addition to DC voltage from the DC source (6). Acquisition (10) and analysis (11) systems measure response to the perturbation. The analysis system (11) measures variation in impedance of the circuit and phase between voltage and current in the circuit. It also uses a frequency domain reflectrometry technique to measure signal reflection modulus of a supply line of the circuit over a defined frequency range.