The RFX power supply system is a combination of electrical devices, su
ch as a.c.-d.c. converters, direct current interrupters, large capacit
or banks and fast closing switches, whose coordinated operation allows
the investigation of plasma in the reverse field pinch (RFP) configur
ation. The large electrical power, required for the pulses, is derived
partly from the 400 kV high voltage Italian grid and partly by the tr
ansfer to the plasma of energy stored before the pulse in the capacito
r banks or in the machine magnets. The circuit operation phases to obt
ain RFP configurations are initially described: after premagnetizing t
he machine and charging the capacitor banks, the plasma current start-
up follows (between 5 and 50 ms long) during which the energy stored i
s transferred to the plasma. Then, by means of the a.c.-d.c. converter
s, the steady state of the plasma configuration is sustained for a max
imum duration of 0.25 s. In the design of the power supply units a few
aspects have been taken into account which have a great impact on the
technical solutions: the possible fault conditions, which can in a fe
w cases heavily damage the devices; the presence of high electromagnet
ic interference (EMI) which can induce noises in signals and errors in
the circuit operations; the request for easy reconfigurations of the
circuits, to cope with different types of plasma investigations. The f
ault protection led in RFX to the development of an active logic able
to command the required protective actions, which have to operate with
in 1 ms of the fault. The EMI reduction was obtained with a careful an
alysis of the EMI source, related to the magnetic stray field caused b
y the machine magnetizing winding and by the operation of converters,
capacitor banks, interrupters and ignitrons; screening of the area wit
h control equipment, the signal insulation and a particular solution f
or the ground system was adopted. Finally, a high level of plant flexi
bility was reached by adopting a modular solution in the design of the
a.c.-d.c. converter system and of the toroidal capacitor bank. In the
paper the design criteria of the power supply system units are presen
ted, together with a discussion concerning the main choices; the struc
tures of the power supply units (the a.c. voltage network, the a.c.-d.
c. converters, the capacitor bank, the energy transfer and the switchi
ng units and the pulse discharge cleaning system) are described in det
ail. Finally, the results of the commissioning and first operation of
the whole power supply system with dummy loads are also reported.