This paper presents the main engineering aspects of the ISTTOK alternating inductive plasma current (AC) operation, aiming at the achievement of long duration multicycle discharges with flat-top plasma current and null dwell time.

These discharges were obtained by feeding the windings of the horizontal magnetic field with an independent power supply and the primary of the transformer as well as the vertical field windings with currents provided by an on-site developed power supply based on an high-voltage capacitor bank for the first pre-discharge, an electrolytic bank and an H-bridge of four fast IGBTs. The operation of this power supply is controlled by a circuit composed by a TTL logic block for the interpretation of the optical signals provided by the ISTTOK central timing system and four drivers (HTK0030) to interface the TTL logic to each IGBT and to switch off the IGBTs when an overvoltage at their collectors is detected. This power supply provides an alternating square wave voltage, with a small rise time (<20 micro-s) and programmable half-cycle duration (0-1 s) and amplitude (0-300 V) for about 5 seconds.

AC discharges were obtained with a maximum of seven half-cycles at I_p=±4 kA, without dwell time, in a total time span (220 ms) of about five times the maximum duration of a single forward discharge. The Ip flatness is increased by adequate feedback control of the gas puffing, vertical magnetic field and current on the primary of the transformer. The later is based on a discret DAC implemented with a set of IGBTs and power resistors.

The ISTTOK AC operation was simulated using an equivalent electric circuit of the tokamak ohmic circuit with the plasma referred to the primary of the transformer. The numerical results are in good agreement with the experimental ones.