Factory and site tests on the 25 MW GTO power amplifier for the control of the JET plasma vertical position

The vertical position of the JET plasma is normally unstable and feedback stabilisation is therefore needed. A new power amplifier (fast radial field amplifier-FRFA) based on GTO inverters has been procured to cope with configurations characterized by plasma having a high degree of vertical instability, beyond the stabilising capability of the phase controlled thyristor power converter used so far. The new power amplifier is composed of four identical subunits which can be connected in two different configurations to achieve output voltages up to 10 kV (at 2.5 kA) and output currents up to 5 kA (at 5 kV). The amplifier is characterised by a peak output power of 25 MW and by switching frequencies of the individual GTOs of 1 kHz. The series connection of more inverters allows more voltage levels (up to nine) to be made available on the load. The paper presents the results of the extensive series of tests on a dummy load performed on the amplifier both at factory, where a complete subunit was assembled, and at the JET sites. The tests included the achievement of the full performances and the assessment of the correct operation of all the required control modes