Design requirements for the plasma edge control system in WENDELSTEIN 7-X

Ten normal conducting control coils will be installed inside the plasma vessel of the WENDELSTEIN 7-X (W7-X) stellarator to modify the magnetic configuration at the plasma edge. The aim is to correct small error fields and to distribute the plasma power on the target plates over a wider area (sweeping). If this two functions are to be combined the power supplies have to deliver a direct current with a superimposed alternating current, the first to correct the error fields and the second for the sweeping. The paper describes possible solutions to combine these two main requirements. The main parameters of the power supply will be presented and some points to be taken into account in the design of the control coils are discussed. The synchronisation of the 10 power supplies is performed by a master control system. Requirements for these control system will be pointed out and the final scheme of the control system will be discussed in detail.