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The Wendelstein 7-X Trim Coil System

The magnet system of the fusion experimental device Wendelstein 7-X (W7-X) contains superconducting as well as normal conducting coils. Seventy superconducting coils are forming the steady state main field to confine the plasma. Inside of the plasma vessel, ten control coils, made of copper, will be placed to modify the strike points of the plasma at the divertor. In addition, a set of five normal conducting, water cooled trim coils will increase the experimental flexibility by providing a means to balance the divertor heat loads among the five field periods. The coils will be placed at the outer surface of the cryostat of W7-X. There are four coils (type A) with equal shape; the fifth coil (type B) has a slightly different shape due to space restrictions. The coils have dimensions of 3.5 × 3.3 m with 48 turns and will be operated with currents of up to 1.8 kA (type A). The other coil (type B) has a smaller size of 2.8 × 2.2 m, compensated by a higher number of turns and a higher operation current of 1.95 kA. Five independent power supplies are being fabricated to operate the coils with a maximum of flexibility. The concept is based on four-quadrant power supplies using Insulated-Gate-Bipolar-Transistors. The trim coil package consisting of the five coils plus the five power supplies is being designed and built in collaboration between IPP, Germany and PPPL, USA, partly funded by the Department of Energy.