A New Algorithm for Strike-Point Sweeping at JET

The exhaust particles collected by the divertor structures cause a localized thermal load around the strike points (the intersections of the separatrix with the divertor). To spread on a larger region this thermal load it is convenient to resort to a sweeping, i.e. a periodical movement, of the strike points. This technique has been already applied to JET by injecting 4Hz triangular waveforms on the divertor D2 and D3 coil currents. This strategy is implemented in the architecture of the JET Shape Controller (SC), and has been validated in a number of experiments. However, it suffers from the inconvenience that the movement of the strike points causes, to a certain amount, 4Hz oscillation of the overall shape. These problems are further exacerbated with the JET eXtreme Shape Controller (XSC), which reacts to the strike point movements, since the sweeping frequency is within its bandwidth. This paper introduces an alternative sweeping strategy which address these difficulties. The proposed algorithm has been implemented within the XSC architecture, and has been validated recently at JET.
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