A flux control tool to perform single discharge magnetic configuration sweeping at the TJ-II heliac.

One of the aims of the TJ-II flexible Heliac is to study the influence of magnetic configurations on confinement and stability in a wide range of rotational transform values. This requires numerous discharges with different sets of configuration coil currents being established during the heating/fuelling phase. Plasma discharges are not always easy to reproduce, and the comparison between different types of discharges with different kinetic and magnetic parameters is always cumbersome. A alternative approach is to establish a route in the configuration space that can be swept in a single discharge. This has associated induced currents that change rotational transform profiles on a resistive time scale. To avoid this, a set of poloidal field coils arranged to maximize vector potential and minimize magnetic field in the plasma area (the ohmic coils) are used to counteract the flux variation produced by the configuration coils, so the configuration sweeping is performed minimising the induced currents in the plasma [1] [2] . Previous configuration sweeping experiments at TJ-II [2] could only be performed by establishing ramp rates and flat-tops in the configuration and ohmic coils [3] . The control system has been recently upgraded to accommodate arbitrary waveforms in all the coils, giving a full range of new possibilities. This work presents a computational tool to design arbitrary configuration sweeps with minimum induced ohmic current and its application to the design of the latest experiments performed by the newly commissioned system