Improvement of the RFX-mod toroidal power supply to enhance the control of the toroidal magnetic field in the RFX-mod experiment

The control of the reversal parameter F and of the m=0 plasma modes are of paramount importance for the confinement properties in RFP plasmas. In the RFX-mod experiment, best performances are obtained at very shallow F(-0.05 <=F< 0) and with a low bias toroidal field; nevertheless, whenever Fgets positive, the configuration becomes a paramagnetic pinch with very poor confinement and thus only very small deviations of toroidal field at the plasma edge can be tolerated [1]. Moreover the m=0 low order modes may be responsiblefor the phase locking and energy exchange between high order m=1 modes of Reverse Field Pinchplasmas [2]. As a consequence, a proper control of the m=0 modes is a mandatory tool for the mitigation of the phase locking and the improvement either of the spectral purity of Single Helical Axis (m=1,n=7) states and of the effectiveness of Pulsed Poloidal Current Drive (PPCD) operation. In the RFX-mod device, these two tasks require a precise and fast control of the current in the toroidal field coil system, which is composed by 12 toroidalwinding sectors, each one independently driven by a large four quadrant power converter (6kA, 3kV). The toroidal power supply system of RFX-mod has an inherently inadequate precision and a slow response, since its operation was originally planned at high bias toroidal field and deep F; instead, the evolution of the RFX-mod physics showed that better discharges are obtained at much lower currents and fields, as low as some tens of mT. In addition, the overall control chain is affected by excessive latency and measurement offsets. This work describes the optimization of the toroidal power supply carried out to improve the control of the toroidal magnetic field at the edge. In particular, it shows the analyses carried out to better tune the parameters of the control system of the toroidal field and the preliminary experimental results obtained after the modifications. This process is a mandatory step towards a more effective reversal parameter and m=0 mode controller. References [1] M. Barp et al 2011 Fusion Engineering and Design, Volume 86, Issues 6-8,Pages 1000-1004 [2] P. Zanca and S. Martini 2001 Plasma Phys. Control. Fusion 43121