Characterization of RFX-mod conducting structures to optimize plasma start up and equilibrium control

The load assembly of RFX-mod includes three main toroidal passive components such as the Inconel vacuum vessel, the thin copper shell and the stainless steel supporting structure. It is characterized by a global time constant for the penetration of the vertical field much shorter than the typical pulse length, which is routinely over 300 ms after the active MHD control system started operation. Thus the plasma equilibrium must be feedback controlled throughout the pulse by acting on the currents of the Field Shaping Winding (FSW). Even though the penetration of the field is not any longer an hindrance to the dynamic control of the plasma position, the creation of a bias field before the gas ionization has proven to allow a better fulfillment of the equilibrium requirements. On the other hand, during the charging phase of the magnetizing winding eddy currents are induced in the passive structures which are not completely extinguished at the breakdown and contribute to the overall magnetic field inside the vacuum vessel. The analysis of amplitude and shape of each single contribution is important to assess how they can affect the critical phase of gas ionization and plasma current start-up. Moreover, the evaluation of the vertical field transfer function is necessary to design an effective closed loop control system of the bias field in the discharge chamber by fully exploiting the available power before the pulse. RFX-mod has been equipped with a set of magnetic pickup and Rogowski coils properly aimed at the characterization of the electromagnetic behaviour of the passive structures. In order to calculate currents flowing in the structures a 2D FE model of the machine included in a MHD plasma equilibrium code was used after an extensive validation against experimental data. This was considered an adequate tool since non axisymmetric effects on the plasma equilibrium can be neglected. In particular, simulation results allow to separate the contribution of plasma, passive structures and active windings to the equilibrium field configuration. Schemes of simultaneous control of the FSW and a subset of the saddle coils for the MHD mode control were also studied to achieve the desired bias field configuration. The evaluation of the transfer function between the vertical field reference and the actual average field on the equatorial plane inside the vacuum vessel is presented in the paper along with comparisons with experimental results to test its accuracy. (author)