MHD Dynamics and Error Fields in the RFX-mod2 Reversed field Pinch

RFX-mod is a Reversed Field Pinch device that allowed performing experiments in regimes with a plasma current up to 2 MA. Due to its low value of the safety factor (q<<1) and the central peaking of current density,the RFP is characterized by the presence of MHDmodes that, in RFX-mod, are controlledby a combination of a passive boundary and an active control system. While at low current several MHD modes of comparable amplitudes are simultaneously present (Multiple Helicity), inhigh plasma current regimes,a single resonant m=1 (dominat) MHD mode frequently increases its amplitude while the others (secondary) decrease(Quasi Single Helicity).In order to improve the control of secondary modes, amodificationof the layout of theRFX-mod device is in progress consisting inthe removal of the Inconel vacuum vessel and a modification of the stainless steel supporting structure to be made vacuum tight. In the upgraded device, dubbed RFX-mod2, the shell-plasma distancedecreases from b/a=1.11 to b/a=1.04 and copper, instead of Inconel, isthe conducting structure nearest to the plasma. RFXLocking simulations [1]have shownthat in RFX-mod2 secondary Tearing Modes amplitude and the edge bulging due to their phase locking will decrease; moreover the plasma current threshold for Tearing Modes wall locking will also significantly increase, from the measured RFX-mod 80-120kA to values from 2 to 5 times higher [2].On the other hand, due to the shorter distance from the shell, the plasma will bemore sensitive to magnetic field errors at its boundary, produced by the shell eddy currents near the poloidal cut for the penetration of the electric and magnetic fields andthe holes for diagnostic access. A finite elementcode(CAFEBEM)that computes the induced currents in thin conducting structures with complex 3D geometry, hasbeen used to determineerror fields in RFX-mod2during the plasma start-up phase. No significant variation of wall locking threshold has been found [3].Forplasma currents below the wall-locking threshold, TM will rotate at frequencies in the kHz range, as observed, e.g., in the MST RFP device [4]and in the very low current RFX-mod discharges [5]. The error fields induced by MHD modes in this frequency range are best modelled by a volume integral formulation of induced eddy currents[6].A simplified vacuum approach is adopted where each harmonicis represented by a toroidal surface current densityJ(?,?)=??(?,?)×n?, where the scalar stream functionis given by ?(?,?,t)= sin(m?+n?+?t), where m and n are the poloidal and toroidal numbers respectively. Effects of shell proximity and error fields on edge plasmaproperties in this frequency regimewill be discussed. Moreover, error fields generated by fast MHD dynamics(such as sawteeth crashes or back transitions from Single Helicity to Multiple Helicity states)may cause wall locking if their amplitude is sufficiently high[4]: the impact of error fieldsfor RFX-mod2 will be evaluated by means of RFXLocking simulation.