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Description
See full abstract here
http://ocs.ciemat.es/EPS2019ABS/pdf/P4.1038.pdf
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 MHD modes that, in RFX-mod, are controlled by 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), in high 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, a modification of the layout of the RFX-mod device is in progress consisting in the 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 distance decreases from b/a=1.11 to b/a=1.04 and copper, instead of Inconel, is the conducting structure nearest to the plasma.
RFXLocking simulations [1] have shown that 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 be more 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 and the holes for diagnostic access. A finite element code (CAFE BEM) that computes the induced currents in thin conducting structures with complex 3D geometry, has been used to determine error fields in RFX-mod2 during the plasma start-up phase. No significant variation of wall locking threshold has been found [3].
For plasma 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 harmonic is represented by a toroidal surface current density J(theta,phi ) = GradPhi(theta,phi ) × n, where the scalar stream function is given by (theta,phi,t) = sin(mtheta+nphi+t*omega), where m and n are the poloidal and toroidal numbers respectively. Effects of shell proximity and error fields on edge plasma properties in this frequency regime will 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 fields for RFX-mod2 will be evaluated by means of RFXLocking simulation.
[1] Zanca, P., et al, Plasma Phys. Control. Fusion 51 (2009) 015006 [2] Marrelli, L. IAEA 2018 [3] Marrelli, L., et al., Fus. Eng. Des., In press (2019) https://doi.org/10.1016/j.fusengdes.2019.01.054 [4] Almagri, A.F. et al., Phys. Fluids B 4 (1992) 4080 [5] Innocente, P., et al., Nucl. Fusion 54 (2014) 122001 [6] Bettini P., et al., IEEE Trans. Mag. 53 (2017) 7204904
