Impact of helical magnetic boundary in 3D nonlinear MHD physics of the helical RFP

Though nominally axisymmetric devices, both the tok amak and the reversed-field pinch (RFP) plasmas are observed to develop long-lasting helical states , under certain conditions. One example is the snak e phenomenon in the core of the tokamaks [1, 2], whil e global RFP helical states with a quasi-single helicity (QSH) spectrum of MHD modes are observed i n all major RFP experiments [3, 4]. These helical states are associated with peculiar (favora ble in the RFP case) confinement properties [5]. An innovative picture is thus put forward for the RFP as a helical configuration [6], which can share similarities also with the stellarator concept. Mor eover, while helical RFP states appear in the experiment at increasing plasma current [7], a cert ain degree of 3D shaping has been observed to help the helical self-organization in the RFX-mod device [8]. 3D nonlinear MHD modeling of the RFP has long since been a useful tool to interpret and even predict experimental findings. In particular, helical RFP s tates have been observed to emerge in a self- organized way as nonlinearly saturated kink instabi lities in 3D nonlinear MHD long before their experimental discovery [9, 10, 11]. The accordance with the experiment has been recently further improved by introducing helical boundary conditions for the edge radial magnetic field resembling the experimental ones, instead of standard ideal wall c onditions [12, 13]. The impact of helical magnetic boundary in 3D nonli near MHD dynamics of the RFP is the subject of this talk. We will report results from two cross-be nchmarked MHD codes [14]. Magnetic boundaries allowing for a small helical component of the edge radial field turn out to substantially enlarge the parameter space of global quasi-helical RFP solutio ns. In particular, first MHD simulations showing a clear systematic repetition of QSH states with same dominant mode in between relaxation events, a distinctive feature of experimental QSH plasmas, ha ve been obtained. MHD modeling also predicts (as clearly experimentally confirmed in RFX-mod) the po ssibility to select the geometric helicity of the helical RFP configuration, within a certain range. As a result the RFP can be conceived as a customizable helical device. The magnetic chaos hea ling effect occurring in the different helical RFP configurations will be discussed, in particular wit h respect to the plasma edge topology when considering toroidal geometry effects. Tools common ly employed in the stellarator community will be used for the analysis of numerical helical RFP stat es, like flux coordinates for the computation of a safety factor/iota profile associated with the glob al helical structure. Finally, the future steps tow ards a more complete physical modeling of RFP plasmas will be discussed. References [1] Cooper W A etal 2010 "Tokamak magnetohydrodynamic equilibrium stat es with axisymmetric boundary and a 3D helical core" Phys. Rev. Lett. 105 035003 [2] Delgado-Aparicio L etal 2013 "Formation and stability of impurity "snakes" in tokamak plasmas" Phys. Rev. Lett. 110 065006 [3] Martin P etal 2003 "Overview of quasi-single helicity experiment s in reversed field pinches" Nucl. Fusion 43 1855 [4] Chapman B etal 2012 "Direct diagnosis and parametric dependence o f 3D helical equilibrium in the MST RFP" Proc. IAEA-FEC 2012 S. Diego, USA EX/P6-01 http://www- naweb.iaea.org/napc/physics/FEC/FEC2012/papers/622_ EXP601.pdf [5] Lorenzini R etal 2009 "Self-organized helical equilibria as a new p aradigm for ohmically heated fusion plasmas" Nature Phys. 5 570 [6] Escande D F etal 2000 "Single helicity: a new paradigm for the reve rsed field pinch" Plasma Phys. Control. Fusion 42 B243 [7] Piovesan P etal 2009 "Magnetic order and confinement improvement in high-current regimes of