Self‐Organized Helical Equilibria in the RFX‐Mod Reversed Field Pinch

Abstract

With operation at high plasma current (I p ∼ 1.5 MA), the plasma in the RFX-mod reversed field pinch selforganises in a 3D helical state with almost conserved flux surfaces featuring strong electron temperature barriers. Up to now the equilibrium of such states was obtained by a perturbative solution of the Newcomb equation in toroidal geometry. This allowed for the mapping of the electron temperature, density and SXR emissivity profiles on helical flux surfaces thus proving the correlation between kinetic plasma quantities and the underlying helical magnetic topology.In order to obtain a full 3D equilibrium reconstruction, the magnetic topology of a SHAx state has been also reconstructed with the VMEC code adapted for the RFP using experimental profiles as input data. Previous results for the axisymmetric case were used as benchmark. It is found that helical equilibria are characterised by a particular q profile with a region of zero magnetic shear as this has been found to be a key ingredient to trigger internal transport barriers.

Theory of tokamak disruptions

“…II, when a disrupting plasma becomes sufficiently nonaxisymmetric to drive strong asymmetries in the normal field to the wall, the disruption may be proceeding too rapidly compared to the wall time for a strong asymmetry inB Án to occur. Spontaneous helical distortions of a toroidal plasma with an axisymmetric normal field distribution on the wall have been calculated 73,74 using VMEC but without halo currents. If that code were generalized to include halo currents in the non-axisymmetric equilibria, then a calculation of the toroidal peaking factor could be made with the field normal to an axisymmetric wall assumed axisymmetric.…”

“…In particular, if the halo plasma is assumed thin compared to the radii of curvature of the plasma surface, the halo plasma can be represented as a jump in the magnetic field at the plasma surface, Appendix E, so a thin-halo model can be added to any equilibrium code. For example, the VMEC code could be modified for this purpose, and it is known that the VMEC code can represent the spontaneous kinking, 73,74 which appears to be an essential element in the drive for destructive halo currents. The degree of axisymmetry of the normal magnetic fieldB Án on the walls may play an important role in the toroidal peaking factor of the halo current and the degree and quickness with which the plasma can resymmetrize in the later phases of a disruption.…”

MHD instabilities in 3D tokamaks

“…Since the standard numerical tools used for tokamak equilibrium computations and for subsequent stability studies are restricted to axisymmetric plasma configurations, the numerical treatment of non-axisymmetric tokamak configurations can greatly benefit from stellarator research. Magnothydrodynamic (MHD) equilibria with an axisymmetric boundary and helical core [2][3][4][5], as well as 3D free-boundary tokamak equilibria taking into account ripple and magnetic perturbation fields [6][7][8][9][10] have been calculated successfully with the VMEC/NEMEC code [11].…”

Visualization of the Plasma Shape in a Force Free Helical Reactor, FFHR

“…The 3D Variational Moments Equilibrium Code (VMEC) [17,18] is applied to FFHR computing the magnetohydrodynamics (MHD) equilibrium problem. Due to the relatively high speed of the VMEC, the code has become the standard code for calculating 3D plasma equilibria [19]. The code provides the 3D magnetic field distribution inside the vessel.…”