arXiv subjects

Paolo Zanca

Publications and source records attributed to Paolo Zanca.

3 recordsLinked to original sources

arXiv2025

A novel computation of the linear plasma response to a resonant error field in single-fluid rotating visco-resistive MHD

This paper reexamines the linear plasma response to a static resonant error field (EF) in the single-fluid rotating visco-resistive magneto-hydrodinamic (MHD). A tearing-mode stable, rotating plasma shields a resonant static EF by a current sheet at the resonant surface. This response is encapsulated within the delta prime (Δ'), a quantity which measures the magnitude and phase of the current sheet. However, if EF exceeds an amplitude threshold this equilibrium breaks down and a wall-locked tearing mode is formed. Several basic aspects of the problem are addressed. First, we assess the validity of the radial Fourier transform method, commonly used to solve analytically the problem, by comparison with a completely different technique. Second, we derive a new analytical Δ' global formula valid in a wide range of plasma parameters. This formula describes the Δ' features much better than previous asymptotic regimes modelling. Third, we derive the EF amplitude threshold for producing a locked mode, pointing out the crucial role of the neoclassical poloidal flow damping effect. The result is almost identical to recent two-fluids outcomes, showing that the choice between single-fluid and two-fluids MHD is not crucial in this specific problem.

physics.plasm-ph↗

arXiv2011

Equilibrium reconstruction for Single Helical Axis reversed field pinch plasmas

Single Helical Axis (SHAx) configurations are emerging as the natural state for high current reversed field pinch (RFP) plasmas. These states feature the presence of transport barriers in the core plasma. Here we present a method for computing the equilibrium magnetic surfaces for these states in the force-free approximation, which has been implemented in the SHEq code. The method is based on the superposition of a zeroth order axisymmetric equilibrium and of a first order helical perturbation computed according to Newcomb's equation supplemented with edge magnetic field measurements. The mapping of the measured electron temperature profiles, soft X-ray emission and interferometric density measurements on the computed magnetic surfaces demonstrates the quality of the equilibrium reconstruction. The procedure for computing flux surface averages is illustrated, and applied to the evaluation of the thermal conductivity profile. The consistency of the evaluated equilibria with Ohm's law is also discussed.

physics.plasm-ph↗

arXiv2004

Reconstruction of the magnetic perturbation in a toroidal force-free circular plasma: application to the Reversed Field Pinch

A new method to obtain the radial profile of the magnetic perturbation in a toroidal force-free plasma having circular cross section is developed. The method is quite general and can be applied to any circular low-beta plasma once suitable boundary conditions are imposed. In particular, due to its characteristics, it is appropriate for describing the Reversed Field Pinch (RFP) configuration. The curvilinear metric of the toroidal geometry is described by adopting flux co-ordinates, and the magnetic field is expressed by the contravariant representation with two independent potentials. The method leads to a system of Newcomb-like equations in the two independent potentials for modes with the same toroidal n-number and poloidal mode number m coupled by toroidicity. The advantage of this description is that the potentials together with the metric coefficients can be related in a straightforward manner to the magnetic field. Moreover it is formally simple and maintains a similarity with the cylindrical treatment. Indeed it is customary in RFPs to approach the analysis of MHD instabilities by using cylindrical geometry. Nonetheless the effect of a more realistic toroidal geometry can play an important role, and indeed by applying the method to the Reversed Field eXperiment (RFX) plasma we found that the toroidal effects on the magnetic perturbations are not negligible.

physics.plasm-ph↗