arXiv subjects

David Terranova

Publications and source records attributed to David Terranova.

4 recordsLinked to original sources

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↗

arXiv1996

Post-Newtonian Cosmological Dynamics in Lagrangian coordinates

The non-linear dynamics of irrotational dust in General Relativity is studied in synchronous and comoving coordinates. All the equations are written in terms of the metric tensor of spatial sections orthogonal to the flow, which allows an unambiguous expansion in powers of $1/c^2$. To lowest order, the Newtonian approximation in Lagrangian form is derived. At this level the evolution is governed by the Raychaudhuri equation for the Lagrangian-to-Eulerian Jacobian matrix. The Lagrangian spatial metric reduces to that of Euclidean 3-space in time-dependent curvilinear coordinates. A Lagrangian version of the Bernoulli equation for the evolution of the `velocity potential' is also given. At the post-Newtonian (PN) level, an exact and general formula is derived for gravitational-wave emission from non-linear perturbations. It is shown that, in the anisotropic collapse of homogeneous ellipsoids, the ratio of the PN tensor modes to the Newtonian metric tends to diverge like the mass density. It is finally argued that a stochastic gravitational-wave background is produced, with present-day closure density $Ω_{gw} \sim 10^{-5}$ -- $10^{-6}$ on $1$ - $10$ Mpc scales.

astro-ph↗

arXiv1996

Lagrangian Dynamics of Collisionless Matter

The non--linear dynamics of self--gravitating irrotational dust is analyzed in a general relativistic framework, using synchronous and comoving coordinates. Writing the equations in terms of the metric tensor of the spatial sections orthogonal to the fluid flow allows an unambiguous expansion in inverse powers of the speed of light. The Newtonian and post--Newtonian approximations are derived in Lagrangian form. A general formula for the gravitational waves generated by the non--linear evolution of cosmological perturbations is given. It is argued that a stochastic gravitational--wave background is produced by non--linear cosmic structures, with present--day closure density $Ω_{gw} \sim 10^{-5}$ -- $10^{-6}$ on Mpc scale.

astro-ph↗