Lucio Barbato

EAST alternative magnetic configurations: modelling and first experiments

Heat and particle loads on the plasma facing components are among the most challenging issues to be solved for a reactor design. Alternative magnetic configurations may enable tokamak operation with a lower peak heat load than a standard single null (SN) divertor. This papers reports on the creation and control of one of such alternatives: a two-null nearby divertor configuration. An important element of this study is that this two-null divertor was produced on a large superconducting tokamak as an experimental advanced superconducting tokamak. A preliminary experiment with the second null forming a configuration with significant distance between the two nulls and a contracting geometry near the target plates was performed in 2014. These configurations have been designed using the FIXFREE code and optimized with CREATE-NL tools and are discussed in the paper. Predictive edge simulations using the TECXY code are also presented by comparing the advanced divertor and SN configuration. Finally, the experimental results of ohmic and low confinement (L-mode) two-null divertor and SN discharges and interpretative two-dimensional edge simulations are discussed. Future experiments will be devoted to varying the distance between the two nulls in high confinement (H-mode) discharges.

Coupling of nonlinear axisymmetric plasma evolution with three-dimensional volumetric conductors

With reference to toroidal fusion devices, we solve axisymmetric nonlinear evolutionary equilibrium equations, describing the plasma behaviour, self-consistently coupled to eddy currents equations, describing surrounding three-dimensional (3D) structures. This formulation allows the analysis of nonlinear plasma quasi-static evolution in the presence of 3D volumetric conductors. Several validations and test cases are presented, suggesting the potential applications of the proposed method to the analysis of various situations of scientific and technical interest for future fusion devices such as ITER and DEMO, like for instance disruptions.

Solution and Extension of a New Benchmark Problem for Eddy-Current Nondestructive Testing

We report on further measurements and a numerical solution for a proposed benchmark problem involving an eddy-current nondestructive evaluation configuration. This is a simplified version of the inspection of fastener holes in aircraft structures, comprising combinations of plates with through holes and a crack. Previously, only line (B-scan) data were provided for the coil impedance, while in this paper, we provide precision surface (C-scan) data as well as details on the evaluation of the critical parameters of the problem. Furthermore, the experimental data are cross validated with numerical ones using a model that combines an integral formulation and the FEM approach.

Effect of three-dimensional conducting structures on vertical stability in EAST

The vertical stability of the Experimental Advanced Superconducting Tokamak (EAST) is studied in detail in the present paper. It is found that correct modelling of the detailed three-dimensional (3D) features of the conducting structures surrounding the plasma is essential in order to get reliable estimates of the growth rate of vertical displacement events. The numerical model developed, based on the CarMa0 code, can take 3D effects into account, and provides results very close to experimental values for a wide range of plasma configurations. A deep insight about the current density patterns induced in passive structures is gained.

Equivalent axisymmetric plasma response models of EAST

A strategy is presented for the derivation of simplified equivalent axisymmetric models of vertical instabilities in the EAST tokamak device. The qualitative understanding and the quantitative results obtained from a 3D model can suggest and quantify suitable modifications of the axisymmetric structures in order to fit both the growth rate and the stability margin for a wide range of different experimental configurations.

3D passive stabilization of n = 0 MHD modes in EAST tokamak

Evidence is shown of the capability of non-axisymmetrical conducting structures in the Experimental Advanced Superconducting Tokamak (EAST) to guarantee the passive stabilization of the n = 0 MHD unstable mode. Suitable numerical modeling of the experiments allows a clear interpretation of the phenomenon. This demonstration and the availability of computational tools able to describe the effect of 3D conductors will have a huge impact on the design of future fusion devices, in which the conducting structures closest to plasma will be highly segmented.

Error Field Impact on Plasma Boundary in ITER Scenarios

Discrepancies in magnetic field maps produced by confinement coils in thermonuclear fusion reactors may drive plasma to lose stability and must be carefully controlled during the whole time evolution of each shot using suitable correction coils. Even when kept below safety thresholds, these error fields (EFs) may alter the geometry of magnetic flux lines, defining the plasma geometry. This paper, using high accuracy 3-D magnetic field computations for confinement coils, addresses the issue of evaluating the effect of EFs on the plasma boundary shape during the shot, modeled as a sequence of equilibrium configurations. In particular, a procedure that can compute the shape perturbations due to given deformations of the coils has been set up and used to carry out an analysis of relationship between the EF and shape perturbations during the time evolution of International Tokamak Experimental Reactor programmed scenario.

Fast computational techniques applied to numerical modeling of RFX-mod fusion device

This paper presents the application of fast computational techniques to the RFX-mod fusion device. The input-output transfer functions are computed.

Numerical behavior of models of composite materials in E'NDT at "Low" frequencies

Introduction In this paper the numerical modelling of composite materials in view of non-destructive testing (NDT) is considered. The numerical modelling is critical especially in the “low-frequency” regime because of a strong ill-conditioning of the relevant stiffness matrix. The proposed numerical model is robust with respect to this underlying ill-conditioning. The conductivity of new composite materials, specifically Carbon Fiber Reinforces Polymers (CFRPs), is reasonably high for allowing nondestructive evaluation based on eddy currents testing. However, the complex electrical structure of the CFRP composites, characterized by higher electrical conductivity along the direction of the carbon fibers and lower conductivity perpendicular to the fibers, poses very difficult problems for their electromagnetic description and modelling. The classic electromagnetic models based on the magnetic (electric) field integral equations, may fail to provide the correct solution at low frequencies such as those of NDT applications [1], [2], [3]. This is due to the low-frequency break-down problem consisting of a strong ill-conditioning of the relevant stiffness matrix. This strong ill-conditioning is due to the different scaling of the solenoidal and non-solenoidal components of the unknown fields, w.r.t. the frequency.